Quick answer: BRE Digest 365 Soakaway design (2016) sizes a rainwater soakaway in four steps. Measure the ground's infiltration rate with a full-size trial pit (time the fall from 75% to 25% of its depth, three times, use the slowest). Take the site's design rainfall for a 1-in-10-year storm at every duration from a few minutes to a day, with a climate-change uplift. For each duration, storage = inflow − outflow; the duration needing the most storage governs. Then check the soakaway half-empties within 24 hours. Approved Document H points to it, so it is what Building Control expects.
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What BRE Digest 365 is, and where it is required
BRE Digest 365 is a design guide published by BRE — the Building Research Establishment — for soakaways that dispose of rainwater by letting it infiltrate the ground. The current edition is 2016; it replaced the 1991 original, which is still quoted on older drawings. It is not a regulation. Its authority comes from Building Regulations Part H: Requirement H3 says rainwater from a roof or paved area should go to a soakaway or other infiltration system where that is practicable, and Approved Document H names BRE Digest 365 as the design method. Lead local flood authorities lean on the same document when they discharge a surface-water planning condition, and the soakage test is often the thing they ask to see.
The digest is a paid publication and is not reproduced here. What follows is the method in our own words, with the equations, so you can see what the calculation does and what it needs from you.
Step 1 — the soakage test (the "BRE 365 percolation test")
Everything rests on one number: the infiltration rate, f, in metres per second — how fast the ground at the soakaway's depth takes water. BRE 365 does not let you read it from a soil description; it is measured with a trial pit representative of the soakaway you intend to build.
- Dig the pit where the soakaway will go, to the depth it will reach. The digest specifies a pit 0.3 to 1 m wide and 1 to 3 m long. Record its length, width and effective depth — measured from the base up to the level the inlet pipe will enter — because the calculation uses the pit's geometry, not just the times.
- Fill it rapidly with water to at least 75% of the effective depth.
- Time the fall from 75% to 25% of the effective depth. This is the "75–25" time, tp75–25. Very slow ground may need the test left overnight.
- Repeat three times, on the same day or on consecutive days, so the ground around the pit is saturated. Dry ground drinks the first fill and flatters the result — which is why a single fill is not a BRE 365 test.
- Design on the slowest fill.
The infiltration rate is the volume drained between the 75% and 25% levels, divided by the pit's internal surface area at 50% effective depth (the base is excluded, because it silts), divided by the time:
f = Vp75–25 ÷ ( ap50 × tp75–25 )
Worked example. A pit 1.5 m long, 0.6 m wide, effective depth 1.2 m. The volume between 75% and 25% is 1.5 × 0.6 × 0.6 = 0.54 m³. The internal surface area to 50% depth is the perimeter times half the effective depth, 2 × (1.5 + 0.6) × 0.6 = 2.52 m². Three fills fall 75% → 25% in 38, 52, 61 minutes; the slowest, 61 minutes, is 3660 seconds. So f = 0.54 ÷ (2.52 × 3660) = 5.85 × 10⁻5 m/s — a free-draining sand or gravel, and comfortably workable. The percolation test calculator does this arithmetic from the three times, and has a stopwatch for the pit.
Step 2 — the design rainfall
The storm a soakaway is designed for is a 1-in-10-year event — a return period of 10 years — unless the approving body asks for more. But no single storm is "the" storm: a short cloudburst delivers water faster than the ground can take it, while a long, moderate storm delivers more water in total. BRE 365 therefore checks a range of durations, from a few minutes to a day or more, and keeps the worst case.
Rainfall depths for each duration come from the Flood Studies Report method: two figures read for the site from national maps — M5-60, the 5-year 60-minute depth, and the ratio r of the 60-minute to the 2-day depth — are converted to the 10-year depth for every duration with the digest's growth factors. To that a climate-change uplift is added: the Environment Agency publishes peak-rainfall allowances by region and epoch, and 40% is what most local authorities now ask for on new drainage. SoakMonkey looks up the depths for the site and applies the uplift you set.
