RoofViews

Building Science

Solar Power is Now Competitive

By Thomas J Taylor

March 07, 2019

Solar Power

For Commercial Roofing, Careful Design and Material Choices Can Make Solar Compelling

Each energy production type, such as coal and solar, has a range of costs depending on location, the efficiency of the particular equipment used, etc. Today the range of costs of photovoltaic — or solar —power now overlap those of conventional sources of electricity as shown in the following graphic (2021 data courtesy of Lazard).

Unsubsidized Cost of Energy

To better compare true costs, the data shown above does not include any tax or other subsidies. The term "Levelized Cost" is defined as an economic assessment of the average total cost to build and operate a power-generating asset over its lifetime divided by the total energy output of the asset over that lifetime. It includes initial capital, as well as maintenance, operating, and fuel costs. It is the breakeven cost without any margins for profit.

An immediate conclusion from the chart is that solar energy, especially utility scale and commercial and industrial roof installations, is competitive with conventional power generation. For the remainder of this article, solar on commercial and industrial roofs ("solar C & I") will be examined more closely.

The power cost of solar C & I varies between $85 and $194 per megawatt hour, or MWh. The design and engineering of such rooftop solar arrays are now highly specialized with a focus on increasing performance (i.e., output or production) and lowering costs. A key metric is the energy density, or energy production per unit area, which will be examined in detail here together with the expected productive life of such installations. Energy density and overall life cycle are important drivers of levelized cost. Some specific parameters vary depending on whether an array is comprised of standard solar panels or bifacial panels. These are discussed separately.

Standard Solar Modules

Maximizing Energy Density

The array design plays a key role in determining energy density. For maximum power production per panel, the array orientation is directly perpendicular to the sun's energy. This results from having south-facing panels angled at 30° (for southern US locations) as shown here:

maximum energy yield per panel copy

However, this arrangement doesn't maximize the number of panels that can be mounted in a given area. In other words, the energy density isn't maximized. The following schematic shows the panels closer together, but at a less optimum angle to the sun. The panels are at a lower angle to avoid shading and so there is a trade-off between maximizing power from each individual panel versus that of the entire installation.

maximum energy density

Of course, the sun's angle changes with location, time of day and throughout the year. However, solar system designers now model an array's output to maximize the annualized energy density for each specific location.

The Daily Energy Curve

If an array is oriented directly south, then power output would rise during the day as the sun rose in the sky. Peak power would be produced between noon and 1pm when the sun is most directly overhead, after which it would taper off. This might seem optimal but there are two situations that could change that perspective:

  • Some utility companies prefer that solar arrays be designed to produce peak power closer to mid-afternoon, to coincide with peak power demand due to air conditioning loads. In such cases, solar arrays are oriented slightly westerly so that the sun is more perpendicular to the panels in mid-afternoon.
  • Solar C&I installations are sometimes made with the express intention of supplying as much of a building's power as possible. In such cases, it might be desirable to smooth out the power curve so that power is provided more uniformly throughout the day. This is done by arranging the panels in a so-called east-west orientation, shown in the following schematic:

The Daily Energy Curve

Daily energy output from such a configuration, compared to that of a more conventional south facing array is shown in the following plot:

Power output

Examination of the east-west panel arrangement suggests that energy density, while not maximized, could be fairly high. Such an installation essentially avoids shading except around sunrise and sunset.

Solar Array Lifetime Assumption

As described earlier, levelized cost calculations assume a certain lifetime during which the asset will produce power. In the case of the data shown in the initial graph, solar arrays were assumed to have a twenty year lifespan. There is some evidence that most solar panel failures occur during the initial years of operation, as manufacturing defects and the like cause breakdowns. However, once those few defective panels have been replaced, there is significant anecdotal evidence that arrays can produce useful power for several decades.

Inverters, which are necessary to convert an array's direct current to alternating current, may also experience initial failures due to manufacturing and wiring defects. Inverters may also experience longer-term failures, but as inverter costs continue to fall, replacement/repair of these devices becomes part of regular system maintenance.

Roof membrane life can be a significant factor in determining the long term economic life of a solar C & I installation. If the membrane requires replacement, the cost of removing and then re-installing the array could prove to be prohibitive. GAF EverGuard Extreme® TPO was developed for demanding installations and for those situations where a longer roof service life was desired. With guarantee coverage available up to 35 years (depending on installation method and membrane thickness) and exceptional accelerated weathering performance, the long term risk of failure is lower for GAF EverGuard Extreme® TPO than for other TPO membranes.

