Every box has an i beside it saying where the number comes from. The same guidance is built into the tool under ? Help.
What the method actually does
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, because it silts up. Then the critical check: the soakaway must half-empty within 24 hours.
The percolation test — the one number you cannot look up
Never design a soakaway from an assumed soil rate. A published soil type is not a substitute for a test, and it is the most common reason a soakaway has to be dug up and done again.
The BRE 365 method, in full:
- Dig a trial pit — roughly 0.3 to 1 m wide, 1 to 3 m long, to the depth the soakaway will work at.
- Fill it and let it drain. Do this three times. The first fill wets the soil up and flatters the result.
- For each fill, time how long the water takes to fall from 75% to 25% of the effective depth.
- Record the pit's length, width and effective depth alongside the times.
f = V(75→25) ÷ ( ap50 × t(75→25) )
The tool computes f for each test and adopts the lowest. Real soils land roughly between 10-6 and 10-3 m/s; a figure far outside that usually means a units slip or a pit that had not finished wetting up. There is a stopwatch built into the app, and the app installs and works offline, because this measurement happens in a field.
Rainfall — the quick route and the proper one
Use the region quick-pick unless you hold site-specific data. It sets the FSR ratio r and the growth-factor region together, and for most domestic jobs that is what gets submitted. r describes how peaky the local rainfall is — 0.12 to 0.45 across the UK — and it decides which storm duration governs. M5-60, the one-hour five-year depth, is simplified by BRE 365 to 20 mm anywhere in the UK.
The return period is fixed at 1 in 10 years: that is the method's design storm, not a choice. The rarer 1-in-100-year event is not stored by the soakaway — it is managed as exceedance flow within the site boundary.
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 authority rather than assuming.
Choosing a form
| Form | Void ratio | When it suits |
|---|---|---|
| Filled granular pit | ≈ 0.30 | Traditional and cheap. Only three-tenths of the excavation is storage, so the hole is large |
| Modular crates | ≈ 0.95 | A far smaller dig for the same storage. Use the manufacturer's declared figure |
| Perforated rings | ≈ 0.30 surround | Where a ring unit suits the ground. The tool also checks the pit can contain the rings |
| Trench | ≈ 0.30 | Long and shallow — useful on a tight plot, and often empties faster because it is nearly all side area |
Deeper is not automatically better. Water soaks out through the sides; the base is ignored. A wider, shallower soakaway usually passes the 24-hour check more easily than a deep narrow one.
Reading the verdict
- DESIGN OK — it holds the critical storm and half-empties within 24 hours.
- FAILS 24h — it holds the storm but drains too slowly. Go wider and shallower to buy side area, or accept the ground is too tight for a soakaway.
- CHECK GEOMETRY — a dimension is outside a physically meaningful range, or the solve went negative.
- RING TOO SMALL — the rings cannot hold the required volume even in the smallest pit that physically contains them.
- AWAITING INPUT — something essential is still blank.
The storm-duration table shows every duration tested, and the highlighted row is the critical storm — the one that sizes your soakaway. It is often not the longest.
What the tool does not do
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 oil interceptor first, and manage the 1-in-100-year plus climate-change exceedance event within the site boundary. It is not a substitute for site-specific engineering judgement.
Frequently asked questions
Can I design a soakaway without a percolation test?
No. The soil infiltration rate 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. BRE Digest 365 sets out the method: dig a trial pit, fill it, and time the fall from 75% to 25% of the effective depth, three times over.
Why three tests, and why does the tool use the lowest?
The first fill wets the soil up and flatters the result, which is exactly why BRE asks for three. Taking the lowest infiltration rate of the set is the conservative choice and the one the method specifies.
What climate change uplift should I use?
0% gives you BRE Digest 365 exactly as published. 40% is the allowance most lead local flood authorities now require over a residential lifetime. It is the single number most likely to be queried, so take it from the drainage condition or the authority's standing advice rather than assuming — the Environment Agency publishes allowances by river basin district.
What is the rainfall ratio r, and where do I get it?
It is the FSR ratio of the 60-minute to the 2-day rainfall depth — a measure of how peaky local rainfall is, running from about 0.12 to 0.45 across the UK. It matters because it decides which storm duration ends up governing your design. Take it from the FSR or FEH rainfall map, or use the region quick-pick in the tool, which sets a representative value. BRE 365 simplifies M5-60 to 20 mm anywhere in the UK.
What does the 24-hour check actually mean?
A soakaway has to be ready for the next storm. BRE 365 requires that half the stored volume drains away through the infiltration surface within 24 hours. This is usually the test that fails, not the storage one — and because water soaks out through the sides while the base is assumed to silt up, a wider, shallower soakaway often passes where a deep narrow one does not.
Which soakaway form should I choose?
A filled granular pit has a void ratio of around 0.30, so the excavation is roughly three times the storage. Modular crates are about 0.95, needing a much smaller dig. Perforated concrete rings are checked additionally to make sure the pit can physically contain them. A trench is long and shallow, useful on a tight plot and often quicker to empty because it is nearly all side area.
What does the tool not check?
Siting. BRE 365 sizes the soakaway; it does not place 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 be managed within the site boundary. None of that is in the arithmetic.
Where the method comes from
The documents behind the method.
- BRE Digest 365 — Soakaway design The method itself: the trial-pit percolation test, the FSR rainfall ratios, the inflow/outflow volume balance across storm durations, and the 24-hour half-empty criterion.
- Flood risk assessments: climate change allowances The published peak rainfall intensity allowances by river basin district — the source for the climate-change uplift your lead local flood authority will expect.