RoofViews

Building Science

Edge metal
Building Science

Edge Metal Design Wall Zones 4 and 5

Keeping water out of a building is undoubtedly the primary function of a roof system. But one could argue that ensuring a building's roof stays in place during high-wind events is equally important. Let's face it, without a roof, it's hard to keep water out! This blog takes a look at one of the subsets of wind design of roof systems: Wall Zones 4 and 5 and their relationship with roof perimeters.IntroductionArchitects, specifiers, and roof system designers are generally focused on the wind-uplift capacity of the roof system itself. Wind resistance of perimeter edges and parapets might not be front of mind, especially given the myriad roof-system Approval Listings that can be found through DORA, FM, and UL. However, rooftop perimeters and corner areas are most vulnerable to high wind, and perimeter edge metal and copings are part of the first line of defense. Codes now include wind-design and system testing for edge metal and copings. FM also just recently (late 2021) updated RoofNav's Wind Rating Calculator to include fascia, copings, and gutters.Edge metal and copingsThe term 'edge metal' encompasses three foundational shapes that are used at a roof's perimeter: L-shaped metal, gravel stop metal, and copings for parapets. The figures below show generic shapes; ones that are often contractor-fabricated. Additionally, there are many manufacturers that provide edge metal. Some of the manufacturer-fabricated shapes are similar to those shown below. However, some are a bit more distinct and some are extruded to achieve more unique shapes.Graphic adapted from National Roofing Contractors AssociationSome examples of GAF's metal details are shown here:Steel and aluminum are common materials used for edge metal shapes and copings. Some are galvanized; some are painted. Commonly used thicknesses range from 20 gauge to 24 gauge for steel and 0.032" to 0.040" for aluminum. The continuous cleat is typically one gauge thicker than the edge metal and coping.Why is wind design of edge metal important?The roofing industry has been investigating high-wind events, primarily through a group called the Roofing Industry Committee on Weather Issues (RICOWI). RICOWI was established in 1990 and has published numerous reports based on post-wind-event investigations of damage caused by hurricanes. RICOWI's most recent report, released November 19, 2019, covers their investigation of the damage caused by Hurricane Michael. RICOWI has published five reports covering their investigations of 6 hurricanes since 2004.One of the most consistent conclusions throughout the series of 5 reports of post-event investigations is that the majority of localized roof damage and roof system failures due to high winds commonly begin at perimeters and corners. This is not surprising as the highest wind loads are at rooftop perimeters and corners. This blog about wind design and ASCE-16, among other topics, discusses the process and factors used to determine wind loads, and it provides additional information about roof zone layout. Localized roof damage and roof system failures due to high winds commonly begin at perimeters and corners. Not recognizing the importance of edge metal design relative to the overall wind performance of a roof system can result in edge metal installations that may not have the appropriate wind-resistance capacity. This could possibly result in localized damage and/or system failures, even when the roof system (i.e., deck, insulation, membrane) is appropriately designed for design wind loads.The following information is intended to supplement the wind design concepts that were discussed in GAF's earlier blog about wind design and ASCE 7-16.Roof and Wall ZonesWind design of metal edges and copings includes an upward and an outward component, unlike the primary roof system which includes an upward component only. (The Edge Metal Testing section of this blog has more information on that topic). ASCE 7 calls the outward pressures acting on metal edges and copings Wall Zones 4 and 5. Wall Zone 4 correlates and is aligned with Roof Zone 2 (the perimeter zones), and Wall Zone 5 is aligned with Roof Zone 3 (the corner zones). The figure shows one example of a building's roof and wall zones. Case studies from this blog provide more specific information related to the figure below.What do the codes say?The International Building Code (IBC) includes requirements for determining the wind-load capacity for metal edges and copings. This requirement has been included since the 2003 version of the IBC. In other words, edge metal and copings should have wind-resistance capacities greater than the design wind pressures. This concept is just like wind design for the primary roof system—the capacity of the system needs to be greater than the anticipated loads.Chapter 15, Section 1504.5 from the 2015 IBC includes requirements for determining the capacity of metal edges and copings."1504.5 Edge securement for low-slope roofs. Low-slope built-up, modified bitumen and single-ply roof system metal edge securement, except gutters, shall be designed and installed for wind loads in accordance with Chapter 16 and tested for resistance in accordance with Test Methods RE-1, RE-2 and RE-3 of ANSI/SPRI ES-1, except Vu1t wind speed shall be determined from Figure 1609A, 1609B, or 1609C as applicable."Chapter 16 of the IBC indirectly includes requirements for determining the wind loads acting on metal edges and copings. In Section 1609.1 Applications, the IBC states "Buildings, structures and parts thereof shall be designed to withstand the minimum wind loads…" The "parts thereof" encompasses metal edges and copings. The requirement in Chapter 15 to design and install metal edges and copings means the outward pressures for Wall Zones 4 and 5 need to be determined.It's worth noting that the scope of the ANSI/SPRI ES-1 test method does not include gutters, which is why gutters are specifically excluded in the code language through 2018. However, SPRI, in 2016, published ANSI/SPRI GT-1, Test Standard for Gutters, which was first included in model codes in the 2021 IBC.Edge metal testingDetermining the design wind pressures (in pounds per square foot) for Wall Zones 4 and 5 is generally the responsibility of the design professional, such as the architect or structural engineer. On the other hand, determining the capacity of metal edges and copings is generally the responsibility of the manufacturer, which may be a manufacturing company or a roofing contractor that fabricates their own metal edges, coping, and clips and cleats.The IBC specifically lists ANSI SPRI ES-1, Test Standard for Edge Systems Used with Low Slope Roofing, as the test method to be used to determine capacity for metal edges and copings. ES-1 includes three (3) test methods (RE-1, RE-2, RE-3), each for a different edge condition.The RE-1 test method is for 'dependently terminated roof membrane systems'. Essentially, a mechanically attached or ballasted membrane is considered to be dependently terminated if a "peel stop" or row of