Soakaway calculator: BRE 365 size and crates
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Areas draining to the soakaway | Roof: area in plan 50 m² |
| Storms: duration and rainfall depth | Storm 1: duration 15 min, rainfall depth 15 mm |
| Infiltration rate f | 0 m/s |
| Fill | Crates |
| Void ratio of the fill | 0.95 fraction |
| Pit shape | Rectangular |
| Pit length | 2 m |
| Pit width | 1 m |
| Storage depth you plan | 1 m |
| Half-emptying time to aim for | 24 h |
| Count the base in the outflow area | No |
Storage depth 1.00 m (0.74 m³ of storage for the critical storm)
- Within the guidance Storage against the critical storm: 1.9 m³ UK-wide: BRE Digest 365 (2016): S ≥ I − O for every storm duration. Check with the source.
- Within the guidance Time to half-empty: 8.796 h England: BRE Digest 365 (2016); Approved Document H, H3. Check with the source
| Result | Value |
|---|---|
| Storage depth | 1 m |
| Storage needed (critical storm) | 0.739 m³ |
| Critical storm | 15 min, 15 mm of rain |
| Storage at this depth | 1.9 m³ |
| Time to half-empty | 8.8 h |
| Pit volume (gross) | 2 m³ |
| Excavation | 2 m³ |
| Geotextile to wrap the pit (before overlaps) | 10 m² |
| Infiltration rate | 36 mm/h |
| Duration | Rain | Inflow I | Outflow O | I − O | Depth needed |
|---|---|---|---|---|---|
| 15 min | 15 mm | 0.75 m³ | 0.027 m³ | 0.723 m³ | 0.39 m |
Show the working
- Pit: 2 × 1 = 2 m² in plan; perimeter 2 × (2 + 1) = 6 m
- Catchment: 50 = 50 m²
- Infiltration rate: f = 1.00e-5 m/s (36 mm/h)
- Critical storm 15 min: I = 50 m² × 15 mm = 0.75 m³; depth needed = 0.75 ÷ (2 × 0.95 + 6 × f × 900 s ÷ 2) = 0.389 m
- Storage needed: 2 m² × 0.389 m × 0.95 = 0.739 m³
- Your depth 1 m: storage 2 × 1 × 0.95 = 1.9 m³ against the largest I − O of 0.723 m³
- Half-empty: 0.5 × 1.9 m³ ÷ (3 m² × f) = 31,667 s = 8.8 h
- Excavation: 2 m² × (1 + 0) m = 2 m³; geotextile 2 × 2 + 6 × 1 = 10 m²
How it works
- Inflow I (m³) = catchment area (m²) × rainfall depth (m) × (1 + climate allowance), for each storm duration
- Outflow O (m³) = a_s50 × f × D, where a_s50 = pit perimeter × half the storage depth (the sides up to 50% depth), f in m/s and D in seconds
- Storage S (m³) = plan area × storage depth × void ratio; BRE 365 needs S ≥ I − O for every duration
- So the depth each storm needs is d = I ÷ (plan area × void ratio + perimeter × f × D ÷ 2); the deepest is the critical storm
- Time to half-empty t_s50 (s) = 0.5 × S ÷ (a_s50 × f)
BRE Digest 365 is the method; your building control body may ask for the full calculation and a percolation test.
Keep a soakaway at least 5 m from a building, and check Approved Document H and building control for your site.
We leave the base out of the outflow area because it tends to clog; switch it on under More settings if your designer counts it.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Void ratio of the fill: Crates | 0.95 fraction | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 G8: crates void ratio about 0.95 from the maker. Use the crate maker's figure. |
| Void ratio of the fill: Rubble | 0.3 fraction | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 G8: rubble fill void ratio about 0.30. Researcher to source (BRE Digest 365). |
| Half-emptying time to aim for | 24 h | Regulation or HSE figure BRE Digest 365 (2016); Approved Document H, H3 (half-emptying time) Not yet checked against the source | Source to be added by the Researcher brief v2 G8: time to half-empty within 24 h, shown as Advisory. Researcher confirms the criterion and the nations it covers. |
Regulations and standards
- BRE Digest 365 (2016): soakaway design
- Approved Document H, H3 (rainwater drainage)
Explained: BRE Digest 365 (soakaway design), Approved Document H (drainage).
Approved Document H covers England. Wales, Scotland and Northern Ireland have their own drainage guidance; BRE Digest 365 is used across the UK.