SoakMonkey
BRE Digest 365 · Soakaway Design
Fill a trial pit and let it drain three times. Record the time for the water level to fall from 75% to 25% effective depth. The tool computes f for each test and adopts the lowest value for design, per BRE 365. f = V₇₅₋₂₅ ÷ (ap50 × t₇₅₋₂₅)
Free to use on screen. See an example report
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| Duration D | Z1 | M5-D (mm) | Z2 | Design R (mm) | Inflow I (m³) | Outflow O (m³) | Req. dimension | Storage (m³) |
|---|
Highlighted row = critical (largest) storm. Method: BRE Digest 365 (1991, rev. 2016) volume balance I − O = S, with FSR rainfall ratios. The base of the soakaway is excluded from the infiltration area (assumed to silt up).
A soakaway must store storm run-off and let it infiltrate fast enough to be ready for the next storm. BRE Digest 365 sizes it by balancing inflow from the drained area against outflow into the soil across a range of 1-in-10-year storm durations, taking the worst case. This tool implements that method exactly, adds an optional climate-change uplift, and checks the soakaway half-empties within 24 hours. A site percolation (soakage) test is required — never design from assumed soil rates. Soakaways should sit at least 5 m from foundations and above the water table. For paved areas, run-off should pass an oil interceptor first. The 1-in-100-year + climate-change exceedance event must be managed within the site boundary.
A fully-branded, Building Control–ready PDF: percolation test, storm analysis, sizing calculations, half-drain check and a dimensioned summary.
£25 + VAT (20%)
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