fasteners is not included within 12" from the roof edge. Without a peel stop or a row of fasteners close to the edge of the roof, the edge metal is acting as the mechanical attachment of the perimeter of the membrane. (The RE-1 figure below is rotated clockwise 115 degrees to show the as-tested configuration of the metal edge. ES-1 presumes a ballasted or mechanically attached membrane will flutter and apply load to the metal edge at 25 degrees. The rotated configuration accommodates a hanging load.)The RE-2 test method is for essentially all metal edge types as long as the "horizontal component" is 4" wide or less.The RE-3 test method is for copings, and RE-3 includes two tests. One test includes an upward load and a 'face' load; the second test includes an upward load and the 'back leg' load.The wind-resistance capacity of metal edges and copings is provided in "pounds per square foot" (psf). This is appropriate because the design wind pressures are also in PSF values which makes the comparison of design wind pressures to wind-resistance capacity simple.Where to find Approval Listings for edge metalSimilar to approval listings for roof systems, there are approval listings for metal edges and copings. Approval Listings are found on FM's RoofNav and UL's Product IQ. An account (free) is required for both. Additionally, NRCA has Approval Listings for contractor-fabricated metal edges and copings which are housed on UL's Product IQ and Intertek's Directory of Building Products.ULKnowing UL's Category Control Number is key to navigating UL's Product IQ. . For metal edges and coping, UL's Category Control Number is "TGJZ". After logging in, performing a search using "TGJZ" provides a list of the manufacturers that have Approval Listings with UL. Clicking on GAF's Approval Listings allows users to easy find rated Roof-edge Systems, Metal, for Use with Low-slope Roofing Systems.Within UL's TGJZ category, GAF has 16 metal-edge products rated using the RE-2 test method and 8 coping products rated using the RE-3 test method. For example (as shown in item 3 in the screen capture above), GAF's M-Weld Gravel Stop MB Fascia B made with aluminum is rated "190 psf". That means this product can be used when the design wind pressures, which include a safety factor, for Wall Zones 4 and 5 are less than or equal to 190 psf.FM's RoofNavWithin RoofNav, Approval Listings for metal edges and copings can be found under "Product Search" using the "Flashing" category. Most likely, users of RoofNav are familiar with the "Assembly Search" function which is regularly used to locate roof systems based on their wind-uplift ratings.The search can be further refined within "Subcategory" by selecting Expansion Joint, Gutter, or Perimeter Flashing.Currently, GAF has 59 Approval Listings in RoofNav: 12 for Coping, 41 for Fascia, and 6 for Gutter products. A screen capture from RoofNav shows GAF's first 20 products.Looking closely at the Listing, the EverGuard EZ Fascia AR – Steel provides detailed information about the product itself and the installation requirements. As shown below, the listing includes multiple Ratings (i.e., wind-uplift capacity) based on material type and thickness, and face height.While the Listing is for a steel fascia, an aluminum fascia is also shown in the detailed information. It's important to note that the chart with the "steel" listing's detailed information is the same chart that is available for EverGuard EZ Fascia AR – Aluminum, as well. Therefore, it's prudent for designers and specifiers to provide appropriate information in the specification to avoid mis-communiction about intended product use.Take note of the material and gauge of the "retainer" (i.e., the continuous cleat). The continuous cleat is required to be 0.50 aluminum, regardless of fascia material type for this Listing. Because the strength of the cleat is a significant factor to the overall wind-uplift capacity of the metal edge (or coping), increasing the thickness of the cleat proves to be an effective method to increase performance.FM RoofNav and Edge SecurementFM announced on its website on October 28, 2021 that "The Wind Ratings Calculator has been updated to return separate flashing ratings for roofs." The red-highlighted area shows the required capacity for Fascia, Coping, and Gutter products.Comparison of the Minimum Wind Uplift Approval Ratings Needed (1-75, 1-90) to the Perimeter and Corner Ratings of the EverGuard EZ Fascia shows that each product type provides the required capacity, and in most cases the required capacity greatly exceeds the required rating.Load PathThe 3 test methods included in ANSI/SPRI ES-1 standard determine the wind-resistance capacity of edge metal attached to a substrate. In other words, the measured capacity (Rating) is of the metal edge or coping attached to the wood blocking; the tests do not measure the capacity of the attachment of the wood blocking to any substrate. The National Roofing Contractors provide information on this topic. The NRCA Roofing Manual: Membrane Roof Systems—2019, on page 289 states:"Wood Nailers and Blocking: Many of the construction details illustrated in this manual depict wood nailers and blocking at roof edges and other points of roof termination. Wood nailers must be adequately fastened to the substrate below to resist uplift loads. This especially is true at parapet walls/copings and roof edges where edge-metal shapes are fastened to wood blocking.Among other advantages, the nailers provide protection for the edges of rigid board insulation and provide a substrate for anchoring flashing materials. Wood nailers should be a minimum of 2 x 6 nominal-dimension lumber. To provide an adequate base, nailers should be securely attached to a roof deck, wall and/or structural framing. In the design of specific details for a project, a designer should describe and clearly indicate the manner in which wood nailers and/or blocking should be incorporated into construction details. A designer should specify the means of attachment, as well as the fastening schedule for all wood nailers and blocking."To that end, FM Global Property Loss Prevention Data Sheet 1-49, Perimeter Flashings, provides a number of recommendations for anchoring wood blocking to various types of walls and structural framing. One example of a roof/wall intersection shows the bottom nailer bolted to the bar joists to ensure an adequate load path.In SummaryArchitects, specifiers, and roof system designers are required by code (always check specific local requirements) to determine wind loads not only for the primary roofing system, but for the metal edges and copings as well. Manufacturers and fabricators are responsible for determining the wind-uplift capacity of their metal edge and coping products, as well as their primary roofing systems.Given the relatively new requirements in the IBC for edge securement, designers, consultants, and specifiers should become familiar with both UL's and FM's approval listings for metal edges and copings. Manufacturers of metal edge and coping products are available to assist designers with selection of edge securement.

