Building Science

The Details Make the Difference in Wind Design

By James Willits

August 23, 2017

Bright red kite flies high above city skyline on bright summer day

Roofing design encompasses many different factors. The assembly is dictated by the use of the building, the owner's budget, the building's location, local building codes, energy codes, and the forces of nature that are regularly, as well as occasionally unleashed upon it. In addition, a change in one part of the building envelope can adversely affect something else. As this implies, there are often many choices that the designer has to make. It is important to note that the installer also has a great effect on the overall performance of the system. Communication between the designer and the installer is paramount to the success of the system. The designer needs to relay exactly what components should comprise the assembly, as well as how the system should be installed. Conversely, the installer should alert the designer of any conditions or potential changes that do not match the plans and specification, since a small change can affect the entire envelope.

Communication between the designer and the installer is paramount to the success of the system.

Let's talk about golf for a moment. A 300-yard drive has exactly the same value on the scorecard as a 6-inch putt. The same is true on the roof. If the installer omits sealant and a clamp on a pipe flashing detail because the incorrect one was displayed in the plans, it has the same result as a cold-welded seam: Water in the building. So nearly every detail, no matter how small, can have the same effect. One such mishap may be small, but like strokes on the scorecard, they all add up.

There are certain details that often get overlooked. Sometimes specifications and plans don't match. If that happens, which one prevails? Sometimes plans trump details, others the opposite is true. Very often, perhaps in the interest of conserving time or effort, a specification or plan detail will state to comply with an established standard, such as those published by FM (Factory Mutual, which does its own system testing for its member insurance companies), SMACNA (Sheet Metal and Air Conditioning Contractors National Association), or the International Plumbing Code without specifying which exact detail or practice. One very common mistake, for example, is specifying FM 1-105 on an OSB deck, but FM doesn't test over combustible decks. So according to FM, a system over an OSB deck wouldn't be rated to withstand 105 pounds per square foot, or PSF, of uplift pressure. Perhaps, in that case, it would be better to outline specific enhanced fastening patterns or fastener pull out values.

Sometimes specifications and plans don't match. If that happens, which one prevails?

The designer of record can possibly open themselves up to liability if they leave details up to the installer's interpretation. Quite often, trades will mix and match responsibility of interfacing details, such as components of drains, counterflashing roof edge termination, coping cap waterproofing, and HVAC transitions to name a few. Returning to the situation where FM 1-105 over an OSB deck has been specified, the designer should ideally have consulted with the membrane manufacturer to identify options that have been demonstrated to conform to established standards. Good and experienced suppliers do a lot of system testing to understand how to achieve required levels of performance with as many options as feasible.

Wind Uplift — The Basics

Wind Pressure

Wind uplift, in general, is the upward force pulling on the building components as a result of wind blowing around and over the building. The roof is naturally exposed to these forces due to its location. When the wind flow moves over the edge of the roof it creates negative pressure. In addition, positive pressure exerted from inside the building from HVAC and openings such as doors and windows can also contribute to these forces, depending upon the building's construction.

Edges are Critical

Roof Field

Corners and perimeter zones are especially vulnerable to wind uplift forces due to their proximity to the edge. Vortices are created at corners, which can increase the upward pull. The next illustration is a top view of the roof, identifying perimeter and corner zones. As a rule of thumb, attachment (uplift resistance) is enhanced at a rate of 1.5x at the perimeter and 2x in the corner to combat these forces. Roof edge termination is especially critical, since it is at the leading edge holding the roof to the structure.

This fully adhered TPO roof was peeled back from the edge during a wind event, separating insulation layers.

This fully adhered TPO roof was peeled back from the edge during a wind event, separating insulation layers.

Roof edge termination is instrumental in resilience to these forces. Remember the golf analogy? Well, nearly every detail counts the same on the scorecard. Imagine this: You are on the 8th tee just starting your backswing when a meteor the size of a 1966 Volkswagen Beetle crashes in the middle of the fairway leaving a huge smoking crater. This is not simply a stroke, but instead, it is a catastrophic ending to the game (and quite a story). The same is true with the roof edge. A few years ago, the National Roofing Contractors Association, NRCA, independently tested numerous roof edge terminations. Mark Graham, the Vice President of Technical Services for the NRCA stated in an article featured in Professional Roofing magazine, "...flexural failure during edge metal testing is much more common than fastener pull-out:" The act of just adding more fasteners will not suffice, because if the metal is an insufficient gauge for the application, it will flex, allowing wind to lift it. It is reasonable to assume that when the edge catches air, the rest of the system is likely to follow like dominoes.

