Built-up roofs: ¼:12, coal tar ⅛:12
IBC 2021 Section 1507.10.1 says built-up roofs shall have a design slope of not less than ¼:12 (2 percent) for drainage. The exception: coal-tar built-up roofs may have a design slope of not less than ⅛:12 (1 percent).
The trap: applying the coal-tar ⅛:12 exception to every built-up roof. A conventional (non-coal-tar) built-up roof is ¼:12.
Every other membrane is ¼:12
The coal-tar exception belongs to coal-tar built-up roofs only. The other low-slope coverings all need ¼:12 (2 percent):
- Modified bitumen: Section 1507.11.1, ¼:12.
- Single-ply membranes: Section 1507.12.1, ¼:12.
- Sprayed polyurethane foam: Section 1507.13, ¼:12.
Extending the ⅛:12 figure to modified bitumen, single-ply or foam is the common error. If a question asks which covering is allowed as low as ⅛:12, the only one is coal-tar built-up.
Turning slope into drop
A slope of ¼:12 means ¼ in of drop per 12 in of run, which is ¼ in per foot. To find how much a roof must fall, multiply the run in feet by the drop per foot.
Worked example: a single-ply roof drains one way to drains 40 ft from the high edge. Section 1507.12.1 requires ¼:12, so 40 ft × ¼ in/ft = 10 in of drop.
Wrong answers come from using another covering's slope:
- 5 in uses the coal-tar ⅛:12 rate (40 × ⅛). Not allowed for single-ply.
- 20 in uses ½ in per foot, the lapped-metal-with-sealant slope.
- 120 in uses 3 in per foot, the slope for lapped metal panels without sealant.
Keep the neighbours straight
- ½:12: lapped metal panels with lap sealant (Section 1507.4.2).
- 2:12: asphalt shingle minimum (Section 1507.2.2).
On a low-slope question, start at ¼:12 and only drop to ⅛:12 if the words "coal-tar built-up" appear.