By James R Kirby

April 24, 2023

Featured Blogs

  • Architects and the GAF team working together to problem solve.
    Building Science

    GAF and the Design Community: Bridging the Gap with the Designer's Collective

    GAF continues to improve its services to meet customers' needs. This includes thinking more and more about the challenges that designers face and how the company can help offer solutions. As a result, the design community can turn to GAF for comprehensive support from day one of their commercial projects.The Designer's CollectiveIn support of this effort, GAF recently hosted its first Designer's Collective. The three-day event in New Jersey brought 11 of the East Coast's most influential architects and design consultants together with GAF department leaders.Regional Sales Directors Natalia Gallo (Mid-Atlantic), Larry Cargal (Gulf Coast), and Cory Yawger (North East), cohosted and organized the event as a series of roundtables and one-on-one conversations. The intimate forum allowed the design community to express needs and concerns directly to GAF department heads, plus give real-time feedback on existing GAF resources for architects and specifiers.GAF team members Natalia Gallo, Cynthia McArthur, Larry Cargal and Monica Wynn ready to learn from attendees The Outdated Gap"GAF started as a shingle manufacturer," explained Gallo. This meant that the company relied on contractors for an invitation to the table on commercial projects. But having manufacturers participate so late in the game was a disservice to owners, architects, project budgets, and the buildings themselves.GAF can provide expertise across several segments of the roofing industry including Commercial and Solar. With so many resources, GAF knew it was time for the relationship with designers to also evolve and grow. "Architects have a partner in GAF, not just a vendor," said Gallo. "And we will help them guide their own customers to the right solution for their particular building."It's nearly impossible for any single design firm to stay completely abreast of the constant changes in building codes. "That's why so many firms specialize," said Cargal. But GAF wants to and can help relieve the burden of navigating projects alone.That's why GAF territory managers "show the architect, step-by-step," what is needed for a commercial project as well as the support available to them. "It's not about sales," said Cargal. "It's about education."Opportunities for ImprovementGAF already offers architects comprehensive field services, technical support, and architectural services via the GAF Design Services team. But the Designer's Collective created an opportunity to further prioritize the design community's needs. When manufacturers can put more of their focus on the architects, everyone wins.Designer's Collective attendees outside of GAF headquartersBetter Project and Building Outcomes"When we know the building owner's goals, capital budget, and operating expense, we can better match the utility of roof assembly to the goals of their building and budget," said Cargal. Without a manufacturer-designer connection, it's left up to the roofer and general contractor who might have different goals. This means the owner's goals could get lost. As a result, "quality and budget can suffer, and the longevity of the building suffers, too," said Cargal.More Time and Resources Saved for Architects"In today's world, one thing you can't afford to waste is time," said Gallo. A close relationship between architects and manufacturers means GAF can directly address pain points instead of presuming. "When we know exactly what architects need, we can save them time," said Gallo. "That's the difference between service and hospitality. Making our customer important enough that we center their needs in the equation."Learning from the Design Community about Their ChallengesThe Designer's Collective provided the framework for architects to convey their industry needs, while also forging personal relationships with GAF leaders. In contrast to larger GAF events in the past, the Designer's Collective was intimate, explained Gallo. As a result, these designers and the GAF department heads are just a phone call away from each other for requests, concerns, and real-time feedback on GAF products and designs.Here are some key insights attendees shared, which will help the GAF team uncover new opportunities to help minimize these issues and support the customer.Employee ShortagesArchitects are observing labor shortages. Overworked principal architects need employee training support. Firms are having trouble getting existing employees to come into offices.Time ResourcesMany designers find they waste time navigating manufacturer websites to find necessary code and installation information.One existing tool GAF has in place today is our Design Services Resources. In addition to the information and assets available on the website, the Design Services team engages on a national scale with associations like the Construction Specifications Institute (CSI), American Institute of Architects (AIA), and International Institute of Building Enclosure Consultants (IIBEC) to bridge the education gap on the latest codes and trends in building design.Product DifferentiationIt's a challenge to differentiate GAF products and warranties, and explain performance value to building owners.Cargal explained that GAF's warranties are lateralized across all systems. "It can be hard for architects to differentiate between importance of or value of different products and warranties," he said. "This event really helped us open our eyes to how we could help architects to help owners to understand."Closing the GapEvents like the Designer's Collective allow architects to "be more of a stakeholder in the process," said Gallo. "A direct relationship with manufacturers can increase the revenue to architects' own customers, because they can serve those customers better."When the gap is closed, owners get a better finished product and a better performing building, explained Cargal. "We're serving architects exactly where they need us to. Without events like the Designer's Collective, we're in the dark about architects needs."Learn more about events like the Designer's Collective and how to receive technical and design support, sustainable options, system integrations, code approvals, and much more from the GAF team.

    By Annie Crawford

    February 14, 2023

  • Woman sitting in open window under steep-slope shingle roof
    Building Science