Roof edge termination is especially critical, since it is at the leading edge holding the roof to the structure.

Attention to Detail

So, if edges are critical, what is to be done? Ideally two things are recommended; first, instead of a general reference to compliance with SMACNA standards, it would be prudent to call out the exact detail that should be applied in specific locations. Second, the specifier may want to designate which trade is the best to be responsible for each detail, as opposed to leaving the decision up to the trades to decide what to include or exclude within their respective scopes. If a SMACNA detail is to be applied, then perhaps a sheet metal contractor may be the better choice to be responsible for that scope.


TP-3 Courtesy of NRCA Guidelines for Single-ply Membrane Roof Systems

Take a moment to look at one common example from the NRCA, which is generally understood to be considered "good roofing practice." Shown above is Detail TP-3 from the NRCA Guidelines for Single-ply Membrane Roof Systems.

The field membrane extends over the roof edge, and down the wood nailer, and is secured by the fastening of the anchoring cleat on the face. The thermoplastic (TPO or PVC) coated metal is then placed on top of the membrane and fastened based upon the Architectural Metal Flashing Securement options found within the NRCA Roofing manual. That detail is completed with a hot air welded flashing strip that ties the roof membrane to the Thermoplastic coated metal for a watertight assembly. That is a roofing detail to be installed by a roofer. Imagine for a moment that the owner wanted to save some money; would you, as the designer, decide to do it here? Keep in mind that even though the assembly may qualify for a standard warranty, the owner is still exposed to the inconvenience of dealing with replacement, as well as collateral damage such as lost wages due to clean up, lost merchandise due to damage, and lost use of space while waiting for repair. There are other ways for a designer to save money on the assembly that do not significantly increase the risk of the roof blowing off. Remember the meteor? The diagram on the right is from ANSI/SPRI/FM 4435/ ES-1-11.


This document establishes standards for roof edge details as they relate to wind uplift resistance based upon actual testing from collaboration with ANSI (American National Standards Institute), SPRI (Single Ply Roofing Industry), and FM (Factory Mutual). It illustrates one of the methods of testing the edge termination. This demonstrates a mechanically attached system with the same detail as above (NRCA TP-3). A load is applied to the field membrane at a 25 degree angle from the deck to simulate the stresses of the field sheet billowing. How would the less expensive alternate detail fare in this test?

...even though the assembly may qualify for a standard warranty, the owner is still exposed to the inconvenience of dealing with replacement...

The table below is from ANSI/SPRI/FM 4435/ ES-1-11:

ANSI chart

Courtesy of ANSI/SPRI/FM 4435/ES-1-11

Pay special attention to a few things; first, it shows the recommended minimum gauge for each metal (a thicker gauge can be specified for added strength); second, it is based upon the width of the exposed metal, so the wider it is, the thicker it should be. ES-1-11 outlines design criteria for wind uplift for edge details. This document is created as a guide to keep roofs where they belong.

Wrapping it Up

The designer of record, whether an Architect or a Consultant, should be decisive, and choose specific appropriate details. The owner is looking for a roof that is resilient, cost-effective, and does not cause any problems. Keep ANSI/SPRI/FM 4435/ ES-1-11 close, and don't risk your reputation in the hands of the lowest bidder. Ask any golf pro and they will tell you that putting is 40% of your game, so you had better make it 40% of your practice.

About the Author

James has over a decade of experience in the roofing industry as an installer, a project manager, a sales person, and a training manager. As the GAF CARE Training Operations Manager, he translates his industry knowledge and experience into the practical installation of roofing systems. James is well regarded for presenting training and seminars that cover roofing theories and practice to a range of audiences.

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Are Hybrid Roof Assemblies Worth the Hype?

