Rainwater Downpipe Calculator

🌧️ Rainwater Downpipe Calculator

Design flow and downpipe size from roof area and rainfall intensity.

Roof plan area (m²)

Rainfall intensity (mm/hr)

Runoff coefficient

Design Flow

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Recommended Downpipe

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Flow (L/min)

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Downpipes Suggested

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⚠️ Q = area × intensity × runoff ÷ 3600. Downpipe capacity is computed to BS EN 12056-3: Q = 2.5×10⁻⁴ · f^5/3 · d_i^8/3 (L/s) with filling degree f = 0.33 and the internal bore d_i in mm. Use the local design rainfall intensity and gutter outlet capacity; pitched-roof plan area is the horizontal projection. Verify before professional use.

Standards & method

✓ Formula independently verified 12 July 2026

Governing standard

BS EN 12056-3

Clauses applied

Q = 2.5 × 10⁻⁴ · f^5/3 · d_i^8/3
Filling degree f = 0.33
d_i is the internal bore, not the nominal size

Independently verified

12 July 2026 — Formula re-derived from the standard and checked numerically against worked reference cases.

Why this matters: Capacity is computed from the standard, not a rule-of-thumb table. Tables that use the nominal size instead of the internal bore over-rate capacity by 25–37% — and the gutter overflows.

Results are for guidance. Verify against the current edition of the governing standard and have a licensed engineer review before construction or installation.

Roof Drainage Flow — Method

The flow a roof delivers to its gutters and downpipes is the catchment area times the design rainfall intensity, adjusted by a runoff coefficient: Q (L/s) = area (m2) x intensity (mm/hr) x runoff / 3600. For a pitched roof use the horizontal plan (projected) area, not the sloped area. The downpipe is then selected so its capacity (running part-full, per BS EN 12056) exceeds the design flow; large roofs are split across several outlets and downpipes so each stays within capacity.

Frequently Asked Questions

How do you size a rainwater downpipe?⌄

Find the design flow Q (L/s) = roof area x rainfall intensity x runoff / 3600, then pick a downpipe whose capacity exceeds Q. A 120 m2 roof at 75 mm/hr gives 2.5 L/s, handled by a 75 mm downpipe.

What rainfall intensity should I use?⌄

Use the local design value, often a 1-in-50-year short-duration intensity (commonly 50-150 mm/hr depending on region). Higher categories of building use a more severe return period.

Do I use sloped or plan roof area?⌄

Use the horizontal plan (projected) area of the roof for the catchment, since rain falls vertically. Add allowances for wind-driven rain on walls draining onto the roof if relevant.

How many downpipes do I need?⌄

Divide the total flow by the capacity of one downpipe and round up, but also respect the maximum gutter run to each outlet. Spreading outlets keeps gutters shallow and avoids overflow.

What is the runoff coefficient?⌄

The fraction of rain that runs off rather than being absorbed or evaporated. For an impervious roof it is about 1.0; reduce it slightly only for special surfaces.

Rainwater Downpipe and Roof Drainage

Roof drainage has to carry away the heaviest rain the building is designed for without the gutters overflowing. The design flow comes straight from the catchment area and the local design rainfall intensity.

The design flow

Q = area x intensity x runoff / 3600 gives litres per second. Use the projected plan area and a region-appropriate intensity (often a 1-in-50-year value for housing, more severe for critical buildings).

Choosing the downpipe

Pick a downpipe and matching gutter outlet whose part-full capacity exceeds the flow; split large roofs over several outlets. For the connected underground drain, size with the pipe sizing calculator.

Related: Water Demand, Pipe Sizing, Pump Head.

Rainwater Downpipe Calculator

Rainwater downpipe — Quick answer