    Ventilation of Steep-Slope Roof Systems and Transitions

    Ventilation for steep-slope roof assemblies is often misunderstood. One must not only understand the code requirements, but be able to translate them into real-world installations. Building codes have requirements for ventilation of steep-slope attics and enclosed rafter spaces. Balanced ventilation — nearly equal amounts of intake and exhaust — typcially provides efficient ventilation. Transitions between low-slope and steep-slope roof areas require more distinct intake and exhaust details than traditional eaves/soffits and ridges. This blog provides information relating to ventilation for educational purposes only. Designing ventilation to meet the specific needs of a given project remains the responsibility of the architect, specifier, design professional or roofing contractor. Damage due to inadequate ventilation is typically excluded from coverage under manufacturer warranties. Introduction Residential attic ventilation was a requirement in the very first edition of the Building Officials Conference of America's (BOCA's) model building code that was published in 1948! Even though this requirement has been around for decades, it is still often misunderstood. Perhaps it's the words used and perhaps it's because the code isn't quite specific enough. When discussing residential construction, we often hear something like "We need to vent the roof," when we really mean that we need to vent the attic. We don't ventilate steep-slope roofs themselves; we ventilate the space beneath the roof. More specifically, ventilation is needed for the space under the roof system that is above the insulation in the attic floor. That's the space we know most commonly as an attic (when the insulation is located in/on the floor of the attic). Benefits of attic ventilation Ventilation of an attic space provides a couple of benefits: it lowers the attic temperature and also helps reduce excess moisture that can accumulate. These benefits occur when the air in an attic space is replaced by outside air that is a lower temperature and has less moisture in it (i.e., lower relative humidity). While this seems obvious for most parts of the US, even in warm, humid locations like Miami and Houston, the majority of the time the ambient air is cooler and contains less moisture than the air in an unconditioned attic. Code requirements The International Residential Code (IRC) applies to one- and two-family dwellings, and because of that, most in the roofing industry relate attic and rafter ventilation with residential steep-slope construction, which is a valid and correct presumption. However, the International Building Code (IBC), which covers all buildings other than one- and two-family dwellings (e.g., commercial, industrial, institutional, large residential), also includes information about attic and rafter ventilation because a large number of these types of buildings also include steep-slope roof systems. To that end, both the IBC and the IRC have requirements that apply to the ventilation of attics and enclosed rafter spaces. These requirements are included in Chapter 8, Section R806, Ventilation in the 2018 IRC, and in Chapter 12, Section 1202, Ventilation in the 2018 IBC. (Free versions of the codes are found here.) Both the IRC and IBC include nearly identical requirements, albeit the code sections are arranged slightly differently. The following summarizes the requirements: The requirements for ventilation are specific to enclosed attics (insulation on the floor of the attic) and enclosed rafter spaces (where ceilings are applied directly to the underside of roof rafters/framing members and insulation is between rafters above the ceiling). Vents should not allow the entry of rain and snow. Vents are to be protected from the entry of small 'creatures' such as birds and rodents. Corrosion-resistant materials are to be used, and minimum and maximum sizes of vent openings are provided. The minimum net free vent area is 1/150 of the vented space. The minimum net free vent area can be reduced to 1/300 when both of the following conditions are met: In climate zones 6, 7, and 8, a Class I or II vapor retarder1 is installed on the warm-in-winter side of the ceiling (i.e., attic floor). A "balanced ventilation"2 method is used. 1Vapor retarders — An example of a Class I vapor retarder is a polyethylene sheet, and an example of a Class II vapor retarder is kraft-faced fiberglass batt insulation. The polyethylene sheet or the kraft-paper side of the insulation should be installed immediately below the attic floor insulation layer in order to meet the requirements shown above, regardless if it's a traditional attic or an enclosed rafter space. Importantly, but not specifically required in the codes, these vapor retarders should be installed and detailed to also act as air barriers to prevent warm, moist air from the interior spaces from leaking up into the attic. 2Balanced ventilation — "Balanced ventilation" means 40% to 50% of the required ventilation area is located in the upper portion of the attic, and the remainder is used for intake at the eave or within the bottom 1/3 of the attic area. Commonly, exhaust vents consist of continuous ridge vents or static vents no more than 3 feet from the ridge (measured vertically). Intake vents within soffits or eaves are common, and in-plane intake vents (such as GAF Cobra IntakePro®) are used when eaves and soffits are not built to include intake vents. Current construction methods commonly incorporate the balanced ventilation method for residential attic construction and, therefore, the 1/300 ratio is used to calculate ventilation amounts. The 1/300 ratio means 1 square foot of attic ventilation (evenly split between intake and exhaust) is needed for every 300 square feet of attic floor space. The intent of the requirements for balanced ventilation is that there is more intake than exhaust. This is quite important! Having more intake than exhaust means there will be proper convective flow from eave to ridge. Because warm, moist air is more buoyant than dry air, the warm, moist air rises and is exhausted at the upper portion of the attic. When there is less intake than exhaust, the lack of intake can "choke" the system, reducing the overall effectiveness of the attic ventilation system. Balanced ventilation and reroofing Balanced ventilation is not only important for new construction, but it is an important objective for steep-slope reroofing projects, especially for residential construction. During reroofing, if the amount of exhaust is increased (e.g., by adding a ridge vent with more total exhaust capacity than the previous static exhaust vents), the amount of intake ventilation should be determined and increased as necessary to create a balanced system. If the amount of intake is too little, intake air will come from other sources! A lack of intake at the eave/soffit can lead to air being drawn into the attic from the interior of a residence through can-lights, ceiling vents, and attic-access locations. Believe it or not, air can be pulled from basements and crawl spaces through the cavities in interior walls up into the attic spaces. These "interior" sources of air can contain warm, moist air that can be detrimental to attics, causing condensation and other moisture problems that didn't previously exist. The interior air may not have been drawn into the attic if the system was previously balanced, even if undersized. So, be cautious when increasing the exhaust amounts on existing buildings without assessing the intake amounts. Addressing any 'intake' deficiencies during steep-slope reroofing projects can help ensure that ventilation is balanced and functioning as intended. This post isn't going to dive into calculating the required