What roof pitch do shingles need?
Short answer. Asphalt shingles need a roof pitch of at least 2 in 12 under the residential code, and slopes from 2 in 12 up to 4 in 12 require a double layer of underlayment or an approved alternative. Anything flatter needs a low-slope roofing system instead, and very steep roofs follow the shingle manufacturer's steep-slope fastening and sealing rules.
Why pitch matters
Shingles shed water by overlapping like fish scales. On steeper roofs water runs off quickly; on low slopes it moves slowly, and wind or debris can push it up under the courses. That is why low-slope shingle roofs need extra protection.
The rules by pitch
- Below 2 in 12: no asphalt shingles. Use a low-slope system such as modified bitumen or a single-ply membrane.
- 2 in 12 to under 4 in 12: shingles are allowed with two layers of underlayment, a 19 inch starter and 36 inch sheets lapped 19 inches, or an approved alternative such as a self-adhered membrane where permitted.
- 4 in 12 and up: standard single-layer underlayment.
- Very steep slopes: commonly above about 21 in 12, manufacturers call for extra nails and hand-sealing so shingles do not slip or unseal.
Measuring pitch
Pitch is the rise in inches over 12 inches of horizontal run. A level and tape measure in the attic, a smartphone app, or an aerial measurement report will tell you, and many roofs have several different pitches.
Common Georgia trouble spots
- Porch roofs and additions that flatten below 4 in 12 where shingles were laid without double underlayment.
- Sections shingled below 2 in 12, which leak no matter how well they are installed.
- Valleys where a low porch roof meets a steep main roof.
On a replacement
A good roofer measures each section's pitch and switches materials where needed, rather than shingling everything. See how roofers measure a roof and ice and water shield in Georgia.