Water pools on new shingles over a low-slope patio roof. Was the pitch too flat for shingles?

The owner described the problem in a post on Reddit's r/Roofing. The low section over the patio had a flat roof until this summer's reroof. The roofer said the slope was enough for shingles, although some contractors who bid the job had advised against shingling it. Water now sits on the lowest course.

The code is specific. Section R905.2.2 of the International Residential Code allows asphalt shingles only on slopes of 2 inches of rise per 12 inches of run or steeper, and from 2:12 up to 4:12 it requires a double layer of underlayment. The Asphalt Roofing Manufacturers Association's residential manual sets the same 2:12 floor and says shingles between 2:12 and 4:12 need special low-slope application. The manual says shingle roofs work by shedding water, so the slope has to carry it off.

GAF tells contractors it cannot guarantee a roof will not leak if shingles go below 2:12, or if a low-slope section is missing the double underlayment. Its guidance also calls for a leak barrier at the eaves on low slopes and warns installers not to shorten shingle exposure to make up for a flat pitch. For slopes under 2:12, the ARMA manual lists roll roofing down to 1:12 with the concealed-nail method, and the IRC does not allow shingles there at all.

The first step is a number. GAF says slope can be measured from the underside of the roof deck with a tape and simple geometry. If the patio section is under 2:12, the shingles there do not meet the IRC. If it falls between 2:12 and 4:12, the owner can ask the roofer for photos or invoices showing two layers of underlayment and a leak barrier at the eave, and should make that request in writing.