Roofing Materials, Systems, and Installation Methods
Slope selects the system
Slope decides the assembly, and choosing wrong fails inspection no matter how good the workmanship. Asphalt shingles (ASTM D3462) are permitted down to 2:12; between 2:12 and 4:12 the code requires double underlayment and the manufacturer's low-slope detail. Below 2:12 you must use a low-slope system: built-up roofing, modified bitumen, single-ply membrane, or standing-seam metal. Metal panels lap at 3:12 or steeper; wood shingles at 3:12; wood shakes and most tile at 4:12 or 2.5:12 depending on the listed product.
Fastening is a specification
Four nails per shingle in the manufacturer's nail line for ordinary exposures, six where the listed wind speed demands it. Nails must penetrate the deck at least 3/4 inch, be driven flush - not over-driven, which cuts the mat - and be placed above the exposure line so the next course covers them. Massachusetts coastal sites carry basic wind speeds at the high end of the state map, so high-wind nailing and starter-course detail are normal requirements, not upgrades.
Worked example
A 3:12 porch roof with rafters at 24 inches on center. Shingles are permitted, but a double layer of No. 15 felt (or an equivalent listed underlayment) is required between 2:12 and 4:12, plus ice barrier at the eave and drip edge at eave and rake. Nailing: six per shingle in the coastal wind zone, per the printed instructions on the bundle.
Traps
Applying shingles at 1:12 because it never leaked before; substituting an unlisted synthetic underlayment that voids the Class A assembly; using staples; and relying on caulk at the ridge instead of a listed ridge vent with hip-and-ridge cap.