Low-slope roofs

During the design phase of a rooftop solar system project, the rooftop should be analyzed for sources of shade before the solar system layout is designed. Ideally, no shade should be cast on a south-facing solar array on the day when the sun is lowest in the sky, which is Dec. 21 in the Northern Hemisphere. Shading or otherwise obscuring only a small section of a solar module or panel can significantly reduce its output. Solar systems should not be shaded from rooftop penthouses, equipment, or adjacent trees or buildings, especially midday. Early morning or late afternoon shading generally is considered acceptable. Inter-row shading between rows of sloped solar systems is a concern on rooftops where adjacent solar systems may cast a shadow or shade on adjacent solar systems. Partial shading can cause "hot spots" in a solar array because shaded cells resist current flow from other series-connected cells in the device. This can damage solar modules or panels and may affect the roof system. Solar component manufacturers should be contacted for specific design guidelines and recommendations.

NRCA recommends designers of low-slope roof systems for solar applications specify the following enhancements:

  • Any above-deck insulation should be high-compressive-strength, rigid board insulation.
  • For membrane roof systems, a thermal barrier board should be located directly underneath the roof membrane.
  • For single-ply membrane roof systems, membrane thicknesses should be increased.
  • Reflective roof surfaces or coatings that provide enhanced protection against the effects of ultraviolet (UV) radiation and high service temperatures should be used.

Building owners and designers should recognize the installation of rack-mounted solar systems in close proximity to low-slope roof system surfaces limit accessibility to perform roof system preventative maintenance, repairs and, if necessary, replacement. For this reason, NRCA recommends building owners and designers install rack-mounted solar systems on roof systems that have equivalent service lives or greater service lives than the solar systems. Also, building owners and designers should recognize it may be necessary to disassemble or remove solar systems to perform roof system maintenance or replacement.

For specific design details, drawings and more NRCA recommendations, the Rooftop Solar Resource suggests you purchase NRCA Guidelines for Roof Systems with Rooftop Photovoltaic Components.

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