Soakaway Sizing Calculator — UK Building Regs Part H

This calculator uses the design method in BRE Digest 365 “Soakaway design” (revised 2016), together with UK Building Regulations Approved Document H, to estimate the storage a soakaway needs.

The design method (I − O = S)

BRE 365 balances the inflow of rainwater against the water that infiltrates into the soil during the storm. The required storage S is the inflow I minus the outflow O:

S = I − O, where I = A × R and O = a_s50 × f × D

• A = impermeable area drained (run-off taken as 100%)
• R = design rainfall depth for the storm (10-year return period)
• a_s50 = the pit’s internal side area to 50% of its storage depth — the base is excluded because it silts up
• f = soil infiltration rate (m/s) from a site soakage trial
• D = storm duration

Crucially, S is worked out for a range of storm durations (from a few minutes to many hours) and the largestvalue is taken. There is no single fixed “30-minute, 50 mm/hr” design storm. For slow-draining ground, the critical case is a long, less intense storm.

Design rainfall (BRE 365 Tables 1 & 2)

The rainfall depth is M10-D = (20 mm × Z1) × Z2. The 5-year, 60-minute base rainfall (M5-60) is 20 mm everywhere in the UK; Z1 comes from Table 1 using your local rainfall ratio r (read off the BRE 365 map, England & Wales ≈ 0.35–0.45) and the storm duration; Z2 is the 10-year (or 100-year) growth factor from Table 2.

Suitability & important notes

• The pit must discharge from full to half-volume within 24 hours, ready for the next storm — this is the BRE 365 suitability rule
• UK SuDS practice (CIRIA 753 SuDS Manual / council infiltration standards) puts a number on that rule: ground below about 1×10⁻⁶ m/s is treated as unsuitable for a soakaway
• Soakaways must be at least 5 m from any building and not in made ground (Approved Document H)
• Crate soakaways have ~95% void ratio; rubble fill only ~30%, so the excavation must be larger
• The soil rates here are indicative pre-test estimates — a site soakage trial is mandatory before design