Bifacial Solar Modules

So far, this blog has been focused on conventional solar panels. However, bifacial solar panels could provide more power than standard modules and are growing in popularity. However, bifacial modules change some of the considerations we've been discussing. Bifacial solar panels are able to absorb solar energy from both sides, with the general concept shown here:

bifacial absorption

Bifacial panels should be installed above highly reflective surfaces, as shown here:

Reflective Surface

It is important to ensure that the underlying substrate is not shaded too much. In fact, bifacial modules require a different set of considerations to maximize power density as compared to the conventional panels discussed above. There are three factors that can increase the energy production of a bifacial installation: the module height above the membrane, the tilt angle, and module row-to-row spacing.

Bifacial Module Height

Module height is important because of its relationship with shading of the membrane. As the following schematic suggests, modules that are further above a highly reflective membrane will produce more energy than those closer.

Bifacial Module

Design guidelines such as those provided by LG* and Prism* suggest the following relationship between height, measured to the lowest point, and bifacial energy gain.

Bifacial Gain by Module Elevation

The array designer will need to balance factors like wind resistance and cost against the potential energy gain. In most situations, it would be wise to keep the modules at or below the parapet wall height to minimize wind loads.

Bifacial Tilt Angle and Row Spacing

As with standard modules, the tilt angle of bifacial panels can be increased to maximize energy density, but membrane coverage must be considered. This is best explained by referring to the following schematic.

Membrane Coverage Ratio

Standard module rows can be closely spaced, with the limiting factor being the degree of shading of the modules themselves. With bifacial modules, shading of the membrane is the limiting factor. As with module height, the bifacial gain in energy can be modeled as a function of membrane coverage ratio. The following graphic shows a generalized response for a bifacial module with a high rear side efficiency:

Membrane Coverage Ratio

Membrane Reflectivity

As stated earlier, the output of bifacial solar panels is dependent on the reflectivity of the substrate. In the case of TPO single ply roofing membranes, there are generally only minor differences between TPO membranes from different manufacturers in terms of initial reflectance. The critical measure is solar reflectance since it is visible light that provides energy for conversion to electricity. Solar Reflectance Index, or SRI, is not appropriate because it includes an emittance term which is a measure of heat being radiated from the surface.

The independent Cool Roof Rating Council shows GAF EverGuard® TPO to have an initial reflectivity of 0.76, in line with other standard TPO membranes. The three year aged reflectivity is shown as 0.68, again in line with other TPO membranes. However, GAF EverGuard Extreme® has an initial reflectivity of 0.83, i.e., 7 percentage points higher than the standard TPO. The three-year aged value is stated to be 0.72.

Many in the solar industry use albedo as a measure of reflectance, instead of solar reflectance used by the roofing industry. In practice, the two measures are very similar, with albedo being a total spectrum reflectance while solar reflectance is primarily measured across the visible region of the sun's energy. The following chart shows the bifacial energy gain as a function of roof albedo, using data from LG*.

Bifacial Energy Gain by Albedo

Clearly, high albedo or solar reflectance increases the energy output from bifacial modules in a solar C&I application. It is thus beneficial to have a membrane like GAF EverGuard Extreme® TPO, which has the potential to maintain a high reflectance. As discussed in the previous blog, TPO generally can maintain a higher level of reflectance versus other membrane types, but also GAF EverGuard Extreme® TPO in particular could resist dirt pick-up for a longer time than other TPO membranes.

Solar Array Lifetime Assumption

For bifacial module installations, membrane life is as important as for standard solar modules, i.e., a long system life can lower the levelized cost of energy. This can then contribute to levelized costs that are more competitive versus conventional energy sources.

Summary

  • Solar energy costs are now very competitive with respect to conventional energy sources such as coal and gas-powered generation.
  • Optimization of the levelized cost of solar power is key to making such power a compelling choice.
  • Maximizing energy density from solar C&I installations, although somewhat different for standard solar modules versus bifacial modules, is an important factor in lowering overall energy costs.
  • Roof membrane choice is important for two reasons:
    • Membranes like GAF EverGuard Extreme® TPO provide a longer service life than standard TPO membranes, which can reduce or eliminate costs of removal and reinstallation of the solar array associated with roof replacement
    • The high reflectivity and possible long term maintenance of that reflectivity makes GAF EverGuard Extreme® TPO a compelling choice for bifacial module installations. This is due to the bifacial energy gain resulting from higher albedo substrates.