How can roofing assemblies contribute to a building's energy efficiency, resiliency, and sustainability goals? Intentional material selection will increase the robustness of the assembly including the ability to weather a storm, adequate insulation will assist in maintaining interior temperatures and help save energy, and more durable materials may last longer, resulting in less frequent replacements. Hybrid roof assemblies are the latest roofing trend aimed at contributing to these goals, but is all the hype worth it?What is a hybrid roof assembly?A hybrid roof assembly is where two roofing membranes, composed of different technologies, are used in one roof system. One such assembly is where the base layers consist of asphaltic modified bitumen, and the cap layer is a reflective single-ply membrane such as a fleece-back TPO or PVC. Each roof membrane is chosen for their strengths, and together, the system combines the best of both membranes. A hybrid system such as this has increased robustness, with effectively two plies or more of membrane.Asphaltic membranes, used as the first layer, provide redundancy and protection against punctures as it adds overall thickness to the system. Asphaltic systems, while having decades of successful roof installations, without a granular surface may be vulnerable to UV exposure, have minimal resistance to ponding water or certain chemical contaminants, and are generally darker in color options as compared to single ply surfacing colors choices. The addition of a single-ply white reflective membrane will offset these properties, including decreasing the roof surface temperatures and potentially reducing the building's heat island effect as they are commonly white or light in color. PVC and KEE membranes may also provide protection where exposure to chemicals is a concern and generally hold up well in ponding water conditions. 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By Authors Kristin Westover

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Building Science

Thermal Bridging Through Roof Fasteners: Why the Industry Should Take Note

What is going on here?No, this roof does not have measles, it has a problem with thermal bridging through the roof fasteners holding its components in place, and this problem is not one to be ignored.As building construction evolves, you'd think these tiny breaches through the insulating layers of the assembly, known as point thermal bridges, would matter less and less. But, as it happens, the reverse is true! The tighter and better-insulated a building, the bigger the difference all of the weak points, in its thermal enclosure, make. 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Likewise, The National Energy Code of Canada for Buildings: 2020 addresses thermal bridging of a number of building components, but also explicitly excludes fasteners: "in calculating the overall thermal transmittance of assemblies…fasteners need not be taken into account" (Section Admittedly, point thermal bridges are often excluded because it is challenging to assess them with simple simulation tools.Despite this, researchers have had a hunch for decades that thermal bridging through the multitude of fasteners often used in roofs is in fact significant enough to warrant study. Investigators at the National Bureau of Standards, Oak Ridge National Laboratory, the National Research Council Canada, and consulting firms Morrison Hershfield and Simpson Gumpertz & Heger (SGH), have conducted laboratory and computer simulation studies to analyze the effects of point thermal bridges.Why Pay Attention Now?The problem has been made worse in recent years because changes in wind speeds, design wind pressures, and roof zones as dictated by ASCE 7-16 and 7-22 (see blogs by Jim Kirby and Kristin Westover for more insight), mean that fastener patterns are becoming denser in many cases. This means that there is more metal on average, per square foot of roof, than ever before. More metal means that more heat escapes the building in winter and enters the building in summer. 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The results might surprise you.First, it's no surprise that the fastener reduced the R-value of the 2' x 2' sample of ISO alone by 4.2% in the physical sample, and 3.4% in the computer simulation (Case B-I compared to Case A-I).When the HD ISO was added (Cases II), R-value fell by 2.2% and 2.7% for the physical experiment and computer simulation, respectively, when the fastener was added. In other words, adding the fastener still caused a drop in R-value, but that drop was considerably less than when no cover board was used. This proved what we suspected, that the HD ISO had an important protective effect against the thermal bridging caused by the fastener.Next, we found that the steel deck made a big difference as well. In the physical experiment, the air contained in the flutes of the steel deck added to the R-value of the assembly, while the computer simulation did not account for this effect. That's an item that needs to be addressed in the next phase of research. 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Extra insulation beyond the code minimum can be specified to make up the difference.Where Do We Go From Here?Some work remains to be done before we have a computer simulation that more closely aligns with physical experiments on identical assemblies. But, the two methods in our recent study aligned within a range of 0.8 to 6.7%, which indicates that we are making progress. With ever-better modeling methods, designers should soon be able to predict the impact of fasteners rather than ignoring it and hoping for the best.Once we, as a roofing industry, have these detailed computer simulation tools in place, we can include the findings from these tools in codes and standards. These can be used by those who don't have the time or resources to model roof assemblies using a lab or sophisticated modeling software. With easy-to-use resources quantifying thermal bridging through roof fasteners, roof designers will no longer be putting building owners at risk of wasting energy, or, even worse, of experiencing condensation problems due to under-insulated roof assemblies. Designers will have a much better picture of exactly what the building owner is getting when they specify a roof that includes fasteners, and which of the measures detailed above they might take into consideration to avoid any negative consequences.This research discussed in this blog was conducted with a grant from the RCI-IIBEC Foundation and was presented at IIBEC's 2023 Annual Trade Show and Convention in Houston on March 6. Contact IIBEC at or GAF at for more information.

