There are other detailing options available for use with modified bitumen systems that provide flexibility depending on the application. Liquid applied flashings are such an option and there are multiple chemistry options on the market such as polymethyl methacrylate (PMMA), polyurethane methacrylate (PUMA), and styrene ethylbutlyene styrene (SEBS). These flashings are often applied in multiple layers and may require an embedded fabric between them. Liquid applied flashings can be used for penetrations, parapet transitions, unusual wall formations and at unique details.

Bitumen and modified bitumen roofing systems have multiple options for recover if they do not contain any trapped moisture within the roof assembly. Avoiding a complete tear off allows for a more cost-effective way to extend the life of the existing roof. Liquid applied coatings can extend the life of a roof system. Liquid applied membranes allow for greater flexibility during installation and are especially adept in recover situations. These coatings and membranes can be made of PMMA, PUMA, acrylic, silicone among others. Liquid applied membranes allow for flexibility both on the full recoat and when recoating and resealing the transition details on the roof. Another option for recover is to apply a fleece-backed single ply membrane such as TPO, PVC or PVC-KEE over the existing roof. These membranes can be adhered to the asphalt roof system in a similar manner as an original cap sheet would be applied over the roof system.

The Hybrid Roof Assembly

This leads to a discussion of the benefits of a hybrid roof assembly. Hybrid roof assemblies offer another way for roof assemblies to contribute to a building's energy efficiency, resiliency, and sustainability goals. Hybrid assemblies combine the benefits of modified-bitumen roof systems with those of single ply roof systems. This intentional combination of materials aims to increase the robustness of the assembly, including UV protection, resistance to ponding water, ease of repair, and resistance to certain chemicals. Hybrid roof assemblies are the latest roofing trend aimed at meeting the demands of modern roofs.

What is a hybrid roof assembly?

A hybrid roof assembly is where two roofing membranes, composed of different materials and product technologies, are used in one single roof system. One type of hybrid assembly consists of a modified bitumen base layer with a cap layer of a thermoplastic single-ply membrane, such as Thermoplastic Polyolefin (TPO) or Polyvinyl Chloride (PVC) or PVC-KEE. Each component of the roof system is chosen for its strengths, and together, the system combines the best of all the technologies present. A hybrid system such as this has increased robustness, with effectively two plies or more of membrane.

The combination of an asphaltic base sheet covered by a single-ply membrane increases the overall system thickness and robustness compared to a traditional single-ply system. It provides additional redundancy and protection against punctures, which can be a concern with single-ply applications. As mentioned previously, asphaltic systems are vulnerable to UV exposure without surfacing. They also have minimal resistance to ponding water and certain chemical contaminants; these material shortcomings can be overcome with the addition of a single-ply membrane. If this is a priority for a project, a higher SRI value can be achieved using a single-ply membrane with both high new and aged SRI values than can sometimes be found with modified bitumen cap sheets. When higher SRI value options are chosen, the single-ply membranes placed as a cap layer over bitumen systems can decrease the roof surface temperatures and potentially help reduce the heat island effect of the building. PVC and PVC-KEE (Ketone Ethylene Ester) membranes may also provide protection where exposure to chemicals is a concern, and they generally hold up well in ponding water conditions. Therefore, the combined attributes of bitumen systems and single-ply membranes complement each other, providing a durable, efficient, and redundant roof system.

Common hybrid roof applications

Hybrid roof assemblies can be a logical choice for data centers, education facilities, and hospitals due to their strong protection against leaks and multi-ply system redundancy. The redundancy of the second layer of membrane in the form of an asphaltic base sheet provides a secondary protection against leaks if the single-ply membrane is breached.

Additionally, the reflective, UV stable single-ply membrane can provide a cap sheet that is more durable against certain environmental situations such as specific chemicals that is necessary for specific projects. The addition of a reflective membrane over a dark-colored asphaltic membrane can greatly increase the Solar Reflectance Index (SRI) of the roof surface. SRI is an indicator of the ability of a surface to reflect solar energy back into the atmosphere. In general, roof material surfaces with a higher SRI will be cooler than a surface with a lower SRI under the same solar energy exposure. A lower roof surface temperature can result in less heat being absorbed into the building interior during the summer months, thus reducing building cooling loads.

Hybrid roof installation advantages and considerations

Hybrid systems are not confined to new construction. A key benefit of a hybrid roof assembly occurs in retrofit or recover scenarios where there is an existing-modified bitumen or built-up roof (BUR) that is in overall fair condition and with minimal underlying moisture present. A single-ply membrane can be installed on top of the existing roof system without an expensive and disruptive tear-off of the existing assembly. The addition of the single-ply membrane adds reflectivity to the existing darker colored membrane and increases the service life of the roof assembly due to the additional layer of UV and weather protection. Additionally, the single-ply membrane can be installed with low VOC options that have minimum odor and noise disturbance if construction takes place while the building is occupied. This can include mechanical attachment, induction welding or adhesives.

Whether it is a new roof or a recover of an existing roof, there must be something separating the two materials. This is due to the chemical incompatibility of the single-ply membrane and the modified bitumen. In both scenarios, the easiest way to separate the materials is to use a fleece-backed membrane where the factory-applied fleece backing acts as the separator. In re-roof applications, a coverboard or polymat can be used as a separator with a smooth backed membrane. Any type of attachment method can be used provided it meets the other requirements for the project.

Demonstrable benefits support the marriage of new and old technology

The possibility of combining the best aspects of two different roofing technologies makes a hybrid roof assembly worth the hype. Putting bitumen systems together with a single-ply membrane provides the best aspects of each for the roof. The single-ply membrane integrates a reflective surface for improved energy efficiency, and increased protection against chemical exposure and ponding water, while the asphaltic base increases overall system waterproofing redundancy, durability, and protection. The ability for this method to be used in both new construction and recover scenarios makes a multi-ply hybrid roof an assembly choice that is here to stay.

Summary

In summary, asphaltic or bitumen-based roof assemblies remain an important roofing solution. Modified bitumen systems are simple, practical, versatile, and durable. They provide multiple options for the final assembly look including granules and single-ply membranes as well as the protection required for roofs with overburden.

Modified-bitumen roofs (both APP and SBS) are tear and puncture resistant, have multiple installation options and have choices in scrim to improve specific other properties. As a comparison, built-up roof systems use relatively inexpensive materials compared to modified bitumen, but modified bitumen systems are easier to quality control in the field. All bitumen roof systems are resistant to overall damage from roof traffic, temperature shifts, and weather, and can be considered for many different types of roofing projects.

END NOTES

¹Boëda, E.; Connan, J.; Dessort, D. (March 1996). "Bitumen as a hafting material on Middle Palaeolithic artefacts". Nature. 380 (6572): 336–338. doi:10.1038/380336a0.
²McIntosh, Jane. The Ancient Indus Valley. p. 57
³“Great Bath". Britannica. Retrieved 7 March 2024. .
⁴Abraham, Herbert (1938). Asphalts and Allied Substances: Their Occurrence, Modes of Production, Uses in the Arts, and Methods of Testing (4th ed.). New York: D. Van Nostrand Co., Inc. Full text at Internet Archive (archive.org)
⁵Roofing Contractors Association of British Columbia. “Template: ROOFTOP EQUIPMENT & WALKWAYS (BUR).” Web. 4 April. 2022.

Andrea Wagner Watts is the Building Science Education Manager for GAF, engaging with industry professionals providing guidance, technical support and education for building enclosure assemblies.