amounts of ventilation. To better familiarize yourself with that calculation, use the GAF Attic Ventilation Calculator. The calculator determines the minimum amount of exhaust and intake, and the minimum lineal feet of specific GAF products, such as Cobra Rigid Vent 3 for warmer climates, Cobra SnowCountry for cold and snow climates, and Master Flow Undereave Vents, is provided to meet those calculated amounts per the 1/300 ratio. Modern changes to construction: Cathedral ceilings Historically, given that attic ventilation requirements go back decades, the code originally applied only to the traditional attic space under a steep-slope roof — that is, attics with insulation located at the floor of the attic/in the ceiling of the upper floor of a residence. Today, and in the recent past, the traditional attic space is often now a usable, conditioned space. That means the ceiling is attached to the underside of the sloped rafters creating a cathedral ceiling, or some form of that. The traditional attic is turned into occupied space, and the result is an enclosed rafter space. (Remember the code language from earlier that says "attics and enclosed rafter spaces"?) Chapter 8, Section R806, Ventilation in the 2018 IRC, and Chapter 12, Section 1202, Ventilation in the 2018 IBC provide an option for ventilation when a cathedral ceiling is installed with insulation under the roof deck in the enclosed rafter space. The specific requirement for this type of construction states that there must be a minimum 1" vent space in each rafter space directly beneath the roof deck above the insulation. This can be somewhat difficult to construct and maintain continuous air flow. Also, once constructed, inspection and repair is difficult without removal of interior drywall and/or exterior soffits and eave components. The graphic, from the International Association of Certified Home Inspectors, is an example of ventilation of the construction method that incorporates enclosed rafter spaces. The 1" minimum required air space (under the deck between the rafters) is considered to be the vented space, and that means the requirements for the protection of openings from snow, rain, and small creatures, as well as corrosion resistance and sizes of vent openings, are applicable. The minimum net free vent area requirements may also apply when there is a vent cavity/air space under the deck and above the insulation between the rafters. In other words, the vent space size is calculated the same way as the traditional attic space. Specifically, the 1/150 ratio still applies, and in order to reduce the amount of ventilation to 1/300, the additional requirements for Class I and II vapor retarders in Climate Zones 6, 7, and 8, and balanced ventilation also apply. At no time can the vent space between the rafters above the insulation and below the roof deck have less net free vent area than is required for intake and exhaust vents. The depth of the air space may need to be greater than 1" deep to accommodate enough air flow to provide proper ventilation. For example, if the 1/300 ratio determines that 10 square inches per lineal foot of net free vent area (NFVA) is required, a 1" deep air space is appropriate. However, if 20 square inches per lineal foot of NFVA is required, then a 2" deep air space is needed to provide appropriate air flow. Calculating the required depth of the air space to match the amount of NFVA for eave intake and ridge exhaust should take into account the ratio of rafter-to-open air space for continuous eave and ridge vents. Tricky transitions There are many options to vent eaves and ridges on traditional residential construction. However, where a steep-slope roof transitions to a low-slope roof (and vice-versa), the methods to provide intake and exhaust ventilation can be a bit trickier. Where a low-slope roof abuts the low edge of a steep-slope roof, a good option for intake vents is to use a "deck-level" intake vent, such as GAF Cobra Intake Pro. This type of intake vent is intended for use where there are no eaves or soffits available to install traditional intake vents. Due to the potential for water to build-up at the transition from the low-slope roof to the steep-slope roof due to rain, sleet, or snow, or some combination thereof, it's logical to install a "deck-level" intake vent up-slope at least 2 courses. It is best to locate an intake vent far enough up-slope to help prevent snow from blocking the vents, as well. The National Roofing Contractors Association (NRCA), in The NRCA Roofing Manual: Steep-slope Roof Systems—2017, provides the following detail for a "Steep- To Low-Slope Roof System Transition." A key element is that NRCA shows the bottom edge of the shingle roof is a minimum of 10" from the low slope transition point. This helps prevent water intrusion through the steep-slope roof. And if the "deck level" intake vent is up 2 courses, the intake is some 20" from the surface of the low-slope roof (albeit measured along the slope, not vertically). Where a low-slope roof abuts the upper portion of a steep-slope roof, detailing and constructing the exhaust vent is needed in order to properly terminate the low-slope roof. The concept, in general, is to use one-half of a ridge vent, and that likely means this detail is built in place (it does not appear that there are pre-manufactured vent devices for this type of installation). A gap is needed at the top of the sloped deck to allow air to move from the attic or enclosed rafter space up and out the vent material. As shown in the detail below, wood blocking and vent materials are installed on top of and along the upper edge of the steep-slope roof covering. A nailable top layer (e.g., a 2x6) is installed to keep the vent material in place and to act as a nail base for the termination of the low-slope roof. In addition to the ventilation details needed at these types of transitions, it's important to remember the transition details need to consider the continuation of the water, air, thermal, and vapor boundary conditions. You can refresh your knowledge with this GAF blog post. What the codes mean but don't say Simply put, ventilation of attics and enclosed rafter spaces occurs outside of the thermal layer. The code requirements have been developed and instituted based on this, but codes don't explicitly state it. That leads to confusion by some who ask if low-slope roofs need to include ventilation. Let's think about that. For membrane roofs with insulation above the deck (that is, compact roofs), where exactly would the ventilation space be located? Between the insulation and the membrane? That's not how low-slope roofs are constructed. The next possible location for a ventilation space would be under the deck, which means the ventilation is on the conditioned side of the thermal layer for a low-slope, compact roof system, and that is illogical. Expensive conditioned air would easily escape from the building, and unwanted exterior air would easily enter. That would be like leaving doors and windows wide open while air-conditioning or heating a space. One very important point — even if there was a way to provide intake and exhaust vents as part of a low-slope roof system, a horizontal air space provides no path for warm moist air to rise to an exhaust vent. Another way to say it — natural convective flow does not really happen in a horizontal space. In conclusion We ventilate our attics and enclosed rafter spaces to remove unwanted heat and moisture. According to the GAF Pro Field Guide for Steep-slope Roofs, attics can reach up to 165° F, and for asphalt shingles, excessive heat can reduce shingle life. The Guide provides information why venting makes sense, and there are a couple other details available for review and use. Keep your ventilation balanced!

