Roof drainage flashings: gutters, downspouts, scuppers, conductors, expansion joints and overflow provisions
Drainage flashings collect and discharge roof water without ponding, overflow or damage from thermal movement. The exam tests sizing, slope, outlet redundancy and expansion details.
Core rules
- Size gutters and leaders for the design rainfall and roof area using the adopted code; a common rule of thumb is 1 sq in. of leader area per 100 sq ft of roof, but verify local rainfall and code.
- Slope gutter bottoms to outlets, commonly at least 1/16 in. per ft. Provide continuous hangers or brackets at the manufacturer's spacing; do not rely on the roof edge.
- Use strainers, cleanouts and accessible conductor openings. Internal conductors need expansion joints, secure supports and overflow protection.
- Provide overflow scuppers or secondary drainage at the maximum design water level so a blocked primary outlet cannot cause structural overload.
- Gutter expansion joints are required at intervals set by design and manufacturer; fix one side and allow the other to slide. Do not rigidly anchor both ends of a long run.
Worked example
A 2,000 sq ft roof drains to a gutter with 3-in. round leaders. The area rule gives 20 sq in. of leader area. One 3-in. round leader has about 7.1 sq in., so three give about 21.2 sq in., nominally adequate, but a single leader would be undersized. Check the local rainfall intensity, gutter slope and overflow scupper; a 6-in. wide scupper with 2 in. design head has roughly 12 sq in. of opening, but opening area is not equivalent to leader capacity. Size overflow independently and keep it clear.
Traps
Using one downspout for a long gutter; sloping gutters the wrong way; sealing expansion joints rigid; forgetting strainers; sizing overflow by the primary leader rather than by the design overflow rate.