By Authors Elizabeth Grant

November 17, 2023

very severe hail
Building Science

Defending Against Very Severe Hail

Think that your roof doesn't need protection against hail? Think again.Severe hail events are increasing in geographic footprint and are no longer just in hail alley. The geographic region that experiences 1 inch or larger hailstones has expanded to be nearly two-thirds of the United States. Nearly 10 percent more U.S. properties, more than 6.8 million, were affected by hail in 2021 than in 2020. Coinciding with the increase in properties affected by a damaging hail event in 2021, there was also an increase in insurance claims, which rose to $16.5 billion from $14.2 billion in 2020.Figure 1: The estimated number of properties affected by one or more damaging hail events. Source: NOAA, graphed by VeriskAccording to data from Factory Mutual Insurance Company (FM Global), a leader in establishing best practices to protect buildings, the review of client losses between 2016-20, showed that the average wind/hail losses averaged $931,000 per event. That's a significant impact on a business, and it doesn't account for the other effects that a disruptive loss could have such as headaches from the process of repairing or replacing damaged roofs. As a result, designing the roof to withstand damage from hail events has become not only a best practice, but a necessity.Why does hail size matter?FM Approvals is a third-party testing and certification laboratory with a focus on testing products for property loss prevention using rigorous standards. FM Global, through the loss prevention data sheets, requires the use of FM Approved roof systems. FM Global estimates their clients lose about $130M each year on average from hail events in the United States. Given the increasing volume of severe hail events and the resulting property loss, damage, and financial impacts, FM Global added to the requirements in the FM Loss Prevention Data Sheet (LPDS) 1-34 Hail Damage in 2018. Loss Prevention Data Sheets provide FM's best advice for new construction and for Data Sheet 1-34, this includes new or reroofing projects on existing buildings. Data Sheet 1-34 provides guidelines to minimize the potential for hail damage to buildings and roof-mounted equipment. FM Global intends that the data sheets apply to its insured buildings; however, some designers use data sheets as design guidelines for buildings other than those insured by FM Global.FM's LPDS 1-34 identifies the hail hazard areas across the United States: Moderate Hail hazard area, Severe Hail hazard area, and Very Severe Hail (VSH) hazard area which are defined by hail size. Note that the VSH area roughly correlates to Hail Alley. Hail Alley receives more hailstorms, and more severe ones, compared to other parts of the country.Figure 2: FM's LPDS 1-34 map outlining the different hail categories: moderate, severe, and very severe. The Very Severe area is most commonly referred to as "Hail alley".The hail hazard areas are divided by hail size, with the Very Severe hail hazard area being the largest hail size of greater than 2 inches. As a result, roofing assemblies have to meet the most stringent hail testing for designation in the Very Severe hazard area.Figure 3: Description of FM Approval hail regions.Even if you are not in hail alley, or one of the states in FM's Very Severe Hail area, hail larger than 2 inches still has the potential to occur throughout the contiguous United States. The National Oceanic and Atmospheric Administration (NOAA) tracks weather events throughout the United States, including hail. NOAA's hail database includes information such as location, date, and magnitude (size) of the hail stone for each event. A sampling of typical data is provided below; note that several states that are outside of FM's VSH zone, had hail events that would qualify as VSH, where hail stones were recorded to be larger than 2-inches in size.Figure 4: Hail events in states that are outside of the VSH area, but qualify as VSH by size.How Do I Design For Very Severe Hail?In order for a roof assembly to achieve a hail rating, the assembly must pass a hail test. FM Approvals designs the hail tests including a different test for each hail hazard area. Hail testing generally includes the use of steel or ice balls that are dropped or launched at roof assemblies in a laboratory setting. Pass criteria vary by the test, but generally visual damage cannot be present to either the membrane