Prepping a residential roof for solar PV
Guest contribution by Joan Crowe, director of technical services for the National Roofing Contractors Association.
The roofing industry continues to see rooftops used as platforms for roof-mounted photovoltaic (PV) systems and this is especially true in residential construction. However, based on calls received by The National Roofing Contractors Association (NRCA), there are is still confusion among roofing contractors, PV installers and homeowners on what they need to know before installing a roof-mounted PV system. This article will briefly discuss some of the important considerations when contemplating a roof-mounted PV system.
Equivalent Service Life
Installation of a roof-mounted PV system is a commitment to a long-term arrangement between a rooftop and PV system. A roof system’s primary function is to keep a building weatherproof. Ideally, a roof system and roof-mounted PV system should have similar life expectancies. Therefore, NRCA recommends homeowners install rack-mounted PV systems on roof systems that have equivalent or greater service lives than the PV components. But keep in mind that the estimated service life of a PV system is reported to be 25+ years and a conventional residential roof system’s service life is understood to be about 15 to 20 years.
If a new roof system is not installed, NRCA recommends a roof-mounted PV system be installed only with an existing roof system with equivalent service life or provisions be made during design that allow minimal disturbance to the PV panels and electrical output during reroofing.
For all roof-mounted PV system installations, NRCA recommends involving a professional roofing contractor and the roof system manufacturer to ensure roof system and manufacturer warranties are appropriate. This is especially true for roof-mounted PV system installation over existing roof systems. The roof system manufacturer should be contacted for written approval of a PV system installation to ensure a roof system warranty will not be voided.
Roof-mounted PV components on residential structures are required to meet building code requirements and they are addressed in the International Residential Code, (IRC). Roof-mounted PV systems were introduced in the 2015 edition of the IRC. In the context of the code, rack-mounted PV components are addressed in Section R907—Rooftop-mounted Photovoltaic Panel Systems. Requirements for roof-mounted PV systems also are found in the National Electric Code (NEC) published by National Fire Protection Association (NFPA). This document is referenced as NFPA 70. The most recent NEC was published in 2014. Specific requirements for PV systems are found in Article 690, Solar Photovoltaic Systems, and include circuit requirements, disconnecting methods, wiring methods, grounding, marking, batteries and safety requirements.
Additionally, requirements for fire- and uplift-resistance of roof-mounted PV systems are addressed in Section R907. This section specifies that roof-mounted PV systems should be the same fire classification as the roof assembly. For uplift resistance design, roof-mounted photovoltaic panel or modules systems are considered as components and cladding in IRC. Accordingly, they are to be designed and installed per the requirements for components and cladding.
Impact (hail) resistance is not specifically discussed in Section R907. However, NRCA recommends PV panels should be tested and labeled to withstand impact from hail, especially in hail-prone areas.
A very important consideration is a roof structure’s structural capacity. All PV systems will add weight to rooftops, and NRCA recommends a structural engineer verify a roof system’s structural capacity. This is especially important when installing a PV system on an existing rooftop. For new construction, however, the PV system’s weight should be included during design of the roof structure. Rack-mounted PV panels can add significant weight to a rooftop; adhered PV panels without racks or rigid substrates are not as heavy. Regardless, the capacity of a roof’s structure must be able to support the additional load of all PV systems.
Rooftop loads can also include sliding loads, or drag loads, from the additional weight of the PV panels, and snow loads from snow that is held on rooftop when PV panels act as a snow fence which prevents the snow from leaving the rooftop.
Placement of roof-mounted PV panels is important to achieve the largest amount of solar exposure and reduce or prevent shading. Because solar exposure is largest on south-facing rooftops, PV panels facing south will produce the greatest amount of electricity. Panels should not be placed in proximity of trees, chimneys, parapet walls and other roof penetrations that would provide shade. Ideally, a roof-mounted PV system should be shaded at any time during the course of a year.
