Roof Drain Sizing Calculator - Gutters & Downpipes

ℹ About This Calculator

Roof drainage must carry away the peak rainfall without ponding or overflow. This calculator sizes the roof drain outlets, gutters and downpipes from the roof area and the local rainfall intensity, following IPC practice.

The design flow is the roof area multiplied by the rainfall intensity (a runoff coefficient near 1.0 for roofs). US design rainfall is high - often 75-150 mm/h for a short-duration storm - so downpipes and outlets are generously sized. Vertical downpipes carry far more than horizontal gutters, and a minimum number of outlets is set so a single blockage does not flood the roof.

Roof Drainage Formula

IPC

Q = A × i × C / 3600 Q = design flow, L/s
A = roof plan area, m²
i = rainfall intensity, mm/h
C = runoff coefficient (~1.0 for roofs)
Number of outlets = Q ÷ outlet capacity

Worked Example

Example: A 400 m² flat roof in a region with 120 mm/h design rainfall generates Q = 400 × 120 × 1.0 / 3600 ≈ 13.3 L/s. At about 4-5 L/s per 100 mm outlet, this needs three to four outlets, each served by a 100 mm downpipe, with gutters sized to the flow reaching each outlet.

Downpipe Capacity & Rainfall (IPC)

Downpipe capacity depends on diameter; the roof area each serves depends on the local rainfall:

Downpipe dia	Approx. capacity (L/s)	Roof area at 120 mm/h
75 mm	~2.5	~75 m²
100 mm	~4.5	~135 m²
150 mm	~12	~360 m²
200 mm	~25	~750 m²

Design Notes

Use the local short-duration design rainfall intensity, not the annual average - US cities commonly design to 75-150 mm/h. Provide at least two outlets per roof area so a blockage does not cause flooding, add overflow provisions (weirs or scuppers) on parapet roofs, and fall gutters toward the outlets. Siphonic systems can move much more water through smaller pipes for large roofs.

Indicative capacities - confirm against IPC and the local rainfall data.

Frequently Asked Questions

How do I size a roof drain? +

Multiply the roof plan area by the local rainfall intensity (with a runoff coefficient near 1.0) to get the design flow, then divide by the outlet capacity to find the number and size of outlets. A 400 m² roof at 120 mm/h needs about 13 L/s of drainage.

What rainfall intensity should I use for roof drainage? +

Use the local short-duration design rainfall, not the annual average. US cities commonly design roof drainage to 75-150 mm/h for a 5-10 minute storm of a chosen return period; higher-risk sites use a longer return period.

What size downpipe do I need? +

It depends on the flow. A 100 mm downpipe carries roughly 4.5 L/s (about 135 m² of roof at 120 mm/h), 150 mm about 12 L/s, and 200 mm about 25 L/s. Size the pipe for the roof area draining to it.

How many roof drains do I need? +

Divide the total design flow by the capacity of one outlet, and always provide at least two so that a single blockage does not flood the roof. Large or subdivided roofs need outlets distributed to suit the falls.

What is a runoff coefficient for a roof? +

For impervious roof surfaces the runoff coefficient is close to 1.0 - almost all rain runs off. Gravel or green roofs have lower coefficients because they retain some water, reducing the peak flow.

Do I need roof overflows? +

Yes, on roofs with parapets. Overflow weirs or scuppers set above the drain outlets prevent water building up dangerously if the primary drains block or a storm exceeds the design. They are a code requirement in many jurisdictions.

What is a siphonic roof drainage system? +

A siphonic system runs the downpipes full-bore, using the siphon effect to move much more water through smaller-diameter pipes with fewer downpipes - useful for large flat roofs like warehouses. It needs specialist design.

What standard covers roof drainage in the US? +

IPC (building drainage) and IPC (Plumbing Services) cover roof and rainwater drainage, giving design rainfall guidance and pipe sizing. Local municipal storm-water rules also apply.

Free Online Roof Drainage & Gutter Sizing Calculator