RoofViews

Building Science

Overburden

June 15, 2022

Since its inception a roof's primary purpose has been to shelter its inhabitants from the elements, but now the underutilized potential of roof surfaces is being realized. For roofs with large surface areas, the potential for large overburden installations, such as solar, vegetative roofing, or amenity decks can be exceptional. Even smaller roofs can have overburden that make a significant impact on the sustainability goals of a building including: increased energy efficiency, stormwater retention, energy generation, biohabitat restoration, food production, reduced urban heat island effect, and outdoor space.

However, selection of the overburden system is only part of the design. Selection and design of the roof membrane, the waterproofing layer that protects the building, is critical for a long-lasting installation. Failure of the membrane, whether it requires repair or replacement, may necessitate removal of the overburden. The removal of the overburden can result in lost energy generation for solar installations, and loss of rainwater capture for both vegetative and blue or purple roof assemblies. Appropriate selection of the entire assembly, including proper detailing and integration of the roof assembly, as well as installation, are paramount to the overall success and longevity of the overburden system.

Overburden Assemblies

Roofing overburden is defined as "any manner of material, equipment or installation that is situated on top of, and covering all or a portion of, a roof or waterproofing membrane assembly."^* This excludes thermal insulation but includes: planters, vegetative roof assemblies (both trays and built-up), loose gravel, water tanks, void fill (like EPS, XPS, and/or Polyiso rigid foam), tiles, pavers, supporting pedestals, equipment, and solar PV arrays.^*

While each property is unique, there are many overburden options and roof assembly considerations to meet sustainability goals.

Vegetative Roofs: Vegetative roofs are plants that are installed in trays or built in-place on the roof with extensive or intensive plant configurations. Tray systems and extensive roofs are generally in depths of less than six inches and consist of shallow rooted plants, such as sedums. Intensive vegetative roof systems have deeper soil depths and can accommodate larger plant installations.
Blue Roofs: Blue roofs are systems that are designed to provide stormwater detention. Rainfall onto the roof is managed using orifices, weirs, or other outlet devices that control the discharge rate of rooftop runoff.
Blue-Green Roofs: A blue-green roof is a blue roof with a vegetative roof assembly.
Purple Roofs: A purple roof is a "sponge" roof that incorporates a sponge-like layer made of hydrophilic mineral wool, a dense polyester fabric detention layer, and may or may not include an additional honeycomb layer to increase the volume of rainwater that can be retained and detained .
Rooftop Solar: Solar PV panels come in both single sided and bifacial (double-sided) modules. A bifacial module can produce power from both sides thereby increasing the energy generation. Modules are supported by racking systems on the rooftop that are either mechanically attached to the roof, mechanically attached to a structural canopy that is attached to the roof, or held in place with ballast.
Agrivoltaics: The emerging practice of agrivoltaics, or agriculture combined with photovoltaics, is showing promise as a mutually beneficial symbiotic solution to not only increase the efficiency of solar panels, but also increase plant size and crop yields by shading and limiting soil evaporation

Roofs have the potential for many types of overburden installations including vegetative assemblies, plaza decks, and incorporation of multiple types of assemblies.

Roof System Considerations

Selection of the roof membrane, the waterproofing layer that protects the building, is critical for the success of overburden installations. The membrane performance and roofing assembly configuration, including location of the membrane in the assembly must be considered. Improper selection of membranes can result in leaks, costly repairs, or even replacement. Special considerations like the type of insulation and cover board should be explored. Most importantly, the roof system should have a lifespan that is equal to or greater than the overburden system. Membrane performance is not only dependent on the overburden and associated roofing assembly materials, but also the design and installation. Roof assembly selection and specific design details down to the last termination and penetration detail should be provided to ensure the roof is installed in accordance with the contract documents.

Membrane Selection

Single-ply membranes can be an excellent choice for overburden installations as they can be used at the deck level and at the top of the assembly. Membranes that will be used at the deck level, such as for vegetative assemblies, should be evaluated for durability and longevity since access to them for repairs will be limited. Solar is typically installed directly on the membrane. For blue roofs and water catchment systems, single-ply membranes are commonly used at the top of the assembly and it is recommended that single-ply membranes meet NSF P151 water standards.