or components below. Roof assemblies that pass each individual hail test are FM approved to be installed in each hail hazard area.There are thousands of FM rated assemblies and it can be difficult to choose just one. To start, it is important to note that selection consists of an entire assembly, however consideration of all roof components including the membrane, coverboard, and attachment method each play an important role in how the assembly defends against hail.Membrane selection is critical for Very Severe Hail prone regions. Thicker roof membranes, as well as higher performance grades that will remain pliable under heat and UV exposure over time and will outperform standard grade materials. Fleeceback membranes also provide an added cushion layer that buffers hail impact.Coverboard selection is a critical component of the roof system design. High compressive strength coverboards are an effective means to enhance the performance of the roof system when exposed to hail events. A coverboard will enhance the roof's long term performance by fortifying the membrane when hail strikes as well as providing a firm surface to help resist damage from typical foot traffic. It will also help the roof insulation below withstand damage from hail. While conventional gypsum coverboards and high-density polyiso coverboards provide excellent protection against foot traffic and smaller hail, they are not effective for VSH. Coverboards for VSH systems were originally limited to plywood or oriented strand board (OSB). The use of plywood and OSB is very labor intensive to install as compared to traditional gypsum coverboards, increasing the cost of the installation. Recently, coverboard manufacturers have developed glass mat roof boards which are a reinforced gypsum core with a heavy-duty coated glass mat facer. Not only do these boards provide protection against 2-inch hail and are an important part of VSH assemblies, they are also a FM Class 1 and UL Class A thermal barrier for fire rated assemblies. These boards are 5/8" thick and are 92-96 pounds per 4'x8' board; about 30 percent heavier compared to plywood yet easier to install as they can be scored and cut like a traditional gypsum board.Consideration of roof attachment method is critical for selection of VSH assemblies. Historically, mechanically attached systems were not able to pass the VSH tests; when an ice ball hit the head of the fastener or plate, the result was a laceration in the membrane. To avoid failures of the membrane at the fasteners and plates, the fasteners were traditionally buried in the system; the insulation was mechanically attached and the coverboard and membrane were adhered. This is still a common installation method and as a result, there are a large number of assemblies where the membrane and coverboard are adhered. Additionally, burying the fasteners allows for the installation of a smooth backed membrane. With the development of glass mat coverboards, there are VSH rated assemblies that can be simultaneously fastened (mechanically attached coverboard and insulation) that utilize an adhered fleece-back membrane.Figure 5: VSH systems. Left is simultaneously fastened 60 mil Fleeceback TPO over glass mat VSH roof board and Polyiso Insulation. Right is 60 mil Fleeceback TPO over glass mat VSH roof board adhered in low rise foam ribbons to mechanically attached Polyiso Insulation.Figure 6: A sample of available VSH assemblies.SummaryWhy Should We Design for VSH?Severe hail events are increasing in geographic footprint and storms with hailstones that meet Very Severe Hail criteria are occurring throughout the country. While designing for VSH is a requirement if a building falls within the VSH area and is ensured by FM Global, many owners and designers are opting for roof assemblies that can withstand VSH storms even if they are not insured by FM Global. Material selection, such as coverboard and membrane, are key components to managing this risk. Glass mat coverboards and thicker, higher grade single-ply membranes, such as fleece-back, increase the roof assembly's resistance to damage. Choosing the right roof assembly could be the difference between weathering the storm or significant damage from hail.What are the next steps?Learn about GAF's Hail Storm System Resources, and as always, feel free to reach out to the Building & Roofing Science team with questions.

By Authors Kristin Westover

January 30, 2023

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