Layout of PV panels should follow the requirements for fire-fighting access. Requirements for access paths are addressed in Section R324.7.1—Roof Access Points in the IRC. The layout also should allow reasonable amounts of rooftop traffic access. Conduit, wiring systems and raceways should be located as close as possible to a roof’s ridges, hips, or valleys to minimize possible tripping hazards and maximize firefighters’ ventilation opportunities in the event of a fire. Additionally, roof-mounted PV panel layout should allow for roof system service, maintenance and repair during the service lives of the roof and roof-mounted PV systems.
Types of PV
There are two general types of PV systems that are available and installed on residential roof systems, rack-mounted PV panels and adhered PV modules. Following is a brief description of each type and some of the issues to consider.
Rack-mounted PV panels: Rack-mounted PV panels typically are glass-faced; frames usually are fabricated from aluminum. The racks and supporting devices are aluminum, stainless steel or coated steel.
Rack-mounted rooftop PV panels should be attached to support frames that are properly attached to a roof deck or structure. The racks are installed on support frames that penetrate a roof system or are curb-mounted.
NRCA recommends the support frames or curb mounts be properly flashed and weatherproof. NRCA does not recommend roof-mounted PV panels that are held in place with ballast because of potential wind-uplift resistance issues.
Adhered PV panels or modules: An increasing number of roof-mounted PV systems use PV panels or modules attached directly to roof membranes and surfaces. These systems are attractive to homeowners who prefer low-profile PV systems. Adhered panels use thin-film PV panels or modules. Adhered PV panels or modules are lighter than rack-mounted panels and, because they do not require racks, there often is no need to penetrate a roof system with accessory components such as wiring and conduit. These products may be adhered directly to certain steep-slope roof materials, such as standing-seam metal panels, clay and concrete tiles or asphalt shingles.
Selecting a Contractor
A roofing contractor or electrical contractor may act as the prime contractor during installation of a roof-mounted PV system. Because both trades will be working simultaneously on the rooftop, cooperation between trades is important, and roofing and electrical contractors who have worked together or have an established working arrangement are well-suited to perform this work.
NRCA recommends PV system designs be done by experienced professionals who understand solar design as well as the weatherproofing complexities and building-code requirements for roof systems. If a PV system designer is not experienced in roof system design and roof-mounted PV integration, it is prudent to have an integrated design process that includes a PV system designer, roof system designer and professional roofing contractor.
NRCA recommends the installation be performed by a full-service system integrator who clearly understands the weatherproofing requirements of roof systems, as well as one who understands roof-mounted PV systems. Because electricians likely are not educated in proper roofing and weatherproofing techniques, a professional roofing contractor can appropriately direct the roofing installation and the installation of PV panels and the flashing and weatherproofing of the PV panel support system. Conversely, because roofing contractors likely are not educated in proper electrical installations, a professional electrician should be included on a roof-mounted PV system installation. Using a professional roofing contractor who is also a full-service PV system integrator provides a turn-key, single-source installation for a homeowner.
A roof-mounted PV system should be installed by qualified and licensed (where applicable) professionals. Installers can be from a roofing company with a PV system specialty, a roofing company with a PV subcontractor or a PV installer with a roofing subcontractor.
It is important to remember a roof system’s primary function is to keep a building weathertight. Roof-mounted PV systems are desirable; however, keeping a roof system weathertight is a job only a professional roofing contractor can perform. Homeowners should consider the following: If a roof with a PV system leaks, it’s likely a professional roofing contractor will be contacted to resolve the leak. Therefore, it may be best the prime contractor for a roof-mounted PV system installation be a roofing contractor.
As you can see, there are many factors to consider when deciding to get a PV system. And keep in mind that this article was a brief summary and wasn’t all inclusive. To learn more about roof-mounted PV systems, NRCA has two publications, “Building Owners Guide to Roof-mounted Photovoltaic Systems” and “NRCA Guidelines for Roof Systems with Rooftop Photovoltaic Components.”
Additionally, NRCA University, in conjunction with Penn State, offers “Rooftop Photovoltaic System Installations,” an online study course for roofing professionals that gives the latest information about rooftop photovoltaic (PV) installations. For more information on the online study course and the publications, visit the NRCA online store.
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