Storm Drainage and Subsoil Drainage
Storm Drainage and Subsoil Drainage tests roof runoff, leaders, area drains, subsoil drains, and sump pumps. The key skill is separating storm water from sanitary sewage and sizing drainage for the local rainfall rate.
Core rules
- Storm water must not connect to the sanitary sewer unless the AHJ specifically approves it.
- Roof drains, scuppers, and leaders must handle the design rainfall intensity for the roof area.
- Secondary roof drainage is required where the primary drains can be blocked by debris.
- Subsoil drains collect groundwater and may discharge to a storm drain, sump, or approved location.
- Sump pumps need a check valve and an air gap or approved backflow protection where discharge could contaminate potable water.
Worked example
A roof area is 1,200 ft² and the design rainfall is 5 inches per hour. Runoff is 1,200 × 5 × 0.0104 = 62.4 gpm. Reasoning: 1 inch of rain on 1 ft² is about 0.0104 gpm, so multiply area by intensity by that factor. The result is the minimum flow the roof drainage system must handle; pipe and leader sizes then come from the adopted sizing table, not from the formula alone.
Traps that cost marks
- Connecting roof drains to the sanitary building drain.
- Sizing leaders by roof area alone without rainfall intensity.
- Omitting secondary drainage on a flat roof.
- Discharging a sump pump directly into a potable-water receptor without an air gap.