Storm drainage systems
Domain 7 - Storm drainage systems
This domain covers roof drains, leaders, secondary drainage, subsoil and area drains, and where storm water may lawfully go. Connecticut adds a firm clean-water principle: storm water and groundwater do not belong in the sanitary sewer.
Sizing and layout
- Roof drain and leader sizing begins with the roof area served and the local rainfall rate. Code tables are based on a 1 in./hr rate, so for other rates adjust the area accordingly. Design flow in gallons per minute = roof area (ft²) × rainfall rate (in./hr) × 0.0104.
- Vertical leaders carry more than horizontal runs of the same size; a horizontal leader needs slope and often a larger size.
- Where a roof is enclosed by parapet walls, secondary drainage (overflow drains or scuppers) is required so blocked primary drains cannot pond water beyond the design load. The overflow inlet is set above the primary inlet so it acts only when the primary fails.
- Roof drains need strainers and accessible outlets; conductors must not discharge where they create a slip hazard or undermine a footing.
Where water goes
- Storm water discharges to a storm system, drywell, or other approved point. Sump pumps, footing drains, and roof leaders may not discharge into the sanitary sewer, and Connecticut towns enforce that through sewer-use ordinances.
- Area drains and subsoil drains need backwater valves where subject to backflow, and must be trapped where they connect to a combined system.
- Basement floor drains and elevator pits need lifting or backwater protection.
Worked example
A 60 ft × 40 ft flat roof (2,400 ft²) at a 4 in./hr rainfall rate is drained by two 4 in. leaders. Reasoning: design flow = 2,400 × 4 × 0.0104 ≈ 100 gpm, so each leader handles about 50 gpm. Compare that against the leader capacity at the available depth of water over the drain and the length of horizontal run; if capacity falls short, add leaders or increase size. Because parapet walls allow ponding, secondary drainage is also required.
Where marks are lost
- Using a 1 in./hr table value for a design storm that is much higher.
- Omitting secondary or overflow drainage on a parapet roof.
- Connecting a sump pump, footing drain, or downspout into the sanitary sewer.