Low-Slope Roofs as Platforms for PV Systems

While commercial and industrial rooftops are increasingly used for PV systems, weatherproofing remains the primary goal for any roofing system.

Low-slope roofs are ideal locations for PV systems: the solar resource is good; power is generated in close proximity to loads; the location is secure and unobtrusive; and one- and two-story buildings in particular have favorable ratios of roof-to-wall area. Best of all, low-slope roofs are plentiful. As a platform for PV systems, they represent an excellent business opportunity for both PV and roofing contractors.

Although the roofing and PV industries often work together by choice, this collaboration will soon be mandated. The 2012 edition of the International Building Code will have PV-specific information in Chapter 15, which covers “Roof Assemblies and Rooftop Structures.” This means that, in addition to existing UL 1703 requirements, rooftop PV systems will need to meet the same requirements for fire, wind and impact resistance as are required for the roof systems on which they are installed.

The roof system is the building’s interface with the PV system, so it is important to recognize the unique issues and concerns that come into play when using a rooftop as a platform for PV. In this article I discuss the components of lowslope roof assemblies, the most common material types and typical roof system construction. I introduce some roofing professionals’ concerns about the application of PV systems on rooftops. Finally, I discuss guidelines and best practices for installing rooftop PV systems that are endorsed by the National Roofing Contractors Association (NRCA).

Low-Slope Roof Assemblies

The NRCA defines low-slope roofs as those with slopes equal to or less than 3:12 (see definitions). A low-slope roof assembly consists of a roof deck, insulation and roof membrane or other weatherproof cover. An air or vapor retarder is sometimes included.

Roof deck. The most common roof decks for low-slope roof systems include steel, concrete and wood. There are regional differences in usage. For example, in the Southwest plywood and OSB are used regularly for low-slope roof decks in light commercial construction, whereas in the Midwest steel roof decks are more prevalent. Roof systems commonly are attached to wood decks with fasteners, but may be adhered if a separator layer, such as an asphalt-coated base sheet, is first nailed to the wood deck. Roof systems are commonly attached to steel roof decks with screws and plates. However, it is becoming more common to use adhesives to attach roof systems to steel decks, although NRCA does not recommend this method. Screws and plates may be used to fasten an entire roof system or only the bottom layer, such as a gypsum-based board used for fire resistance. When only the bottom layer is fastened, the insulation and membrane are likely adhered with asphalt, liquid adhesive or foam adhesive.

Roof systems are generally adhered to concrete decks with asphalt. In the past, this meant hot asphalt, but today there are additional choices like cold adhesives and foams. The less common roof deck types include poured gypsum, precast gypsum panels, cementitious wood-fiber panels, lightweight insulating concrete and thermal-setting insulating fills. Fastening standoffs for PV mounting systems to these roof deck types requires knowledge of the unique materials and fasteners necessary for a long-term installation.

Insulation. Insulation for roof systems is best installed in layers with a cover board included as the top surface. The most common insulation material for low-slope roof systems is polyisocyanurate, but many other types can be used: cellular glass, expanded or extruded polystyrene, fiberglass, mineral fiber, perlite and wood fiberboard. Common cover boards include perlite, wood fiberboard and gypsum. Insulation is all too often installed in one layer and without a cover board, thereby reducing the system’s overall R-value.

Membranes and weatherproof coverings. Several weatherproof coverings are used for low-slope roof systems. Built-up roofing (BUR), polymer-modified bitumen (MB), single-ply ( for example, EDPM, TPO, PVC), metal panel and spray polyurethane foam (SPF) roof systems are used in all climate zones of the US, with some expected regional variation in market share. Because BUR, MB and single-ply coverings are rolled goods or sheets, they are considered to be membranes. Metalpanel roof coverings are formed from metal coils and sheets and typically have vertical seams that are locked together to raise the joint out of the drainage plane. SPF is applied on-site to make an insulation-based roof system.

Although roofing professionals may be able to visually determine roof types, subtleties can be hard to discern. There are several white and light-colored single-ply membranes, for example, and each has its own chemical recipe and base polymer. It is important to know the proper method—glue, tape or hot-air welding—to use for long-term adhesion of seams. A BUR roof may be asphalt or coal-tar based; each has different properties. MB sheets come in several common types, and each is installed using different adhering methods.


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