Porch, patio, carport and dormer roofs with shingles often leak because they are too flat for shingles to shed water. Asphalt shingles are allowed only on a 2:12 slope or steeper, and between 2:12 and 4:12 the code requires double underlayment. A shingled roof below 2:12, or a low-pitch section without the extra underlayment, lets wind-driven rain and snowmelt back up under the laps. The fix is either a proper low-slope underlayment under the shingles or switching that section to a membrane roof. This page is general information; your roofer and building department make the final call.
On this page: Signs · Causes · What to check · Repairs · Code basis · When to call
Signs of a too-flat shingle roof
- Leaks on a porch ceiling, patio cover or low dormer while the main roof stays dry.
- Leaks mostly during snowmelt or long, slow rains rather than short storms.
- Shingles that look almost level when you stand back from the house.
- Ice building along the low edge, and dirt lines where water sat.
- Moss, algae or debris collecting on the shallow section.
Why low-pitch shingle roofs leak
- Below the minimum. Under 2:12, shingles are not permitted at all.
- Missing double underlayment on a 2:12 to 4:12 section.
- Slow water. Rain and melt move slowly enough to back up under the laps in wind.
- Ice backup. Melt freezes at the cold edge and dams water on the slope.
- Poor tie-in where the low section meets the main roof or the house wall. See roof-to-wall flashing.
Check the pitch safely
- Stand back and compare. A section much flatter than the main roof is a candidate.
- Measure from inside if you can. Pitch is inches of rise in 12 inches of run, measured on a rafter with a level.
- Look at the ceiling below for stains concentrated near the house wall or the low edge.
- Leave the rooftop measurement to a roofer. Low roofs feel safe and still cause falls.
Our roof slope guide explains pitch and which materials fit each range.
Fixes by pitch
| Pitch | Fix |
|---|---|
| 4:12 and steeper | Standard shingle roof; look for flashing or other causes |
| 2:12 to under 4:12 | Reroof with two layers of underlayment lapped as the manufacturer requires, or self-adhered membrane under the section |
| Under 2:12 | Convert the section to a low-slope membrane such as modified bitumen, TPO or EPDM |
| Any pitch with ponding or deflection | Check framing; see flat roof ponding |
Fixes that fail: coating or tarring the shingles, adding more shingles over the leaking ones, and caulking the laps.
Code and manufacturer basis
The 2024 IRC, section R905.2.2, allows asphalt shingles only on slopes of 2:12 or greater and requires double underlayment for slopes from 2:12 up to 4:12. Owens Corning’s Duration instructions give two low-slope options for 2:12 to under 4:12: two layers of synthetic underlayment, each course lapped half its width plus 2 inches, or self-adhered ice and water barrier with 3-inch laps and metal drip edge. Low-slope membranes have their own minimum: sections R905.11 and R905.12 require at least a 2 percent (1/4 inch per foot) slope for modified bitumen and single-ply roofs.
When to call
Call when a porch or patio roof leaks while the rest of the house does not, or when planning a reroof so the low section gets the right system. Related guides: modified bitumen lap failures, membrane seam failures and the roof problems guide.
Get the low section measured
Integrity Roofing and Painting is veteran-owned and family-run since 1990, an Owens Corning Platinum Preferred contractor with 650+ Google reviews. We measure the pitch, photograph the tie-in and recommend the system that fits that slope. Colorado Springs area: (719) 488-2800 or a free inspection. Denver metro: (303) 209-0152 or a free Denver inspection. Austin: (512) 610-5897 or a free Austin inspection. Same-business-day reply.
Sources: 2024 IRC Chapter 9, sections R905.2.2, R905.11 and R905.12 (UpCodes); Owens Corning Duration installation instructions (low slope underlayment).