Step 3 — the storage calculation
For each storm duration D, the soakaway must hold whatever arrives faster than it can soak away:
Storage S = Inflow − Outflow = ( A × R ) − ( as50 × f × D )
where A is the impermeable area drained (roofs and paving, m²), R the design rainfall depth for that duration (m), as50 the soakaway's internal surface area at 50% effective depth (base excluded, m²), f the infiltration rate (m/s) and D the duration in seconds. The outflow term depends on the soakaway's own dimensions, so the calculation is iterative: guess a size, run every duration, and enlarge until the largest S fits.
Continuing the example. 100 m² of roof and drive drains to a soakaway 2 m × 2 m in plan, effective depth 1.5 m, so as50 = 8 × 0.75 = 6.0 m². With the infiltration rate above and illustrative 10-year depths for a southern-England site uplifted by 40%:
| Duration | Depth with 40% uplift | Inflow A × R | Outflow during the storm | Storage needed |
|---|---|---|---|---|
| 15 min | 28.0 mm | 2.80 m³ | 0.32 m³ | 2.48 m³ |
| 30 min | 36.4 mm | 3.64 m³ | 0.63 m³ | 3.01 m³ |
| 1 h | 44.8 mm | 4.48 m³ | 1.26 m³ | 3.22 m³ |
| 2 h | 56.0 mm | 5.60 m³ | 2.53 m³ | 3.07 m³ |
| 4 h | 70.0 mm | 7.00 m³ | 5.06 m³ | 1.94 m³ |
| 6 h | 78.4 mm | 7.84 m³ | 7.59 m³ | 0.25 m³ |
The 1-hour storm governs at 3.22 m³ — not the heaviest short burst, and not the longest storm either, which the ground keeps up with. The depths here are illustrative; the real ones are read for your site, and the tool sweeps far more durations than this table shows.
Step 4 — what the soakaway is built from
Storage is the water the construction can actually hold, not its excavated volume. A pit filled with clean, coarse rubble stores only the voids between the stones — 30 to 40% of its volume. Modular geocellular crates store 90 to 95%. Concrete ring and trench soakaways sit between. So the 3.22 m³ above means roughly 10.7 m³ of rubble-filled excavation or 3.4 m³ of crates, which is why the same test can produce very different holes in the ground. SoakMonkey dimensions all four construction types from the storage figure; the crate counter turns a storage volume into a number of units and a plan size, and the spreadsheet page lays the same arithmetic out row by row.
Step 5 — the half-drain check
A soakaway that fills in one storm must be ready for the next. BRE 365 requires the time to empty from full to half-full to be no more than 24 hours:
ts50 = 0.5 × S ÷ ( as50 × f ) ≤ 24 h
In the example, 0.5 × 3.22 ÷ (6.0 × 5.85 × 10⁻5) works out at about 1.3 hours — well inside the limit. On slow ground this check, not the storage, is what fails: enlarging the soakaway adds storage and surface area in proportion, so a soakaway that cannot half-empty in a day at one size generally cannot at any size, and the design has to change to attenuation with a controlled outfall.
What sits alongside the digest
Separation. Approved Document H keeps a soakaway at least 5 m from any building or road and out of unstable ground; many authorities also want it clear of the boundary and of other soakaways — on a tight plot the location is settled before the size. Groundwater. The base of the soakaway should sit above the highest expected water table; a pit that holds water on its own is telling you something. Silt. A silt trap or catchpit upstream keeps the infiltration surfaces working; it is why the base is left out of as50. Larger schemes. For areas beyond a house or small development the same infiltration test feeds a full SuDS design to the CIRIA SuDS Manual, and the lead local flood authority may ask for a 1-in-100-year check on top of the 10-year design storm.
What the report to Building Control contains
The trial-pit dimensions and the three fall times; the infiltration rate and how it was derived; the impermeable area and how it was measured; the design rainfall parameters, return period and climate-change uplift; the storm-duration table with the governing case; the construction type and its void ratio; the resulting dimensions; and the half-drain time. That is what a SoakMonkey report sets out, with a report ID that can be checked at /verify/.