    By James R Kirby

    June 07, 2021

  • Polyiso installation
    Building Science

    Designing with Polyiso

    Thermal insulation is an important part of commercial roofing assemblies. As with anything, there are ways to design with and install polyiso insulation — a better way, a best way, and many variations in-between! What may be best in terms of lowest up-front costs, may prove only good or worse over the long-term life of the building.As discussed later, important points to remember about polyiso and its use include:Due to polyiso's very low air permeability, good design and installation practices can result in low risks of condensation, even for buildings with somewhat higher than normal humidity levels.Building designers can use the label R-value, stated at a mean temperature of 75°F, for projects in most ASHRAE climate zones. However, for climate zones 6 and 7, it may be appropriate to use published values at 40°F.Electricity is four times as expensive as gas, on an equivalent Btu basis. Therefore, polyiso's greatest value is seen during summer months when air conditioning usage is at its highest.IntroductionPolyiso has become the go-to insulation for commercial roofing with about 75% market share. The reasons include:Lowest cost per R-value.High R-value per inch, meaning it doesn't require as much thickness as other types of rigid insulation to achieve the same insulation performance.Good fire resistance. Being a thermoset unlike polystyrene foams, it doesn't melt during a fire. Melting of thermoplastic foams and subsequent flowing down of molten material through the deck into a building can be hazardous for occupants and fire crews.Solvent compatibility — adhesives used in adhered systems don't adversely affect polyiso. This is not the case with polystyrene foams.Reduced condensation — When installed correctly in a properly designed system, polyiso helps to resist interior air from reaching the underside of the membrane, thereby reducing condensation risks.However, just because polyiso is specified and used doesn't mean that the ideal outcome is reached. There are installation details that need to be considered.Single-ply membrane combined with polyiso can make for a good roof, but additional factors can improve performance:The BasicsPolyiso boards have straight cut edges (the boards are not tongue and grooved). These butt joints between boards can allow for vertical air movement.Polyiso was often installed as a single layer:As shown in the figure above, a single layer of butt-jointed insulation is generally a bad idea and is no longer allowed by the International Building Code (IBC) in the 2018 and newer versions. For roofs with a single layer of butt-jointed insulation boards:There is little restriction for air flow and so, during wind events, wind uplift forces result in membrane billowing for attached systems.Membrane billowing draws interior conditioned air up to the underside of the membrane and may create a risk of condensation within the assembly.Billowing may place additional stress on membrane fasteners, a factor recognized by membrane manufacturers by the issuance of shorter guarantees or warranties for mechanically attached versus adhered membrane.Essentially unrestricted air flow up into the assembly diminishes the insulation value and lowers energy efficiency.In contrast, two layers of polyiso restrict the free flow of air into the roof assembly. Care must be taken to seal between roof penetrations and the polyiso, or air is still free to move up into the assembly as shown here:Spray foam is a good choice for achieving a seal between the polyiso and the penetrations.Two layers of polyiso with staggered joints and sealed penetrations is a good system, but two of its features still reduce its insulation efficacy. These are shown in the following magnified view of a roof assembly cross-section.This cross-section illustrates lower energy efficiency resulting from:Insulation and membrane fasteners both act as thermal bridges, conducting heat through the assembly.Gaps between insulation boards that allow thermal convection to occurWhen both the insulation and membrane are attached and fastener densities are at their highest, R-value reductions of up to 29% have been predicted.Adhered versus AttachedThe alternative to mechanical attachment is to adhere the insulation with adhesive. Typically, low rise foam is the adhesive used for both insulation and cover board installation. The original method of application was as a ribbon but for labor savings it is often sprayed in a spatter-pattern as shown in the following picture of a typical installation in progress using one of GAF's low rise foamsUsually, when polyiso is adhered, the roof membrane is also adhered. For a steel deck substrate, fasteners are typically only used for the first layer of insulation, and then the adhesive is used on the subsequent layers of insulation and membrane. While it may be possible to adhere directly to a steel deck and eliminate fasteners totally, this approach is more common with concrete decks, as shown in the schematic here:Key features of systems with adhered insulation are:Air flow up through the assembly is limited and the risk of condensation in cold climates is low. Similarly, membrane billowing is minimized due to the restricted air flow up through adhered insulation layers.Thermal bridging is minimized since only the first layer of insulation might be mechanically attached. With concrete decks, the first layer of polyiso can be adhered thereby eliminating the use of fasteners totally.Wind uplift resistance is uniformly distributed across the roof deck. A system with adhered membrane and insulation can act as a monolithic system with excellent wind uplift resistance.When combined with an adhered membrane the finished roof appearance is usually aesthetically pleasing. When applied in accordance with manufacturer's instructions, the membrane can be very flat and there will not be any insulation fasteners to telegraph through and mar the appearance.Induction WeldedInduction welded fasteners are another type of roof attachment, the largest type being the Drill-Tec™ RhinoBond® system. By definition this is a mechanical attachment method but it has many of the features of adhered systems. The technique fastens TPO and PVC membranes to the substrate below using a microprocessor controlled induction welding machine. The thermoplastic roof membrane is welded directly to specially coated fastening plates used to attach the insulation. The picture below shows such a system being used:The induction machine is placed above each plate in turn and activated for approximately 10 seconds. As the machine is moved to the next position, a weighted magnet is placed over the plate and acts to squeeze the membrane down onto the hot fastener plate causing it to weld to that plate's surface coating.For typical mechanical attachment of single-ply membranes with fasteners along the seam lines, the insulation boards are simply secured with five fasteners per 4 x 8 ft. board to keep them flat. When using a Drill-Tec™ RhinoBond® system, the combined insulation and membrane fasteners resist wind uplift forces as shown in this picture during a test of wind uplift resistance:This means that wind loads are more uniformly distributed versus a conventionally attached system.Key features of systems with induction welded membrane attachment are:Performance is very similar to adhered in terms of the distribution of loads.Thermal bridging is reduced compared to traditional mechanically attached membrane systems.There are no application temperature restrictions and so this approach can be used in place of adhesive attachment regardless of how cold it might be.Drill-Tec™ RhinoBond® systems can be cost competitive due to the speed of installation as compared with traditional bucket and roller adhesives.Dew Point CalculationsThe dew point is the temperature at which moisture vapor forms condensation. It's a function of the relative humidity and the ambient temperature. The evaluation of a roof assembly where the dew point might be reached is an important step towards designing a roof with minimized condensation risks. The topic has been covered in greater depth elsewhere, but this section summarizes the key steps. Examine the chart below (this is a simplified form of what is used by HVAC engineers).Locating 40% relative humidity in the first column and then going across to the 70°F, design dry bulb