What counts as low slope
Sources use different cutoffs. Roofing codes treat anything below the shingle minimum, 2/12, as a low-slope roof that needs a membrane or standing seam system. Many roofing trades call 3/12 or less low slope. OSHA uses 4/12 for fall protection. A roof of 1/4:12 to 2/12 is low slope on any of those definitions.
How much drop is 1/4 inch per foot?
Ponding water is the main danger on a flat roof, because it adds weight and finds every weakness. Here is the total drop for a roof drained one way:
| Run to the drain | 1/4:12 | 1/2:12 | 1:12 | 2:12 |
|---|---|---|---|---|
| 10 ft | 2.5 in | 5 in | 10 in | 20 in |
| 20 ft | 5 in | 10 in | 20 in | 40 in |
| 30 ft | 7.5 in | 15 in | 30 in | 60 in |
| 40 ft | 10 in | 20 in | 40 in | 80 in |
A 20 ft wide roof at 1/4:12 drops 5 inches from the high edge to the low edge. That is only 1.2 degrees, so a slight sag in the framing can leave a pond. Many designers choose 1/2:12 or more to allow for movement. Tapered insulation is often used to build the slope into a flat deck.
Roofing that works at low slope
| Roofing | Minimum slope |
|---|---|
| Built-up, modified bitumen, EPDM, TPO and PVC membranes | 1/4:12 |
| Standing seam metal | 1/4:12 |
| Lapped metal panels with sealant | 1/2:12 |
| Mineral-surfaced roll roofing | 1:12 |
| Asphalt shingles, with double underlayment | 2:12 |
| Metal shingles, exposed-fastener metal panels | 3:12 |
These are code minimums from the International Residential Code. The manufacturer's installation instructions can set a higher figure, and a lower one voids the warranty.
Checking an existing flat roof
- Look for standing water 48 hours after rain. A puddle that stays points to a low spot.
- Check that every drain, scupper or gutter is clear and at the low point.
- Measure the slope with a level and a tape. Hold a 4 ft level horizontal, then measure the gap at the far end. A 1/4:12 slope gives a gap of 1 inch over 4 ft.
To turn a measured rise and run into pitch, use the roof pitch calculator. For metal at a shallow slope see minimum pitch for a metal roof.
Frequently asked questions
What is the minimum slope for a flat roof?
1/4 inch per foot (1/4:12), under the International Residential Code. A zero-slope roof is not permitted.
What roof pitch is considered low slope?
Under 2/12 for roofing codes, 3/12 or less in many trades, and 4/12 or less for OSHA fall protection. The cutoff depends on the source.
How much slope does a 20 ft flat roof need?
At 1/4:12, a total drop of 5 inches over 20 ft. Many designers use 1/2:12, which is 10 inches.
Sources
- Jaspector: IRC 2024 roof slope requirements by covering
- Cornell LII: 29 CFR 1926.500, definitions of low-slope and steep roofs