Membrane Color

For roof assemblies where the membrane is exposed and at the top of the assembly, such as for solar panel installations and blue roof assemblies, the color can have a significant impact on the performance of the system and also on the roof surface temperatures. Reflective roof membranes can lower the ambient roof temperature. Using a lighter colored roof can decrease the urban heat island effect in cities, and also may decrease the amount of heat that is able to radiate into a building's interior.

For solar panel installations, light colored or reflective roof membranes can lower the ambient roof temperature which allows the panels to function more efficiently. The temperature of a PV panel can significantly impact how much electricity the panel produces; as panels get hotter, they produce less power. According to an article published by GAF, "It is estimated that the efficiency of a PV panel can be up to 13 percent higher when installed over a highly reflective membrane compared to a dark membrane with low reflectance. Also, the use of bifacial PV panels over reflective roof membranes can increase the efficiency by 20-35 percent, as they take advantage of the reflected light.

For overburden installations, such as blue or purple roofs where the membrane is exposed, reflective membranes are also advantageous. Even though the roof may not always be holding water, or presumably when water is present it is translucent, a reflective membrane contributes to lower roof surface temperatures. Additionally, vegetative roof systems can take advantage of reflective membranes where vegetation is not installed. Codes require that borders and paths are maintained on the roof for fire, access, and maintenance. Reflective membranes at these locations may lower roof temperatures which mitigates interior heat gain, and also decreases the strain of summer heat on vegetation.

For solar panel installations, light colored or reflective roof membranes can lower the ambient roof temperature which allows the panels to function more efficiently.

Membrane Thickness

Roof assemblies should be installed to match or exceed the service life of the overburden systems. The risk of installing a less robust system, such as with a thinner single-ply membrane, could require roof replacement prior to the end of overburden's service life. An unprotected roof membrane should offer enhanced protection against the effects of UV, high service temperatures, punctures, and added foot traffic to help ensure that the roof's life and overburden will match. For single-ply membranes, thicker membranes can provide protection against punctures from the extra foot traffic for overburden service and maintenance. Single-ply membrane thickness significantly improves impact resistance (such as by dropping a tool) by almost 80 percent from 45 mil to an 80 mil membrane. A thicker single-ply membrane also provides additional protection to both UV and high surface temperatures, which is important since a thicker overall membrane provides more thickness over the scrim (reinforcing layer). This portion of the membrane provides weather resistant properties, including UV resistance.

A roof membrane in an IRMA assembly should have added protection to punctures and abrasion from roof elements above it, including root damage in a vegetative assembly. For installations where the membrane is on top, it is a best practice to install walkway pads around solar arrays or exposed areas of single-ply membranes to protect against the added foot traffic to service the installations.

Membrane Attachment

There are two broad categories of roof attachment; mechanically attached via use of fasteners, and adhered.

Attachment methods should be reviewed for ease of installation and energy efficiency. Energy efficiency roof assemblies can be directly related to thermal bridging, which occurs when components allow for heat transfer through the roof assembly. Heat loss means that the mechanical equipment will have to work harder to maintain the desired interior temperatures. Thermal bridging occurs at gaps or discontinuities between materials, such as at fasteners in a mechanically attached system. Particularly where the fasteners penetrate the entire assembly from the membrane through the insulation and into the deck, the fasteners provide a direct thermal path from the exterior to the interior.

Mechanically attached single-ply systems are also subject to billowing in high wind events. Billowing, or fluttering, of a membrane is when wind causes a negative pressure by pulling interior air into the roof assembly creating uplift force on the roof assembly. Over time, it can cause stress and fatigue on the mechanical attachments and membrane. Interior air that is pulled into the roof assembly equates to energy loss since often the temperature controlled air may warm or cool based on the temperature of the membrane. Overburden installed over mechanically attached systems may billow and flutter with the roof membrane during high wind events. As the membrane flutters and moves, the overburden will shift on the membrane, causing abrasion of the membrane surface. The overburden may also experience stress and fatigue due to movement, which could decrease their overall service lives.

