Rainwater Downpipe Size Calculator UK

It depends on your roof's effective area (plan area adjusted for pitch) divided across however many downpipes serve that gutter run. As a rough guide, a single 68mm round or 65mm square downpipe handles up to about 50m² of effective roof area, 100mm round up to about 130m², and 110mm round/square up to about 160m² - enter your figures above for an answer specific to your roof.

A steeper roof sheds the same rainfall faster from the same ground footprint, so BS EN 12056-3's effective area formula - plan area x (1 + 0.5 x tan(pitch)) - scales the area upward as pitch increases. A flat roof (0°) has no multiplier (effective area equals plan area), while a steep 50° roof multiplies plan area by roughly 1.6x.

Plan area is the flat, birds-eye-view footprint of the roof - what you'd measure looking straight down from above, ignoring the slope. This calculator asks for plan area because that's what the BS EN 12056-3 effective-area formula is built around; the formula's pitch factor already accounts for the extra rainfall-shedding effect of the slope, so using the longer sloped surface area instead would double-count the pitch.

Divide your roof's effective area by your chosen downpipe size's capacity and round up - for example, an 81m² effective area needs 2 x 68mm downpipes (2 x 50m² = 100m² capacity) or 1 x 100mm downpipe (130m² capacity). This calculator does that division for you and also shows how many of each standard size the whole roof would need.

Around 12m is a commonly used maximum spacing between downpipes on a typical half-round gutter and 68mm downpipe combination, though this depends on the gutter profile and fall - check your specific gutter manufacturer's guidance for their product's rated maximum run length.

It's a simplified, historic design figure from Approved Document H's Table 2, not a literal prediction of the heaviest storm your area will ever see. BS EN 12056-3's full calculation method uses location-specific rainfall intensities that can be higher in some parts of the UK, particularly upland and western areas exposed to more intense rainfall - this calculator uses the simpler flat rate, which is standard practice for ordinary domestic roofs but leaves less margin in a genuinely high-rainfall location.

68mm round or 65mm square is the most common size fitted on standard semi-detached and terraced houses in the UK, sized around the roughly 40-50m² effective roof area a typical single gutter run drains. Larger detached houses, bungalows with bigger roof spans, or roofs with fewer downpipes than ideal often need to step up to 100mm or 110mm.

Yes - downpipe capacity and downpipe count are interchangeable in this calculation: doubling the number of 68mm downpipes roughly doubles their combined capacity, so 2 x 68mm downpipes can serve a similar effective area to 1 x 100mm downpipe. Whether that's the right choice depends on your gutter layout, roof shape and where outlets can practically go, not just the raw capacity sum.

The pitch-factor formula correctly reduces to effective area = plan area for a flat (0°) roof, since there's no slope to accelerate runoff. Most flat roofs in practice drain via internal outlets and syphonic or siphonic drainage systems rather than conventional eaves guttering, though, which use a different design method entirely - this calculator is built for pitched-roof eaves gutters, and a flat-roof internal drainage design should come from a roofing/drainage specialist.

An undersized downpipe can't clear water as fast as the gutter delivers it in heavy rain, so the gutter overflows - sending water down the external wall, over doorways, or pooling at the base of the building instead of into the drainage system. Over time this can cause damp, staining, foundation saturation, or erosion beneath the downpipe outlet, so it's worth checking rather than assuming an existing pipe is adequate.

Not for this calculator's capacity bands, which are based on the pipe's internal diameter rather than its material - a 68mm PVCu downpipe and a 68mm cast aluminium downpipe of the same bore have essentially the same hydraulic capacity. Material choice affects cost, durability and aesthetics (aluminium and cast iron are common on period or listed properties) rather than the flow calculation itself.

Always round up to the next standard size, or add a downpipe, rather than rounding down - guttering is sized to handle a defined storm intensity without overflowing, and a downpipe running right at its rated capacity leaves no margin for a blockage, leaf build-up, or a storm slightly heavier than the 75mm/hour design rate.

Yes - the same method applies to any pitched roof section with its own gutter run, whatever the building. Just use that specific roof section's plan area and pitch, not the whole property's total roof area, since each gutter run is sized to what actually drains into it.

This calculator uses the same simplified Approved Document H method intended for conventional domestic eaves gutters. Large commercial roofs, valley gutters, box gutters, and syphonic drainage systems need a full BS EN 12056-3 calculation (or engineered syphonic design) from a drainage engineer, since they involve considerations - like specific local rainfall intensity, multiple roof planes draining to shared outlets, and negative-pressure syphonic flow - this tool doesn't model.

This calculator uses widely corroborated UK trade capacity bands for standard 68mm, 100mm and 110mm round/square downpipes. For a specific manufacturer's product (especially anything non-standard, like a decorative profile or a period cast-iron replica), check that manufacturer's own published flow-capacity data, since real-world capacity can vary slightly by profile and outlet design.