Generations of builders have installed asphalt shingle roofing on homes with chimneys, side walls and other structures that introduce a vertical surface above the roof line. To preserve the integrity of the roof and prevent water leaks, roofers tend to install flashing next to these structures with at least a 4-in. overlap on the horizontal plane. As a manufacturer of roofing and solar photovoltaic systems, CertainTeed recommends that when installers affix modules to the roof, they treat the anchoring system like any other vertical rooftop structure and provide at least a 4-in. horizontal overlap.
Why 4 inches? The specification comes from the latest edition of the National Roofing Contractors Association Roofing Manual: Steep-slope Roof Systems, published in 2013. (NRCA will release a new version of the manual next year.) Though there isn’t necessarily empirical data showing the lateral distance that water can travel on a sloped roof, NRCA members have 45 years of accumulated knowledge and experience since the first publication of the manual. They know weatherproofing for shingle roof systems about as well as anyone.
The minimum recommendation of a 4-in. horizontal overlap, also known as a side lap, applies to sloped roofs with a pitch of 4:12 or greater. Water can travel a greater lateral distance on lower slopes. If the pitch is less than 4:12, check with the roofing system manufacturer and be prepared to use flashing with a wider side lap.
For standard, rack-mounted photovoltaic systems, it’s always a good idea to read the product specifications and installation instructions prior to purchasing flashing to ensure that it is sized properly. With CertainTeed’s Apollo II integrated solar roofing system, there’s no need to search for compatible flashing. The product includes flashing designed with a 4-in. side lap.
Because there are many applications in asphalt shingle roofing that call for a 4-in. side lap, it is relatively easy to select appropriate flashing products for this purpose. The same cannot be said about selecting proper adhesives for sealing the roofing material and the flashing. When choosing among the many available adhesives, focus first on products that are compatible with asphalt shingles. It will do no good to purchase a high-quality adhesive if the manufacturer can’t vouch for its performance with your chosen material.
For flashing with asphalt shingles, including the Apollo II flashing, use an elastomeric, a tri-polymer, a urethane or a roofing mastic that meets the ASTM D4586 Type II standard. This still leaves an endless variety to choose from, but you will have narrowed down the options to those adhesives that can support a leak-free installation for the lifetime of the system.
This tip was contributed by Chris Fisher, PV product development leader at CertainTeed.