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Runs in your browser. Pick the 3.6.5 formula or BRE Digest 365 in step 2.
The two routes (clause 3.6.3)
Standard 3.6 covers surface water drainage. Clause 3.6.3 accepts a soakaway constructed in accordance with clause 3.6.5, with BRE Digest 365, or with National Annex NA 4 of BS EN 752:2008. For a single house or an extension, the 3.6.5 formula is the quick route. BRE 365 is the fuller one: it tests storms from 5 minutes to 24 hours with Scotland & Northern Ireland rainfall growth, and it takes a climate-change allowance.
The clause 3.6.5 formula
S = (A × 0.0145) − (a × f × 900)
- A — the area drained, in m².
- a — the internal surface area of the soakaway to 50% effective depth, excluding the base, in m². It depends on the size you are solving for, so it has to be assumed first and checked.
- f — the soil infiltration rate in m/s, from the clause 3.9.1 percolation test: f = (1 ÷ Vp) ÷ 1000.
- S — the storage required, in m³.
Read it as a single 15-minute balance: 14.5 mm of rain on the drained area, less what soaks out of the sides in 900 seconds. SoakMonkey does the circular part for you — it solves the width, length or pit size so that the storage provided equals S.
Worked example
A 60 m² roof on ground with a percolation value Vp of 25 s/mm, so f = (1 ÷ 25) ÷ 1000 = 4.0 × 10⁻⁵ m/s, draining to a stone-filled pit 2.4 m long and 1.5 m effective depth with 30% voids:
| Rain in: A × 0.0145 | 60 × 0.0145 = 0.87 m³ |
| Pit width that balances | 0.65 m |
| Side area a to 50% depth | (2.4 + 0.65) × 1.5 = 4.58 m² |
| Soaks away: a × f × 900 | 4.58 × 4.0 × 10⁻⁵ × 900 = 0.165 m³ |
| Storage S | 0.87 − 0.165 = 0.71 m³ (2.4 × 0.65 × 1.5 × 0.30) |
| Half-empty time | about 32 minutes |
For comparison, the BRE Digest 365 route on the same pit and soil (Scotland & Northern Ireland growth, rainfall ratio r = 0.30, M5-60 = 20 mm) needs 0.77 m³ with no climate-change allowance and 1.23 m³ with +40%. On this site the 3.6.5 formula gives the smallest soakaway of the three; it is only offered for single dwellings and extensions.
The percolation test (clause 3.9.1)
Clause 3.6.5 asks for the ground to be tested with the percolation method in clause 3.9.1 — the test also used for drainage fields. Holes 300 mm square and 300 mm deep are soaked overnight, refilled, and timed as the water falls from 75% to 25% full. The time in seconds divided by 150 mm is Vp, in seconds per millimetre; take the average of at least three runs. This is a different test from BRE 365's trial pit, which is described on our percolation test page — SoakMonkey takes either.
Where it can go
Every part of a soakaway should be at least 5 m from a building and from a boundary (clause 3.6.3), though run-off, strata and permeability may reduce or increase that. For single dwellings and extensions, clause 3.6.5 lets the distance come down slightly on a small site where the ground drains very well. Each dwelling should have its own soakaway.
How SoakMonkey does it
- Pick Scotland — building warrant in step 1 (the button above does it for you). Rainfall growth switches to Scotland & Northern Ireland.
- In step 2 choose the route: the clause 3.6.5 formula (enter Vp) or the BRE Digest 365 analysis (enter the rainfall ratio and a climate-change uplift).
- Pick the soakaway type and the dimensions you want to fix; the tool solves the rest and reports the storage and half-empty time.
- The PDF cites the clause you used, the formula or storm analysis, and the siting rules.
Frequently asked questions
What is the 3.6.5 soakaway formula?
Clause 3.6.5 of the Scottish domestic Technical Handbook sizes the soakaway for a single dwelling or an extension with S = (A × 0.0145) − (a × f × 900). A is the area drained in m², a is the internal surface area of the soakaway to 50% effective depth excluding the base in m², f is the soil infiltration rate in m/s, and S is the storage required in m³. In effect it stores 14.5 mm of rain, less what soaks away in 15 minutes.
How do I get f from a percolation test in Scotland?
From the clause 3.9.1 percolation test: holes 300 mm square and 300 mm deep, soaked overnight, refilled, and timed as the water falls from 75% to 25% full. The time in seconds divided by 150 mm gives Vp in s/mm; average at least three runs. Clause 3.6.5 converts it with f = (1 ÷ Vp) ÷ 1000, so a Vp of 25 s/mm gives f = 4.0 × 10⁻⁵ m/s.
Can I use BRE Digest 365 in Scotland?
Yes. Clause 3.6.3 accepts a soakaway designed to clause 3.6.5, to BRE Digest 365, or to National Annex NA 4 of BS EN 752:2008. BRE 365 is the fuller method — it tests a range of storm durations and takes a climate-change allowance — and it is the one to use for anything beyond a single house or extension.
How far must a soakaway be from a house in Scotland?
Clause 3.6.3 says every part of a soakaway should be at least 5 m from a building and from a boundary, though run-off, strata and permeability may reduce or increase that. For single dwellings and extensions, clause 3.6.5 allows the distance to be reduced slightly on small sites where the ground drains very well. Each dwelling should have its own soakaway.
Does the 3.6.5 formula allow for climate change?
Not explicitly — it fixes the rainfall at 14.5 mm. The introduction to Standard 3.6 says climate change is expected to bring more rain and has to be taken into account. If you want a stated allowance, use the BRE Digest 365 route, which takes a climate-change uplift.
Sources
- Scottish Government, Technical Handbook (domestic), Standard 3.6 surface water drainage — clauses 3.6.0, 3.6.3 and 3.6.5.
- Scottish Government, Technical Handbook (domestic), Standard 3.9 — clause 3.9.1, the percolation test.
- BRE Digest 365, Soakaway design.
Related: the BRE Digest 365 method, what size soakaway do I need?, and soakaways for SAB approval in Wales.