Storm Drainage
Storm Drainage
This domain tests roof drainage, leaders, secondary drainage, and the separation of storm water from sanitary waste. The exam expects you to size roof drains and leaders from roof area and rainfall intensity, and to keep storm drainage independent of the sanitary system.
Core rules
- Storm water must discharge to an approved storm drainage system, dry well, or other approved point. It must not connect to a sanitary building drain or sewer unless the authority specifically allows combined drainage.
- Roof drains, scuppers, and overflow drains must be sized for the roof area served and the local design rainfall intensity. Secondary drainage is required so a blocked primary drain does not overload the roof.
- Leaders and horizontal storm drains need support, protection from freezing, and cleanouts at changes of direction. Roof drains need strainers and proper flashing.
- Traps may be required on storm drains depending on the system and code; verify the adopted requirements rather than assuming.
Worked example
A flat roof has 2,000 square feet of area and the design rainfall is 3 inches per hour. Approximate flow is Q = area x intensity x 0.623 = 2,000 x 3 x 0.623 = 3,738 gallons per minute. Use that flow with the adopted leader-sizing table and manufacturer data to select the leader and roof drain; do not select by roof area alone.
Traps
Do not connect storm drainage to the sanitary sewer. Do not omit secondary drainage on a roof that needs it. Do not size a leader from a straight-line plan dimension without accounting for rainfall intensity. Do not forget strainers and cleanouts.