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.
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