Storm Drainage and Roof Drain Sizing by Rainfall Intensity
Storm drainage is sized for a design storm, not the average day. The two inputs that matter are the catchment area and the local rainfall intensity for the chosen return period.
The rational method
- C — runoff coefficient: 0.95 for roofs and paving, 0.35–0.7 for lawns and gravel.
- i — rainfall intensity for the design return period (often 1-in-100-year for roofs) and a short duration (5–10 min) from the local IDF curve. Philippine urban design values are commonly 200–350 mm/h.
- A — contributing catchment, including any vertical wall that sheds onto the roof (add 50% of wall area).
Roof drains and leaders
Size each roof drain and its vertical leader (downpipe) for the flow from its portion of roof at the design intensity. Typical capacities:
| Leader size | Approx. capacity |
|---|---|
| 75 mm | ~2.5 L/s |
| 100 mm | ~5.5 L/s |
| 150 mm | ~16 L/s |
Horizontal storm drains are sized like sanitary drains but at storm DFU-equivalent or directly by flow and slope.
Secondary (overflow) drainage
Codes require an independent secondary path — overflow scuppers or a second set of drains with inlets 50 mm above the primary — sized for the full design storm assuming the primary system is blocked. This prevents ponding loads from collapsing a flat roof.
On-site detention
Where the outfall cannot accept the peak, a detention tank or pond stores the difference between inflow and the permitted discharge, released slowly through an orifice.
EngEst Pro applies the rational method with your local rainfall intensity to size roof drains, leaders, and storm piping — and lists drains, downpipes, and storm drainage pipe in the BOM alongside the sanitary system.
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