Adhered system where only the first layer of insulation is mechanically attached, significantly reduces interior air loss and thermal bridging.

Adhered systems that do not use mechanical fasteners greatly reduce thermal bridging by eliminating the path from the interior of the roofing assembly to the exterior. Adhering also prevents billowing of the membrane, by mitigating the interior air that can be brought into the roof assembly.

Insulation

Insulation is critical in roofing assemblies for overall energy efficiency of the building. The higher the R-value, expressed per inch, the better the thermal performance of the insulation and its effectiveness at maintaining interior temperatures. Higher R-value per inch means less material is required to achieve the desired insulating value. In overburden systems, the roof system thickness can have an impact on the overall design of the overburden system. For example, roof flashings need to extend a minimum of 8-inches past the completed installation. For vegetative systems, this means that the flashing must extend 8-inches past the vegetation. Flashing heights are of particular importance at mechanical curbs and parapets. For a new construction installation, it is possible to raise the heights of the curbs and parapet walls to the desired height, however, in an existing building, this can be problematic.

Insulation boards should be installed so that the joints are staggered and offset, and several layers of insulation should be installed rather than just one thick layer. Gaps between boards can decrease insulating ability by allowing thermal loss, an increased condensation potential if air travels into the roof assembly. Air brings moisture, which if allowed to condense, can saturate the insulation boards. Wet insulation has an R value of approximately zero, which is like having no insulation at all.

Staggered and offset insulation board joints.

Coverboards

Inclusion of a coverboard must be considered after selection of the overburden. Rooftops with overburden assemblies not only have increased foot traffic on the roof, but are also an added dead load on the roof assembly. High traffic areas should always be protected with walkway pads. Adding a high-compressive-strength coverboard below the roof membrane will enhance system protection, including compressive strength. Coverboards provide added protection against penetration, including tools dropped by service contractors, wind-borne debris, and hail. A thicker coverboard increases its penetration resistance. The use of high-density polyiso insulation coverboards should be considered since they not only add protection to the roof assembly, but also add an R-value of 2.5 per half-inch.

Overburden Layout, Details & Flashings

Qualified design and construction professionals should be involved throughout design and construction phases of roof and overburden systems. Overburden layout should address access for installation and both roof and overburden maintenance throughout the component service lives.

Inadequate detailing or termination heights leave the roof assembly vulnerable to leaks. Improperly detailed or flashed components may be water-tight for a period of time, but will most likely fail prematurely. Removal of the overburden system for repairs can be an added difficulty and cost. It is good practice to allow penetrations, flashings, expansion joints, drains, and other critical roofing interfaces to be accessible for maintenance and repairs. Flashings should be installed in accordance with current NRCA guidelines and the roof manufacturer's construction details. Ballasted assemblies can block or inhibit drainage resulting in trapped water on the roof membrane. The overburden layout should leave portions of the roof around drains open for both access and drainage and be unimpeded by the overburden.

Summary

While the roof is the primary defense to prevent water entering into a building, due to the current market trends and legislation, this previously underutilized asset is being recognized by owners for the opportunity the roof offers. There are many overburden options and roof assembly considerations when meeting sustainability goals. Once the overburden type is selected, the roof needs to be determined, which depends on the type of overburden and the ultimate use of the roof space. The overburden's success depends on the roof assembly beneath. For installations such as solar or vegetation, the plants and solar array will need to be removed for repairs or replacement; so, a more robust membrane for the long-term durability and serviceability of the complete roof may be needed. A performance-based approach for energy efficiency requirements (insulation, thermal bridges, and air tightness) versus a prescriptive code minimum may be warranted to ensure a long-lasting roof and to minimize roof replacements over the life of the building.

^*Resource: Roofing Contractors Associoation of British Columbia

About the Author

Kristin Westover

Kristin Westover, P.E., LEED AP O+M, is a Technical Manager of Specialty Installations for low-slope commercial roofing systems at GAF. She specializes in cold storage roofing assemblies where she provides insight, education, and best practices as it relates to cold storage roofing. Kristin is part of the Building and Roofing Science Team where she works with designers on all types of low-slope roofing projects to review project design considerations so designers can make informed roof assembly decisions.