temperature shows a dew point of 45°F. This means that in an environment that is 70°F and 40% relative humidity (RH), water in the air will condense at a temperature of 45°F. A roof assembly separates the interior conditioned environment from the outside and the insulation layer in the roofing system resists heat loss or gain to/from the outside, depending on the season. Within the insulation layer the temperature has a gradient between the hot and cold side, i.e. between inside and outside.As an example, consider a building in the winter to illustrate the point. The interior is 70 °F with 40% RH, like the example on the chart above. The temperature gradually drops from the innermost part of the insulation until at the outermost part it will be at the exterior, cold temperature. The plotting of temperature through the insulation thickness is referred to as the temperature gradient of that system. Using the example, if the temperature gets to the dew point of 45°F at any point in that system then water would be expected to condense on the nearest surface. This is shown in the following diagram:Summarizing, in this example the interior air has 40% of the total water vapor that it can support. But as the air migrates up through the roof system, it gets cooler until the point where it can no longer hold onto the water vapor and condensation occurs. In the example shown above, that's at 45°F and just inside the insulation layer.As will be discussed later, dew point calculations can be used to inform the placement of a vapor retarder layer when used.When a dedicated vapor retarder layer is not used, it is good practice to ensure that the dew point is in the upper layer of polyiso insulation. This will lower the risk of interior air migrating up through the roof assembly and condensing on a surface below its dew point.Using Vapor Retarders with PolyisoVapor retarders can be used to limit humidity ingress to the roof assembly. Under certain circumstances, depending on the building use and location, condensation can occur as discussed in the previous dew point discussion. In general there are four basic building conditions that can be considered:1. Buildings with Standard Amounts of Occupancy-Generated MoistureThese are the most common situations covering for example, office, retail, and warehouse spaces. The risk of having a condensation issue is low and good roofing practices such as sealing around penetrations may likely suffice.2. Buildings with Larger Amounts of Occupancy-Generated MoistureThis category includes apartments and other multiple residency buildings, paper mills, laundries, buildings with indoor swimming pools, and the like. In fact, anything that doesn't fit into category 1 above should be evaluated to determine humidity levels. The building's air handling and ventilation systems should be carefully specified to take into account the moisture loading.3. Construction–Related MoistureMost construction practices release some amount of moisture into the building space. These can be relatively short term such as drywall installation and painting. However, some practices can release large amounts of water over a considerable time frame into the building. These include poured in place concrete floors and roof decks.4. Concrete Roof DecksThese can present a challenge for roof system designers especially in new construction. Regardless of the type of concrete, significant amounts of water remain after curing is completed. Allowing concrete to thoroughly dry is most appropriate; however, it is often reasonably impractical. Dealing with potential moisture in concrete decks is beyond the scope of this article, but guidance can be found elsewhere.It is recommended that a building science professional experienced in designs for Categories 2, 3, and 4 be involved to determine whether a vapor retarder should be used and what type.Specification of R-ValuePolyiso is manufactured to meet the ASTM C1289 Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board. ASTM C1289 specifies the thermal resistance at a mean temperature of 75°F for various product thicknesses and requires that the values at 40 and 110°F be made available upon request.Important features of manufacturer's published R-values are:The values are an average across a temperature range. The test methods need the insulation specimen to have a hot and a cold side at least 40°F apart. Most reputable test labs use a difference of 50°F for accuracy. A published value at 75°F is actually an average R-value across the range of 50 to 100°F.To take into account the diffusion of gas out of, and air into the polyiso foam, the values are based on projected "Long Term Thermal Resistance" or LTTR, obtained by rapidly aging thin slices of the foam.The Polyisocyanurate Insulation Manufacturers Association, PIMA, conducts a third-party certification program to independently validate LTTR values. This is referred to as the PIMA QualityMark™ program. The LTTR values are considered as "labelled R-values" to be used by building design professionals.Label R-values represent a 15-year time-weighted average value. They can help a design professional estimate a building's energy efficiency without having to be concerned about long-term loss of performance, which is already factored into the value.Building design professionals designing roofs for ASHRAE climate zones 6 and 7 may need to use the R-value reported for a mean temperature of 40°F.Attachment PatternsThe various fastener patterns for polyiso have been mentioned in the previous sections. However, due to the impact of fasteners on thermal bridging and wind uplift resistance, the key points are summarized here:For systems that have both mechanically attached membrane and insulation, the membrane attachment provides the wind uplift resistance. The polyiso insulation fasteners are simply there to hold the insulation flat during the roof installation and to resist long term lateral movement.Typical fastener patterns are shown here:For systems with adhered single-ply membrane and mechanically attached polyiso boards, the insulation fasteners provide the wind uplift load resistance. This is the case whether both layers of polyiso are mechanically attached or only the bottom layer (in which case the upper layer would be adhered).Manufacturers have tested the fasteners per board required to meet wind uplift resistance requirements for these combined mechanically attached and adhered systems. The number of fasteners needed depends on the board size and thickness. For common systems, the numbers are shown in the table below:For each of these combinations above, manufacturers' handbooks provide fastener patterns.The number of fasteners for these combined mechanically attached and adhered systems is very large, for example a 125,000 s.f. big box type roof could require around 50,000 fasteners, resulting in significant thermal bridging.When installing over a steel deck, to reduce thermal bridging and to make for a more robust system with reduced condensation risk, it is advisable to only attach the first layer of polyiso and to adhere all subsequent layers and the membrane.If the first layer of polyiso is attached and the rest of the system adhered, then using a 1.5" thickness for that first layer would help to bury the thermal bridging fasteners. It could also put the dividing line between first and second polyiso layers below the dew point, which is advisable.ConclusionsPolyiso is a cost-effective roof insulation and has the advantage that its permeability is low. Good design and installation practices can result in low risks of condensation even for buildings with higher than normal humidity levels.When designed correctly, mechanically attached components with two layers of polyiso having staggered and offset joints can be part of a successful roof system.Adhered insulation and membrane roof systems have advantages including reduced or eliminated thermal bridging, lowered condensation risks, and better wind uplift resistance.In cases where building use anticipates higher interior humidity levels and/or the local climate suggests higher condensation risk, then a building science professional should be consulted as to vapor retarder use and specification.Building designers and specifiers are advised to use the labelled R-values shown for a mean temperature of 75°F. For projects in ASHRAE climate zones 5 and 6, values at 40°F could be used depending on the building's geometry and local energy costs.

