Soakaway Sizing Calculator — UK Building Regs Part H

Soakaway size depends on your impermeable area (roof plus any paved areas draining into it), your soil's infiltration rate, and the local design rainfall. Under BRE Digest 365 the required storage is the largest value of inflow minus infiltration outflow across a range of storm durations. For slow-draining soils, the worst case is a long storm, not a short one. A typical 3-bed semi with around 70 m² of impermeable area draining to one soakaway in moderately permeable ground needs roughly 2–3 m³ of storage; faster sand or gravel can need under 1 m³. This calculator estimates that storage to BRE 365, but a site soakage trial to measure the real infiltration rate is mandatory before you build.

Building Regulations Approved Document Part H requires surface water to be disposed of via soakaways where ground conditions allow. Soakaways must be at least 5 metres from any building and not in an area of fill or made ground. BRE Digest 365 provides the design methodology for sizing soakaways. All drainage pipes connecting to the soakaway need the correct gradient, so use our Drainage Fall Calculator.

Soakaway crates are significantly more efficient than rubble fill. Crates have a void ratio of approximately 95% (meaning 95% of the volume stores water), while rubble fill only has about 30% void ratio. This means a crate soakaway can be roughly one-third the size of a rubble fill soakaway for the same storage capacity.

Usually not. UK SuDS practice (the CIRIA 753 SuDS Manual and most council infiltration standards) treats ground with an infiltration rate below about 1×10⁻⁶ m/s as unsuitable for infiltration drainage, because the pit will not discharge from full to half-volume within the 24 hours required by BRE Digest 365. Most heavy clays fall below that threshold, so Building Control is unlikely to approve a soakaway. In those cases you typically connect surface water to a watercourse, a public surface-water sewer, or another SuDS feature — or consider a French drain — see our French Drain Calculator. Only a site percolation test to BRE 365 can confirm the true rate, as fissured clays occasionally drain faster than the soil texture suggests.

Dig a hole 300mm x 300mm to below the proposed soakaway invert level. Fill with water to 300mm deep and let it drain. Refill to 300mm and time how long it takes to drain from 75% full (225mm) to 25% full (75mm). The result is used in BRE DG 365 calculations. Tests should be done in winter when the water table is highest. The gravel needed to fill a soakaway can be calculated with our Gravel Calculator.