The roofing industry has long been a competitive space for contractors, but that healthy competition has helped drive innovation and lift up independently-owned businesses across the country. BYLTup, Inc. is one such business, and CEO Trevor Leeds is committed to elevating the greater roofing industry through everything his company does.After many years in roofing, Trevor partnered with his brother, Brady Woodside, in 2022 and co-found BYLTup, Inc. in Southern California. The pair started from scratch with a blank slate, allowing them to create a culture and brand experience that raises the industry bar, which is important to them.Trevor knew GAF would be a part of their success from the start. He experienced GAF-certified contractors'* support through his previous company and knew the organization's vision and innovation would be key to building the business.Enhancing Roofing Professionalism and SafetyTrevor is passionate about the roofing industry and believes everyone—from manufacturers and distributors to the contractors performing the installations—should work together to create a professional and positive customer experience. Even more importantly, Trevor advocates for the industry's employees and their experiences; safety is an area he wants more people to consider."We need to be making sure that we are taking care of employee safety," Trevor says. "Everyone should be focused on that. Everyone should be concerned about their employees going home safe and sound every single night. When an accident happens in the industry, it's devastating to the business owner and the individual, but it impacts the industry too. We all feel that in some form or fashion."Guiding the Next Generation through GAF Roofing AcademyTrevor's concern for employees and the industry's future led to his involvement in the GAF Roofing Academy. While he didn't have an immediate need for labor in his business, he recognized that taking action now could contribute to building a pipeline of skilled workers that would benefit the roofing industry as a whole.Trevor observed the pool of talent and skilled workers dwindling during his time in roofing. "You have the baby boomer generation that is retiring and a younger generation that wants to get into tech and doesn't want to get into the trades," he notes. "We've got to be able to hire and attract young talent back into the industry or we're all going to suffer."Training Students at the SoCal Roofing AcademyTrevor first learned about the GAF Roofing Academy at a regional conference and stepped up to assist. His company was involved in the first SoCal Roofing Academy, which had three tracks: residential roofing, commercial roofing, and sales training. BYLTup hosted the training's residential portion, and Trevor was impressed by the students' interest and engagement."I was blown away at how engaged they were. GAF had certainly done a very good job screening and prepping these individuals coming in, but they were green," he says. "The trainer was fantastic. The students were shingling after three days."Once the groups completed the tracks, they were brought back together for their graduation ceremony, which was a life-changing experience for many. "I realized that many of these people had probably never been recognized like this before," Trevor says. "They were very proud, holding their certificates and sending photos to their families."Making an Unexpected Hire at BYLTupJust before hosting the SoCal Roofing Academy, GAF Roofing Academy Operations Manager Erick Osuna reached out to Trevor and shared recent graduate Jose Navarro's resume. Jose had completed the commercial roofing training program and stood out to Erick, so he connected with Trevor about possibly hiring him."At that time, I wasn't looking for someone on the commercial roofing side. We weren't really expanding that crew, but we were looking for a production role," Trevor shares. "Long story short, we loved him. He was going to be a great fit for the company, and we hired him as our assistant production manager."Jose excelled on the job. BYLTup quickly moved him into a full production manager role, hiring an assistant to work with him. Trevor says Jose was a great hire. "He's young, he's hungry, he wants to learn. I've never seen another individual in the industry that just wants to learn roofing, and he's a sponge."Committing to Supporting GAF Roofing AcademyTrevor has seen the caliber of students attending and graduating from the GAF Roofing Academy, inspiring his commitment to support ongoing trainings. "I was pleasantly surprised at the overall outcome of the [Roofing Academy] event," he notes. "It encouraged me to the point that I said, 'I want to do this, whether we benefit from it or not, I want to support this at least twice a year, and we want to get more contractors involved in it.'"Want to get involved in supporting the future of the roofing industry? Learn how to participate in a future Roofing Academy and discover the training opportunities available to your team through GAF CARE.*Contractors enrolled in GAF certification programs are not employees or agents of GAF, and GAF does not control or otherwise supervise these independent businesses. Contractors may receive benefits, such as loyalty rewards points and discounts on marketing tools from GAF for participating in the program and offering GAF enhanced warranties, which require the use of a minimum amount of GAF products. Your dealings with a Contractor, and any services they provide to you, are subject to the Contractor Terms of Use. Visit www.gaf.com/gaf-contractor-terms-of-use for details.