*Trade and company names or company products referred to herein are intended only to describe the materials and products discussed. In no case do these references imply recommendation or endorsement, nor do they imply that the particular products are the best available for the purpose discussed.

About the Author

Thomas J Taylor, PhD is the Building & Roofing Science Advisor for GAF. Tom has over 20 year’s experience in the building products industry, all working for manufacturing organizations. He received his PhD in chemistry from the University of Salford, England, and holds approximately 35 patents. Tom’s main focus at GAF is roofing system design and building energy use reduction. Under Tom’s guidance GAF has developed TPO with unmatched weathering resistance.

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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. A range of codes and standards are beginning to address this problem, though it's important to note that there is often a time lag between development of codes and their widespread adoption.What Is the Industry Doing About It?Long in the business of supporting high-performance building enclosures, Phius (Passive House Institute US) provides a Fastener Correction Calculator along with a way to calculate the effect of linear thermal bridges (think shelf angles, lintels, and so on). By contrast, the 2021 International Energy Conservation Code also addresses thermal bridging, but only considers framing materials to be thermal bridges, and actually pointedly ignores the effects of point loads like fasteners in its definition of continuous insulation: "insulation material that is continuous across all structural members without thermal bridges other than fasteners and service openings" (Section C202). 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. By making our buildings more robust against wind uplift to meet updated standards, we are in effect making them less robust against the negative effects of hot and cold weather conditions.So, how bad is this problem, and what's a roof designer to do about it? A team of researchers at SGH, Virginia Tech, and GAF set out to determine the answer, first by simplifying the problem. Our plan was to develop computer simulations to accurately anticipate the thermal bridging effects of fasteners based on their characteristics and the characteristics of the roof assemblies in which they are used. In other words, we broke the problem down into parts, so we could know how each part affects the problem as a whole. We also wanted to carefully check the assumptions underlying our computer simulation and ensure that our results matched up with what we were finding in the lab. The full paper describing our work was delivered at the 2023 IIBEC Convention and Trade Show, but here are the high points, starting with how we set up the study.First, we began with a simple 4" polyisocyanurate board (ISO), and called it Case A-I.Next, we added a high-density polyisocyanurate cover board (HD ISO), and called that Case A-II.Third, we added galvanized steel deck to the 4" polyiso, and called that Case A-III.Finally, we created the whole sandwich: HD ISO and ISO over steel deck, which was Case A-IV.Note that we did not include a roof membrane, substrate board, air barrier, or vapor retarder in these assemblies, partly to keep it simple, and partly because these components don't typically add much insulation value to a roof assembly.The cases can be considered base cases, as they do not yet contain a fastener. 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

