Solar-ready design for low-slope roofs

Though photovoltaics are increasingly a key component of commercial projects, they may create challenges for the roof system. AIA partner GAF offers best practices to consider to ensure the roof and solar array perform as designed.

Commercial rooftops are an appealing option for the installation of solar arrays to support energy conservation and generation: It’s estimated that if photovoltaic systems were installed on all commercial buildings in the US with roofs over 5,000 sq. ft., they would provide enough energy to power nearly 60 percent of the total commercial electricity demand.

However, it is important to remember that the roof’s primary function is to protect the building and its inhabitants from the elements. A solar-ready roof is typically a new or replacement roof that will incorporate solar arrays, and there are many important considerations for roof system design and panel layout.

For example, as solar panels get hotter, they produce less power. Installing a solar panel over a highly reflective membrane (versus a membrane with lower reflectance) may boost the panel’s efficiency by as much as 13 percent. Also, the use of bifacial solar panels over reflective roof membranes can increase the panel efficiency by 30 to 35 percent, as they take advantage of the reflected light.

Damage is another important consideration. While ballasted solar panel mounting systems can be cost effective, they can add significant weight to the roof and may also shift and flutter during high winds and seismic activity. This movement could lead to damage of the roof membrane that is “detrimental to satisfactory long-term roof system performance,'' according to the National Roofing Contractors Association (NRCA).

After installation, new challenges may arise when the roof becomes a permanent platform for the continuous operation, service, and maintenance of the solar arrays. It’s imperative that architects carefully consider roof system design, including membrane, coverboards, insulation, and attachments, in correlation with any photovoltaic arrays.

Here are the main considerations to take into account when designing low-slope roof systems for solar:

Choose the right products

Solar arrays have a predicted lifespan of more than 25 years, so it’s important that the roof have a commensurate or greater life expectancy.

According to the National Renewable Energy Laboratory (NREL), "the best roof for a flat application is a fully adhered thermoplastic olefin or polyolefin (TPO) membrane roof," reinforcing the use of adhered membranes as well as an adhered top layer of insulation and coverboard within the roof system. Designers and owners may also want to consider an increased roof membrane thickness to extend the roof’s service life, and using wider rolls will minimize the number of seams buried below the solar arrays.

Regardless of the type of solar array installation, NRCA recommends using a roof membrane that provides enhanced protection against the effects of UV radiation and high service temperatures (for example, GAF’s Everguard Extreme TPO) so that the roof life expectancy will match that of the solar arrays.

Include an adhered high-compressive-strength coverboard directly beneath the roof membrane to withstand increased foot traffic, enhance system durability, and extend the life expectancy of the roof.

For a ballasted system, use high-compressive-strength insulation, a minimum of two layers, staggered and offset. These systems also should include a protection or separation sheet adhered to the membrane.

Lay out and install properly

NRCA recommends using attached or penetrating solar mounting systems through the roof to the structure. Penetrations and flashings must be well detailed and coordinated with the roofing contractor, solar contractor, and electrician. For ballasted solar array supports, additional protection of the roofing system may be required for warranty coverage.

Generally, solar panel layouts require a clear pathway around roof edges, hatches, skylights, service penetrations, between rows of panels, and along both centerline axes of the roof areas. Setting rack heights with enough clearance to service the roof membrane, especially at drains and penetrations, is also important.

Install walk pads for high-traffic areas to prevent damage to the roof during service of the PV panels.

Finally, conduct integrity testing of the roof membrane prior to installing solar overburden.

Consider long-term requirements

In addition to these immediate needs, designers should consider how solar layout requirements align with best practices for roof maintenance.

• Lay out solar arrays to maximize solar energy collection while avoiding high-wind-uplift areas and additional snow accumulation.
• Provide perimeter and maintenance access for roof and solar array maintenance, as well as fire safety and smoke ventilation.
• Set racking systems so that they don’t cross roof expansion joints or block drainage.
• Set solar arrays and rack heights so that drains and penetrations are accessible for maintenance.
• Engage with the roof contractor to inspect (and repair as needed) the roof membrane after solar array installation.

It’s important to note that materials, layout, structure, and installation all go hand in hand for long-term health of your roof and systems.

The good news is that as rooftop solar becomes more popular, there are more resources available to designers, owners, and contractors to help design, install, and maintain a durable roof system that can match or outlast the service life of solar arrays. See GAF’s Roofing and Building Science full publication for more information and key resources.

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