RoofViews

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.

Picture1

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.

Picture2

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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Have you ever thought about building products reducing the carbon dioxide emissions caused by your building? When considered over their useful life, materials like insulation decrease total carbon emissions thanks to their performance benefits. Read on for an explanation of how this can work in your designs.What is Total Carbon?Total carbon captures the idea that the carbon impacts of buildings should be considered holistically across the building's entire life span and sometimes beyond. (In this context, "carbon" is shorthand for carbon dioxide (CO2) emissions.) Put simply, total carbon is calculated by adding a building's embodied carbon to its operational carbon.Total Carbon = Embodied Carbon + Operational CarbonWhat is Embodied Carbon?Embodied carbon is comprised of CO2 emissions from everything other than the operations phase of the building. This includes raw material supply, manufacturing, construction/installation, maintenance and repair, deconstruction/demolition, waste processing/disposal of building materials, and transport between each stage and the next. These embodied carbon phases are indicated by the gray CO2 clouds over the different sections of the life cycle in the image below.We often focus on "cradle-to-gate" embodied carbon because this is the simplest to calculate. "Cradle-to-gate" is the sum of carbon emissions from the energy consumed directly or indirectly to produce the construction materials used in a building. The "cradle to gate" approach neglects the remainder of the embodied carbon captured in the broader "cradle to grave" assessment, a more comprehensive view of a building's embodied carbon footprint.What is Operational Carbon?Operational carbon, on the other hand, is generated by energy used during a building's occupancy stage, by heating, cooling, and lighting systems; equipment and appliances; and other critical functions. This is the red CO2 cloud in the life-cycle graphic. It is larger than the gray CO2 clouds because, in most buildings, operational carbon is the largest contributor to total carbon.What is Carbon Dioxide Equivalent (CO2e)?Often, you will see the term CO2e used. According to the US Environmental Protection Agency (EPA), "CO2e is simply the combination of the pollutants that contribute to climate change adjusted using their global warming potential." In other words, it is a way to translate the effect of pollutants (e.g. methane, nitrous oxide) into the equivalent volume of CO2 that would have the same effect on the atmosphere.Today and the FutureToday, carbon from building operations (72%) is a much larger challenge than that from construction materials' embodied carbon (28%) (Architecture 2030, 2019). Projections into 2050 anticipate the operations/embodied carbon split will be closer to 50/50, but this hinges on building designs and renovations between now and 2050 making progress on improving building operations.Why Insulation?Insulation, and specifically continuous insulation on low-slope roofs, is especially relevant to the carbon discussion because, according to the Embodied Carbon 101: Envelope presentation by the Boston Society for Architecture: Insulation occupies the unique position at the intersection of embodied and operational carbon emissions for a building. 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The input on the far left is a given number of kilograms of carbon dioxide equivalent (CO2e) generated for the flight, from the manufacturing of the airplane, to the fuel it burns, to its maintenance. The output is the flight itself, which creates CO2 emissions, but no durable good. In this case, the only CO2 reduction strategy you can make is to make fewer or shorter flights, perhaps by consolidating visits, employing a local designer of record, or visiting the building virtually whenever possible. Now consider the wallpaper you might specify for your client's building. It involves a discretionary expenditure of CO2e, in this case, used to produce a durable good. However, this durable good is a product without use-phase benefits. In other words, it cannot help to save energy during the operational phase of the building. It has other aesthetic and durability benefits, but no operational benefits to offset the CO2 emissions generated to create it. 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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 3.1.1.7.3). 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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We needed to simulate and physically test these, so we could understand the effect that fasteners have when added to them.We also ran a set of samples, B-I through B-IV, that corresponded with cases A-I through A-IV above, but had one #12 fastener, 6" long, in the center of the 2' x 2' assembly, with a 3" diameter insulation plate. These are depicted below. The fastener penetrated the ISO and steel deck, but not the HD ISO.One visualization of the computer simulation is shown here, for Case B-IV. The stripes of color, or isotherms, show the vulnerability of the assembly at the location of the fastener.What did we find? 