With the growing popularity and versatility of PVC roofing systems, GAF has responded by building a team of PVC experts to aid architects, specifiers, and the design community in project success. These highly trained and experienced professionals offer 1:1 support to help answer customers' questions and unique needs when working with PVC on roofing projects, and are here to help you!PVC, or polyvinyl chloride, is a single-ply thermoplastic roofing membrane. The material's characteristics help it thrive in particularly challenging applications. In fact, PVC roofing systems have a proven track record in the US, first introduced in the 1970s after making their debut in Germany a decade earlier.Here's a look at the benefits of PVC roofing systems as well as the guidance and support offered by the PVC experts at GAF.PVC Roofing Advantages and ApplicationsWhile several materials are available for use in roofing systems, PVC has been identified as a particularly long-lasting option. Accordingly, it's steadily grown in popularity nationwide.David Allor, senior area PVC specialist, joined the GAF team in 2022 as a PVC expert. His role was created specifically to support roof designers. Allor has an extensive background in the commercial roofing industry and extensive knowledge about PVC as a high-performance roofing solution."The industry refers to it as a very spec-driven product, and I believe what they're speaking to is a specifier's preference to work with a product that's been well established in the market and field tested," Allor shares. "It has the ability to provide protection against chemical exposure* and stand up well to challenging environments."While PVC may be a spec-driven product, it's also application-driven. Allor explains that PVC roofs may help provide protection for restaurants, food manufacturers, and packaging plants—any facility using animal fats where oils may be exhausted.* He also says that even if the building itself isn't exhausting gases, grease, or chemicals, it's important to consider the buildings and structures that surround it, as their exteriors may be subjected to exhaust that could degrade roofing systems.*Supporting the Design CommunityAs North America's largest roofing materials manufacturer, GAF has equipped teams with the resources and specialists needed to supply customers with more than just products. In recent years, the company built a solid team of PVC specialists that cover the entire US. These specialists assist customers with PVC specifications, roof system designs, and whatever else designers need to ensure successful project outcomes.Whether your project or design firm is based in the Northeast, Southeast, Southwest, Midwest, or West Coast, a PVC specialist is positioned to help. Allor explains the team of GAF PVC specialists is active in the organizations specifiers belong to, including the American Institute of Architects, the Construction Specifications Institute, and the International Institute of Building Enclosure Consultants.Allor describes the team's role as supporting the customer and connecting them to other GAF project support teams that can help. For instance, he said he called on a leading architectural design firm in the Boston area—and two months later, someone reached out for assistance with a specific project needing a tight turnaround."Literally within 12 hours of them reaching out to us, we replied and committed to three pertinent deliverables, those being a Tapered ISO Design Package, an Assembly Letter confirming all FM Requirements, and last but not least, a conference call the next day with the Building and Roofing Science team to go over three critical transitional details." Allor says.Committing to Successful Project OutcomesThe PVC specialists at GAF are focused on one goal: helping architects, specifiers, and the roofing design community achieve successful project outcomes through a high-performing PVC roofing system. With professionals like Allor providing insight and guidance, you can rest assured your PVC project will be in good hands.Ready to get started on your next PVC roofing project? Explore the different PVC roofing solutions available, and visit this page to find your local PVC roofing specialist who is ready to assist you.*GAF warranties and guarantees do not provide coverage against exposure to chemicals, grease, oils, animal fats or exhaust. Refer to gaf.com for more information on warranty and guarantee coverage and restrictions.