When a soakaway will not work
Below roughly 1 × 10⁻⁶ m/s — heavy clay — the half-drain check fails at any sensible size; the percolation test calculator flags it as soon as the times are in. A high water table, a source protection zone, contaminated ground or simply no room 5 m clear of buildings and boundaries rule a soakaway out too. The alternatives are attenuation with a restricted discharge to a watercourse or sewer, or a permeable-paving system designed to the SuDS Manual — none of which BRE 365 sizes.
Frequently asked questions
What is BRE Digest 365?
BRE Digest 365 "Soakaway design" is the UK method for sizing an infiltration soakaway. The 2016 edition replaced the 1991 original. It sets out a site soakage test to measure how fast the ground takes water, a design-rainfall method for the site, a storage calculation that balances inflow against outflow across a range of storm durations, and a check that the soakaway half-empties within 24 hours. Approved Document H refers to it, and Building Control and lead local flood authorities expect a soakaway to be sized to it.
Is BRE 365 a legal requirement?
Not in itself — it is guidance, not regulation. Building Regulations Part H requires rainwater to be discharged to a soakaway or other infiltration system where practicable, and Approved Document H names BRE Digest 365 as the way to design one. In practice a Building Control body or a lead local flood authority will ask for a BRE 365 calculation, and a planning condition on surface water drainage will usually say so explicitly.
What is the BRE 365 soakage test?
A trial pit, 0.3 to 1 m wide and 1 to 3 m long, dug to the depth the soakaway will reach, is filled with water to at least 75% of its effective depth and the time for the level to fall from 75% to 25% is recorded. The fill is repeated three times, on the same day or consecutive days, so the surrounding ground is saturated, and the slowest result is used for design. The infiltration rate f, in metres per second, is the volume drained divided by the pit surface area at 50% depth and by the time taken.
What is a good infiltration rate for a soakaway?
Roughly 1×10⁻⁵ m/s and above is straightforward: a sensibly sized soakaway will empty within the 24-hour half-drain limit. Between 1×10⁻⁵ and 1×10⁻⁶ m/s a soakaway usually still works but grows quickly, and the half-drain check starts to govern. Below about 1×10⁻⁶ m/s — heavy clay, typically — a soakaway is rarely practical at any size and the design should look at attenuation and a controlled discharge instead.
Can I download BRE Digest 365 as a PDF?
It is a BRE publication sold through the BRE Bookshop, not a free document, so a free PDF found online is unlikely to be a legitimate copy or the current 2016 edition. This page describes the method in full in our own words, and the PDF you actually need for Building Control is the design report — SoakMonkey produces one from your test results for £25 +VAT.
What does a BRE 365 calculation need from me?
The three fall times and the trial pit dimensions from the soakage test; the impermeable area draining to the soakaway (roofs and hard paving); the site location, for the design rainfall; the return period and climate-change uplift the approving body wants — normally 1 in 10 years plus 40%; and the construction you intend to use, because a rubble-filled pit and a crate system store very different fractions of their volume.
How is BRE 365 different from a percolation test for a drainage field?
They are different tests for different things. Approved Document H’s drainage-field test uses a 300 mm square hole and reports a percolation value Vp in seconds per millimetre, used to size a septic-tank drainage field. BRE 365 uses a full-size trial pit and reports an infiltration rate in m/s, used to size a rainwater soakaway. A Vp cannot be converted into a BRE 365 result.
Does BRE 365 apply to a house extension?
If the extension’s roof drains to a new soakaway, yes — the method is the same whatever the size of the area drained, though the numbers are small. Approved Document H also keeps any soakaway at least 5 m from a building or road, which on a small plot decides where it can go before the calculation decides how big it is.
Is the calculator free?
Yes — entering the soakage test and sizing the soakaway on screen is free. You only pay £25 +VAT to export the Building Control-ready PDF report.
Back to the soakaway calculator, or start with the percolation test calculator. The BRE 365 figures are also on the building regulations reference.