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. Despite this anomaly, both approaches showed the same thing: steel deck acts like a radiator, exacerbating the effect of the fastener. In the assemblies with just ISO and steel deck (Cases III), adding a fastener resulted in an R-value drop of 11.0% for the physical experiment and 4.6% for the computer simulation compared to the assembly with no fastener.Finally, the assemblies with all the components (HD ISO, ISO and steel deck, a.k.a. Cases IV) showed again that the HD ISO insulated the fastener and reduced its negative impact on the R-value of the overall assembly. The physical experiment had a 6.1% drop (down from 11% with no cover board!) and the computer simulation a 4.2% drop (down from 4.6% with no cover board) in R-value when the fastener was added.What Does This Study Tell Us?The morals of the study just described are these:Roof fasteners have a measurable impact on the R-value of roof insulation.High-density polyisocyanurate cover boards go a long way toward minimizing the thermal impacts of roof fasteners.Steel deck, due to its high conductivity, acts as a radiator, amplifying the thermal bridging effect of fasteners.What Should We Do About It?As for figuring out what to do about it, this study and others first need to be extended to the real world, and that means making assumptions about parameters like the siting of the building, the roof fastener densities required, and the roof assembly type.Several groups have made this leap from looking at point thermal bridges to what they mean for a roof's overall performance. The following example was explored in a paper by Taylor, Willits, Hartwig and Kirby, presented at the RCI, Inc. Building Envelope Technology Symposium in 2018. In that paper, the authors extended computer simulation results from a 2015 paper by Olson, Saldanha, and Hsu to a set of actual roofing scenarios. They found that the installation method has a big impact on the in-service R-value of the roof.They assumed a 15,000-square-foot roof, fastener patterns and densities based on a wind uplift requirement of 120 pounds per square foot, and a design R-value of R-30. In this example, a traditional mechanically attached roof had an in-service R-value of only R-25, which is a 17% loss compared to the design R-value.An induction-welded roof was a slight improvement over the mechanically attached assembly, with an in-service value of only R-26.5 (a 12% loss compared to the design R-value).Adhering instead of fastening the top layer of polyiso resulted in an in-service R-value of R-28.7 (a 4% loss compared to the design R-value).Finally, in their study, an HD polyiso board was used as a mechanically fastened substrate board on top of the steel deck, allowing both layers of continuous polyiso insulation and the roof membrane to be adhered. Doing so resulted in an in-service R-value of R-29.5, representing only a 1.5% loss compared to the design R-value.To operationalize these findings in your own roofing design projects, consider the following approaches:Consider eliminating roof fasteners altogether, or burying them beneath one or more layers of insulation. Multiple studies have shown that placing fastener heads and plates beneath a cover board, or, better yet, beneath one or two layers of staggered insulation, such as GAF's EnergyGuard™ Polyiso Insulation, can dampen the thermal bridging effects of fasteners. Adhering all or some of the layers of a roof assembly minimizes unwanted thermal outcomes.Consider using an insulating cover board, such as GAF's EnergyGuard™ HD or EnergyGuard™ HD Plus Polyiso cover board. Installing an adhered cover board in general is good roofing practice for a host of reasons: they provide enhanced longevity and system performance by protecting roof membranes and insulation from hail damage; they allow for enhanced wind uplift and improved aesthetics; and they offer additional R-value and mitigate thermal bridging as shown in our recent study.Consider using an induction-welded system that minimizes the number of total roof fasteners by dictating an even spacing of insulation fasteners. The special plates of these fasteners are then welded to the underside of the roof membrane using an induction heat tool. This process eliminates the need for additional membrane fasteners.Consider beefing up the R-value of the roof insulation. If fasteners diminish the actual thermal performance of roof insulation, building owners are not getting the benefit of the design R-value. 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 https://iibec.org/ or GAF at BuildingScience@GAF.com for more information.

By Authors Elizabeth Grant

November 17, 2023

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