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In Your Community

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Companies, organizations, and firms working in the building, construction, and design space have a unique opportunity and responsibility. Collectively, we are contributing to nearly 40% of energy-related carbon emissions worldwide. While the goals, commitments, pledges, and promises around these challenges are a step in the right direction, no one entity alone will make major improvements to this daunting issue.We need to come together, demonstrate courageous change leadership, and take collective approaches to address the built environment's impacts on climate. Collectively, we have a unique opportunity to improve people's lives and make positive, measurable changes to impact:Buildings, homes, and hardscapesCommunity planningConsumer, commercial, and public sector behaviorOur Collective Challenge to Reduce our Carbon FootprintAccording to many sources, including the U.S. Green Building Council (USGBC), the built environment accounts for 39% of global energy-related carbon emissions worldwide. Operational emissions from buildings make up 28% and the remaining 11% comes from materials and construction.By definition, embodied carbon is emitted by the manufacture, transport, and installation of construction materials, and operational carbon typically results from heating, cooling, electrical use, and waste disposal of a building. Embodied carbon emissions are set during construction. This 11% of carbon attributed to the building materials and construction sector is something each company could impact individually based on manufacturing processes and material selection.The more significant 28% of carbon emissions from the built environment is produced through the daily operations of buildings. This is a dynamic that no company can influence alone. Improving the energy performance of existing and new buildings is a must, as it accounts for between 60–80% of greenhouse gas emissions from the building and construction sector. Improving energy sources for buildings, and increasing energy efficiency in the buildings' envelope and operating systems are all necessary for future carbon and economic performance.Why It Is Imperative to Reduce our Carbon Emissions TodayThere are numerous collectives that are driving awareness, understanding, and action at the governmental and organizational levels, largely inspired by the Paris Agreement enacted at the United Nations Climate Change Conference of Parties (COP21) in 2015. The Architecture 2030 Challenge was inspired by the Paris Agreement and seeks to reduce climate impacts from carbon in the built environment.Since the enactment of the Paris Agreement and Architecture 2030 Challenge, myopic approaches to addressing carbon have prevailed, including the rampant net-zero carbon goals for individual companies, firms, and building projects. Though these efforts are admirable, many lack real roadmaps to achieve these goals. In light of this, the US Security and Exchange Commission has issued requirements for companies, firms, and others to divulge plans to meet these lofty goals and ultimately report to the government on progress in reaching targets. These individual actions will only take us so far.Additionally, the regulatory environment continues to evolve and drive change. If we consider the legislative activity in Europe, which frequently leads the way for the rest of the world, we can all expect carbon taxes to become the standard. There are currently 15 proposed bills that would implement a price on carbon dioxide emissions. Several states have introduced carbon pricing schemes that cover emissions within their territory, including California, Oregon, Washington, Hawaii, Pennsylvania, and Massachusetts. Currently, these schemes primarily rely on cap and trade programs within the power sector. It is not a matter of if but when carbon taxes will become a reality in the US.Theory of ChangeClimate issues are immediate and immense. Our industry is so interdependent that we can't have one sector delivering amazing results while another is idle. Making changes and improvements requires an effort bigger than any one organization could manage. Working together, we can share resources and ideas in new ways. We can create advantages and efficiencies in shared R&D, supply chain, manufacturing, transportation, design, installation, and more.Collaboration will bring measurable near-term positive change that would enable buildings and homes to become net-positive beacons for their surrounding communities. We can create a network where each building/home has a positive multiplier effect. 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By Authors Jennifer Keegan

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GAF Building and Roofing Science Team
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Developing Best Practice Solutions for GAF and Siplast Customers

With any roofing project, there are a number of factors to consider when choosing the right design: sustainability profile, potential risks, overall performance, and more. Our Building and Roofing Science (BRS) team specializes in working with industry professionals to help them enhance their roof designs across all of these areas. Leveraging their building enclosure expertise, our BRS team serves as thought leaders and collaborators, helping design professionals deliver better solutions for their customers."We're a consultant's consultant. Basically, we're a sounding board for them," explains Jennifer Keegan, Director of Building and Roofing Science. Rather than solely providing product specifications and tactical support, the BRS team partners with consultants, specifiers, and architects to provide guidance on designing high-performing roofs that don't just meet code, but evolve their practices and thinking. For example, this might include understanding the science behind properly placed air and vapor retarders.As experts in the field, our BRS team members frequently attend conferences to share their expertise and findings. As Jennifer explains, "Our biggest goal is to help designers make an informed decision." Those decisions might be in a number of areas, including the building science behind roof attachment options, proper placement of air and vapor retarders, and how a roof can contribute to energy efficiency goals.Expanding the BRS TeamOur BRS team is accessible nationwide to look at the overall science of roof assembly and all of the components and best practices that make up a high-performance, low-risk, and energy-efficient roof. Our regional experts are positioned strategically to better serve our customers and the industry as a whole. We have the capacity to work with partners across the country on a more personalized level, providing guidance on roof assembly, membrane type, attachment method, or complicated roof details including consideration of the roof to wall interface.Partnering with the Design Services TeamIn addition to our newly expanded BRS team, GAF also offers support through its Design Services team. This group helps with traditional applications, installations, and system approvals. GAF's Design Services team is a great resource to answer any product questions, help you ensure your project meets applicable code requirements, assess compatibility of products, outline specifications, and assist with wind calculations. By serving as the front line in partnership with our BRS team, the Design Services team can help guide the design community through any phase of a project.GAF's Building and Roofing Science team is the next step for some of those trickier building projects, and can take into consideration air, vapor, and thermal requirements that a designer might be considering for their roof assembly. Through a collaborative process, our BRS team seeks to inspire project teams, as Jennifer explains, "to do it the best way possible."Engaging with the TeamsGAF has the support you need for any of your design and roofing science needs. To request support from the GAF Design Services team, you can email designservices@gaf.com. For additional support from our Building and Roofing Science team regarding specialty installations or how a building can be supported by enhanced roof design, contact us at buildingscience@gaf.com.Our Building and Roofing Science team is always happy to support you as you work through complex jobs. You can also sign up to join their office hours here.

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