The Louisiana coast has seen several severe weather events in the last decade, which has made it difficult for many people to rebuild their homes and prepare for future storms. This was the case for the Atakapa Ishak Chawasha tribe, a small community accessible only by boat.The group Rebuilding Together New Orleans recognized the impact a joint effort could have on the region. Together with the GAF Community Matters initiative, and roofing contractor Sunbelt Roofs, the group collaborated with the Atakapa Ishak Chawasha tribe to replace five hurricane-damaged roofs in the tribe's coastal Louisiana homeland.Sowing ResilienceThe week-long roof rebuilding initiative was part of the GAF Community Matters commitment to repair or replace 500 roofs throughout the Gulf region. The goal was to further strengthen and empower the resilience of the families, shelters, and livelihoods of the coastal Atakapa Ishak Chawasha tribe.The Timberline HDZ® RoofCycle™ Series Shingle, which is made with recycled material, was used to protect these homes.Adapting Environmentally"We are stewards of the environment," said Rosina Philippe, a tribal elder of the Atakapa Ishak Chawasha tribe, as she spoke with GAF during the week-long rebuilding initiative. Over a millennium, the tribe has resided in the region, finding all it needs to feed and sustain itself from the environment.However, man-made conditions have significantly impacted the tribe's way of life. The once-forested coastal region is now a salt marsh area, devoid of fresh water and trees, due to levees along the Mississippi River and canals cut through the coastal landscape.These changes have made it "more of a challenge to find the food and plants and medicines that we depend on for our livelihood. But we have been finding them, propagating them, and sharing them with other tribes," said Philippe. "Our way is to accept the natural processes, not change them."Fortifying against Unpredictable WeatherIn addition to the tribe's changing landscape, "what we're dealing with is the unpredictability of the weather," said Philippe. Climate change can cause dramatic temperature swings, which can impact everything from the life cycle of the tribe's crops to the protection of its homes.Situated at the southeasternmost lands of Plaquemines Parish, fingerlike marshland trails out into the Gulf. The homes in the Atakapa Ishak Chawasha community are built on stilts along these watery paths and are accessible only by boat. These homes are often the first line of impact with hurricanes that rip through the Gulf of Mexico.Hurricane Ida—the fifth most expensive hurricane in US history—dealt the water-based community a devastating blow. For nearly two years, the hurricane's impact on tribal homes compounded with each subsequent storm because there wasn't an opportunity to repair its damage.Partnering for StrengthFortunately, through collaborative efforts between the Atakapa Ishak Chawasha, Rebuilding Together New Orleans, GAF, and Sunbelt Roofs, the opportunity to build tribal community resilience through new roofs became available. Bringing in roofing materials and roofers by boat, the Sunbelt Roofs team removed the storm- and hurricane-damaged roofs, repaired roof decking as needed, and installed new GAF roofing systems on the five homes.The roofing materials installed are designed to protect against volatile weather like hurricanes. To provide long-lasting protection against leaks and wind-driven rain, Sunbelt Roofs installed StormGuard® Film-Surfaced Leak Barrier and FeltBuster® Synthetic Roofing Felt. To help prevent shingle blow-off due to high-speed winds, Sunbelt Roofs installed Pro-Start® Starter Strip Shingles.The Timberline HDZ® RoofCycle™ Series shingles that were installed have Dura Grip™ Adhesive, which provides advanced protection from shingle blow-off and wind-driven water creating leaks or water damage.Building Communities TogetherThe initiative with the Atakapa Ishak Chawasha tribe is part of a comprehensive $25 million GAF commitment through 2025 to leverage roofing expertise, resources, and products to help support and empower resilient communities' fundamental needs. Through grants, in-kind donations, and hands-on support, GAF Community Matters is partnering with area organizations that know their communities' needs best.In the Gulf region, GAF partnered with organizations such as Rebuilding Together New Orleans, SBP, Good360, Habitat for Humanity, and Team Rubicon to rebuild or repair 500 Gulf region roofs, train a skilled workforce, and empower communities hard-hit by severe weather.Moving ForwardDespite the challenges of climate change and the changes in their physical landscape, Philippe said, "We still live a rich life here." As the environment changes, the tribe adapts, a demonstration of its resiliency. "These are the new norms that we are having to learn and document and chronicle, this is the information we'll have to pass onto our future generations so that they can continue to inhabit this area," she explained.To learn more about the resiliency work at GAF or to get involved, explore the GAF Community Matters initiative. Special thanks to the Atakapa Ishak Chawasha tribe, a member of the First People's Conservation Council of Louisiana.

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