SoakMonkey
BRE Digest 365 · Soakaway Design
Area is whatever impermeable surface actually drains to this soakaway — roof plus hardstanding, measured on plan. BRE 365 assumes 100% run-off, so there is no coefficient to apply.
The quickest route to the rainfall figures is the region quick-pick below. It sets the ratio r and the growth-factor region together, and for most domestic jobs that is what gets submitted. Replace them only if you hold site-specific FSR or FEH data.
The climate-change uplift is the number people argue about. 0% is BRE 365 as published; 40% is what most lead local flood authorities now expect. Get it from the drainage condition or the LLFA rather than assuming.
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)
No. This is the one number the tool cannot supply and a soil map cannot either. Building Control will ask for the test record, and a soakaway designed on an assumed rate is the most common reason one has to be dug up again.
The test itself is simple: dig a trial pit, fill it, and time the fall from 75% to 25% of the effective depth. Repeat three times — the first fill wets the soil up and flatters the answer, which is exactly why BRE asks for three. The tool takes the lowest rate of the set.
Standing over the pit now? Use the stopwatch button — it times the fall and fills the box in. The app installs and works offline for that reason.
You fix the dimensions the site decides, and the tool solves the one left over.
Deeper is not automatically better. Water soaks out through the sides; the base is ignored because it silts up. A wider, shallower soakaway usually passes the 24-hour check more easily than a deep narrow one.
Two things have to be true: the soakaway must hold the critical storm, and it must half-empty within 24 hours so it is ready for the next one. The second is what usually fails.
BRE 365 sizes the soakaway; it does not site it. Keep it at least 5 m from foundations and above the water table, run paved areas through an interceptor first, and manage the 1-in-100-year exceedance within the site boundary.
Free to use on screen. See an example report
| 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%)
Secure payment via Stripe · £25 + VAT (20%) = £30.00 total. Included: your Building Control–ready PDF with a unique Report ID (verifiable at energycount.co.uk/verify), re-download free for 24 hours and free re-export for genuine corrections within 1 hour (up to 5). A VAT invoice is emailed automatically — add your company name and VAT number at checkout. By paying you agree to our Conditions of Sale — the report is supplied immediately, so the 14-day cancellation right is waived on delivery.
You are sizing one thing: a hole in the ground big enough to hold a storm and empty itself before the next one. SoakMonkey works it out to BRE Digest 365 and tells you whether the design half-empties inside 24 hours.
Five steps, and only two of them really decide the answer: the area draining into it (step 2) and the soil infiltration rate from your percolation test (step 3). Step 4 is you choosing a shape; the tool solves the missing dimension.
Every box has a small i beside it. Click it and it says what the number is and where to get it.
BRE Digest 365 balances what runs in against what soaks away, across a whole range of storm durations, and takes the worst case:
storage S = inflow I − outflow O, for every storm duration — the biggest S wins
Inflow is the drained area times the design rainfall depth. Outflow is the infiltration rate times the wetted side area times the storm length. The base is excluded from the infiltration area, because it silts up. Then the critical check: the soakaway must half-empty within 24 hours, or it is not ready for the next storm.
Area is the impermeable surface actually connected to this soakaway — roof plan area plus any hardstanding — measured on plan, not up the slope. BRE 365 assumes 100% run-off from it, so there is no run-off coefficient to apply.
Rainfall. Use the region quick-pick unless you have site-specific figures: it sets the FSR ratio r and the growth-factor table together. r describes how "peaky" the local rainfall is (0.12 to 0.45 across the UK); M5-60 is the one-hour depth with a five-year return period, which BRE 365 simplifies to 20 mm anywhere in the UK.
Climate change uplift. 0% gives you the original BRE method; 40% is what most lead local flood authorities now require. Check your drainage condition or the LLFA's standing advice — this is the number most likely to be argued about.
This is the one number you cannot look up. Never design a soakaway from an assumed soil rate — a published soil map is not a substitute for a test, and Building Control will ask for the test record.
The BRE 365 method: dig a trial pit, fill it, and time how long the water takes to fall from 75% to 25% of the effective depth. Do it three times. The tool computes the infiltration rate f for each and adopts the lowest — the conservative one.
f = V(75→25) ÷ ( ap50 × t(75→25) )
Real soils land somewhere around 10-6 to 10-3 m/s. A figure far outside that usually means a units slip or a pit that was still wetting up.
You fix the dimensions you know and the tool solves the remaining one. Change the form and the field set changes with it.
The storm-duration table shows every duration tested; the highlighted row is the critical storm, the one that sizes your soakaway. It is often not the longest one.
BRE 365 sizes the soakaway. It does not site it. A soakaway should sit at least 5 m from foundations and above the water table, paved areas should drain through an oil interceptor first, and the 1-in-100-year plus climate-change exceedance event must still be managed inside the site boundary. None of that is in the arithmetic.
It implements BRE Digest 365 (1991, revised 2016) with the FSR rainfall ratios, adds an optional climate-change uplift, and checks the 24-hour half-empty criterion, with the arithmetic shown so a drainage engineer can follow it. It is not a substitute for site-specific engineering judgement, and a published soil type is never a substitute for a percolation test.
Stuck? Get in touch or call 01202 623236.