Dimensional asphalt shingles can be problematic when installed on slopes below 3.5/12. This is due to the way they decrease the slope at the edge of the shingle because of the extra shingle thickness. While permitted by building standards down to 2/12, even per manufacturer’s instructions, roofing contractor industry associations recommend against it. It has been demonstrated that more leaks occur on roofs with dimensional shingles installed below 3.5/12. For additional information see WSRCA_LamShingle_TechBulletin.
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