Rainwater Downpipe Calculator — How Many Downpipes Do I Need?

Effective roof area accounts for wind-driven rain on the vertical elevation. Per the effective-catchment-area method in BS EN 12056-3:2000 (UK National Annex NC): Effective Area = Plan Area × (1 + 0.5 × tan(pitch)). At 30° pitch the factor is 1.29; at 45° it is 1.5. Steeper pitches collect more wind-driven rain.

BS EN 12056-3:2000 does not set a fixed metre limit between outlets — the run is hydraulically limited by the gutter and outlet capacity, not a constant length. As a conservative rule-of-thumb, UK industry guidance (MGMA, manufacturer sizing guides) commonly allows up to about 12 metres of 112mm half-round gutter before an outlet at a typical fall, with two outlets falling from a central high point used for longer runs. This calculator defaults to a slightly more conservative 10 metres so it never under-specifies; you can adjust the Max Gutter Spacing in the advanced options. Longer runs risk overflowing during heavy rain.

Most UK houses use 68mm round or 65×75mm square downpipes, which handle roof areas up to about 50m² per downpipe. For larger roofs, extensions, or flat roofs with internal outlets, 110mm downpipes are used. The choice depends on your effective roof area and local rainfall intensity.

75mm/hour (≈ 0.021 L/s per m²) is the historic fixed UK design rainfall intensity, still used in Approved Document H Table 2 and widely in practice. BS EN 12056-3:2000 instead uses location-specific intensities: external eaves gutters that can overflow safely without entering the building are designed to a Category 1 one-year return period, using a 2-minute critical storm duration for pitched roofs (the time taken for runoff to fill the gutter). Higher intensities of 100-150mm/hour are used in high-rainfall areas (western Scotland, the Lake District, Snowdonia) or where overflow would enter the building, e.g. flat-roof internal outlets.

Yes, multiple downpipes can connect to a shared underground drain, but the pipe must be sized for the combined flow. A 110mm underground pipe at 1:80 gradient can typically handle 3–4 standard 68mm downpipes. Each connection should have a back-inlet gully or direct connection with a rodding point. Use our Surface Water Drainage Calculator for underground pipe sizing.