Roof Drainage, Slope, and Waterproofing Details
This domain tests minimum slopes by profile, drainage sizing, and detailing that keeps water moving off the roof. Questions often give a run length and ask for fall, or give a profile and ask whether the specified slope is acceptable.
Slope by profile
- Structural standing seam with hydrostatic seams: 1/4:12 minimum; 1:12 typical.
- Architectural snap-lock: usually 3:12.
- Through-fastened: 3:12, with some manufacturers allowing 2½:12 with sealant.
- Below 3:12, treat the roof as low-slope: two underlayment layers or a membrane.
Drainage design
Size gutters and downspouts from the 100-year, 1-hour rainfall intensity for the site, using manufacturer or SMACNA tables. Slope to drains and scuppers; add crickets behind curbs and at valleys. The roof must drain and not pond; standing water for more than 48 hours signals a design or framing problem.
Waterproofing details
Step flashing at sidewalls with 4 in. exposure and 2 in. concealed. Keep underlayment continuous through valleys and hips.
Worked example
A 20 ft run at 1/4:12 needs 20 × 0.25 ÷ 12 = 0.42 ft, about 5 in. of fall. If the framing provides only 3 in., the specified hydrostatic seam cannot be justified and the detail must be redesigned rather than sealed.
Traps
Assuming seams are waterproof at any slope; forgetting crickets at curbs; and sizing drainage with a 1 in./hr rule in a high-rainfall region.