    By Thomas J Taylor

    March 17, 2021

  • GAF Cedar City plant
    Building Science

    How will ASCE 7-16 affect the low-slope roofing industry? A Tale of Two Buildings

    A common question being asked in the roofing industry is whether or not the 2016 version of ASCE 7 is going to increase the design wind pressures acting on a building. The answer is "yes" in many cases. So, the follow up question is "by how much?" And, that leads to the next question, "how much more capacity will roof systems be required to have when wind design follows ASCE 7-16?" This blog looks at 2 buildings of different sizes and heights in two cities in the U.S., one centrally located and one along the Gulf coast, and compares design wind pressures based on ASCE 7-10 and ASCE 7-16. By assessing the required roof-system capacity for all roof zones in conjunction with the increases in the size of roof zones, we can begin to put some real numbers to the question "by how much will ASCE 7-16 affect the low-slope roofing industry?" While analysis of two building types in two cities doesn't represent a significant study, this study offers an example of how the 2016 version of ASCE 7 is going to affect installed roofing systems over the next decade. Case Studies Design wind pressures (DWP) were determined for two building types in two cities. DWPs were based on varying the Risk Category and Exposure. The following table shows the building types, cities, variables, and constants. The big box store is 290' long x 169' wide x 24' tall, and the apartment building is 100' long x 40' wide x 55' tall. Wind speeds were determined using an online tool. DWPs can be determined by using proprietary third party methods (e.g., RoofNav), simplified calculation tools (e.g., RoofWindDesigner), or hand calculations following the methods within the ASCE 7 standard. For this case study, DWPs were determined using hand calculations via an Excel spreadsheet. The DWPs are slightly more refined since there was no rounding of wind speeds for ASCE 7-16. Both RoofNav and Roof Wind Designer round to the nearest 5 or 10 mph. The maps in ASCE 7-16 have been revised, not only from a wind speed perspective but from the number of wind isobars on the maps. The ASCE 7-16 maps are more refined which allows for more exact wind speed values. ASCE 7-10 Map, Risk Category II, showing a limited number of wind-speed isobars. ASCE 7-16 Map, Risk Category II, showing many additional wind-speed isobars relative to the ASCE 7-10 wind map shown in the above figure. In total, 72 different sets of DWPs were calculated. Buildings over 60' tall are designed the same in ASCE 7-16 as in ASCE 7-10; therefore, this article focuses on buildings less than 60' tall. Design Wind Pressure Results The following charts provide the DWPs for each of the scenarios that were analyzed for this article. Design wind pressures based on ASCE 7-10 and ASCE 7-16 for a big box store in Kansas City, MO that is 24' tall. Design wind pressures based on ASCE 7-10 and ASCE 7-16 for a big box store in Mobile, AL that is 24' tall. Design wind pressures based on ASCE 7-10 and ASCE 7-16 for a 5-story apartment building in Kansas City, MO that is 55' tall. Design wind pressures based on ASCE 7-10 and ASCE 7-16 for a 5-story apartment building in Mobile, AL that is 55' tall. Comparing the 2010 and 2016 versions The primary intent of this article is to start generating data to help answer the question "how much did the DWP increase from the 2010 version to the 2016 version of ASCE 7?" Using the DWP calculated for each roof zone within each of the four scenarios, we calculated the percent increase in DWP per zone. The percent increases in DWP were calculated by dividing the ASCE 7-16 value with the corresponding ASCE 7-10 value for each roof zone. However, because there are 4 roof zones in ASCE 7-16 (and 3 roof zones in ASCE 7-10), both Roof Zones 1' and 1 were divided by the value for ASCE 7-10 Roof Zone 1 to determine the percent increase. Values in the following charts that are below 100% (shaded gray) indicate a reduction in DWP from ASCE 7-10 to 7-16. Percent increases in DWP per roof zone for a big box store in Kansas City, MO that is 24' tall, focusing on Exposure C values. Percent increases in DWP per roof zone for a big box store in Mobile, AL that is 24' tall, focusing on Exposure C values. Percent increases in DWP per roof zone for a 5-story apartment building in Kansas City, MO that is 55' tall, focusing on Exposure C values. Percent increases in DWP per roof zone for a 5-story apartment building in Mobile, AL that is 55' tall, focusing on Exposure C values. For the big box store scenarios, the percent increases in DWP per roof zone varies from 84% to 178%. The percent reductions (values less than 100%) are all in Roof Zone 1' while Roof Zones 1, 2, and 3 have increased DWP. As seen in the Figures, Roof Zone 1 percent increases are greatest while Roof Zone 3 increases are smallest. For the 5-story apartment building scenarios, the percent increases in DWP per roof zone varies from 84% to 177%. The percent increases in Roof Zone 1 are the largest. The percent reductions are all Roof Zone 1'. It is worth noting that the increases in Pressure Coefficients from ASCE 7-10 to 7-16 are, in fact, relatively indicative of the increases in DWP. For more on Pressure Coefficients and the wind design process, see this GAF blog. The case studies in this blog also show that the 2016 version of ASCE 7 imposes higher DWPs in Roof Zones 1, 2, and 3. However, given some of the substantial reductions in Roof Zone 1', it could be expected that installed roof system capacity may be lower. While that is hopeful, the roofing industry has a minimum-capacity backstop of 60 psf. This means that any calculated DWP at or below 60.0 psf (even for DWP values as low as 23 psf) will have a 60-psf-capacity roof assembly installed to meet building code requirements. The reality is many buildings are required to use a 60-psf-capacity roof system when in fact '60 psf' is more capacity than needed based on calculated DWPs. Let's take a closer look at one subset for each of the 4 scenarios. Example 1: Big Box; Central US The big box store in KC, MO (Risk Category II, Exposure C) saw the DWP increase from 38 psf to 56 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 32 psf in Roof Zone 1'. Regardless of the increases or decreases, a 60-psf-capacity roof should be installed in both Roof Zone 1 and Roof Zone 1' per 7-16, which is unchanged relative to ASCE 7-10. Roof zones and pressures for a 24' tall big box store in Kansas City, MO based on ASCE 7-10 using Risk Category II and Exposure C. Roof zones and pressures for a 24' tall big box store in Kansas City, MO based on ASCE 7-16 using Risk Category II and Exposure C. For Roof Zones 2 and 3, the DWPs are not only increased, but the sizes of the roof zones that require higher capacity are also increased. Roof Zone 2 per ASCE 7-10 required a 75-psf-capacity roof, and a 75-psf-capacity roof will be needed per ASCE 7-16. However, approximately 9% of the roof area will need a higher capacity because of the increase in roof zone size. The following chart shows roof area percentages based on the required roof system capacity for this scenario. Chart showing required roof capacity and the roof area (%) where each is required. Example 2: Big Box; Gulf Coast A big box store in Mobile, AL (Risk Category II, Exposure C) saw the DWP increase from 69 psf to 109 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 63 psf in Roof Zone 1'. A 75-psf-capacity roof should be installed in Roof Zone 1', and higher-capacity roofs will be required in Roof Zones 1, 2, and 3 relative to ASCE 7-10. Roof zones and pressures for a 24' tall big box store in Mobile, AL based on ASCE 7-10 using Risk Category II and Exposure C. Roof zones and pressures for a 24' tall big box store in Mobile, AL based on ASCE 7-16 using Risk Category II and Exposure C. For Roof Zones 1, 2, and 3, the DWPs are not only increased, but the size of the roof zones that require higher capacity are also increased. A larger perimeter roof zone (combining Roof Zones 1 and 2) with higher required capacity is created (relative to 7-10). Approximately 30+% of the roof area will need a higher capacity. The following chart shows roof area percentages based on the required roof system capacity for this scenario. Chart showing required roof capacity and the roof area (%) where each is required. Example 3: 5-story; Central US The 5-story apartment building in KC, MO (Risk Category II, Exposure C) saw the DWP increase from 45 psf to 66 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 38 psf in Roof Zone 1'. However, because the method used to determine the sizes of roof zones in ASCE 7-16 is changed, Roof Zones 1' and 1 do not exist on this building! Decreases of DWP in Roof Zone 1' are rendered meaningless, and the lack of a Roof Zones 1' and 1 means the entire roof consists of only Roof Zones 2 and 3, the perimeter and corner roof zones. Roof zones and pressures for a 55' tall apartment building in Kansas City, MO based on ASCE 7-16 (top) and ASCE 7-10 (bottom) using Risk Cat II and Exposure C. The increases in DWPs for the roof zones significantly increase the roof system capacity requirements for this scenario. Almost the entire roof area will need a higher capacity roof. The following chart shows roof area percentages based on the required roof system capacity for this scenario. Chart showing required roof capacity and the roof area (%) where each is required. Example 4: 5-story; Gulf Coast The 5-story apartment building in Mobile, AL (Risk Category II, Exposure C) saw the DWP increase from 81 psf to 129 psf in Roof Zone 1 per ASCE 7-10 vs ASCE 7-16; and reduce to 74 psf in Roof Zone 1'. Similar to Example 3, due to the size of the roof zones relative to the size of the roof, Roof Zones 1' and 1 do not exist in this scenario. Roof zones and pressures for a 55' tall apartment building in Mobile, AL based on ASCE 7-16 (top) and ASCE 7-10 (bottom) using Risk Cat II and Exposure C. The increases in DWP for the roof zones significantly increase the roof system capacity requirements for this scenario. The entire roof area will need a higher capacity. The following chart shows roof area percentages based on the required roof system capacity for this scenario. Performance versus Prescriptive The case studies in this analysis are presented using a performance method for wind design. That is, a specific DWP is determined for each roof zone and a roof system with appropriate capacity is selected for use in the associated roof zone. Another method that is often used in the roofing industry is a prescriptive method for wind design. The DWP for Roof Zone 1 (or Roof Zone 1' in certain localities) is used to select a roof system with an appropriate capacity for Roof Zone 1 (similar to a performance method), and then Roof Zones 2 and 3 use an installation method/layout that is prescriptively increased relative to Roof Zone 1 installation methods. Prescriptive installation methods for Roof Zones 2 and 3 are regularly used with mechanically attached (MA) roof systems, and for MA roofs, Roof Zone 2 increases are commonly 50% and Roof Zone 3 increases are commonly 100%. Conclusion ASCE 7-16 DWPs are generally increasing relative to ASCE 7-10, but this impact is better understood by comparing the required roof system capacity for different building types and dimensions in different cities. Looking only at DWPs does not provide enough information; a comparison of required roof system capacities is needed. Not only did the DWPs change, but the methodology for determining the size of the perimeter and corner roof zones changed significantly in the new ASCE 7-16 version; the amount of roof area contained by the perimeter and corner roof zones increased. The case studies—of two buildings less than 60 ft tall in two cities—and corresponding analysis in this blog are just one additional small step to understanding how the ASCE 7-16 will affect wind design over the next decade.

    By James R Kirby

    February 21, 2021