Rainwater soakaway in the UK: where roof water may go and how to size it with BRE 365
Roof water goes first to a soakaway or other infiltration system, then to a watercourse, and only then to a sewer [1], never into a septic tank or cesspool [2]. Build the soakaway at least 5 m from any building or road [3] and size it for a storm that happens once in 10 years using BRE Digest 365 [4]. Example calculation: a 100 m² roof in London on moderately draining soil needs about 3 m³ of water storage.
Updated 2026-09-29 · every regulation and price links to its source at the bottom of the page. A guide, not a construction design.
Where can you drain rainwater from your roof?
In England, Approved Document H sets the order: “(a) an adequate soakaway or some other adequate infiltration system; or, where that is not reasonably practicable, (b) a watercourse; or, where that is not reasonably practicable, (c) a sewer.” [1] Collecting rainwater in a butt or tank for use sits outside this order.
Scotland starts with storage: a tank with an overflow, then SuDS, then a soakaway, then a sewer, and a watercourse only with SEPA’s agreement [5].
Can rainwater go into a septic tank?
No. “Rainwater or surface water should not be discharged to a cesspool or septic tank.” [2] Designers of treatment systems exclude it too [6], and Natural Resources Wales asks owners not to connect roof or surface water run-off [7].
Example calculation: 20 mm of rain on a 150 m² roof is 3 m³ of water, more than a whole 2,700-litre tank [8]. A storm like that washes sludge out onto the drainage field and clogs it. It works the other way too: sewage must never go into a rainwater soakaway [9].
How far from the house should a soakaway be?
Approved Document H says infiltration devices should not be built within 5 m of a building or road, on unstable land, where groundwater reaches the bottom at any time of year, too close to drainage fields or other soakaways, or where runoff could pollute groundwater [3]. It sets no distance from the boundary, so check your council’s local guidance.
In Scotland every part of a soakaway should be at least 5 m from a building and from the boundary, with a separate soakaway for each house [5]. For planning applications, Edinburgh council also wants a soil test before a soakaway is chosen and storm rainfall taken from FEH22 [10].
How do you size a soakaway? The BRE 365 method
Approved Document H refers larger soakaways to BRE Digest 365; for small ones draining 25 m² or less it lets you assume 10 mm of rain in 5 minutes [11]. The method is a water balance [12]:
- Inflow I = A × R: drained area times rainfall depth, with 100% run-off [13].
- Outflow O = a_s50 × f × D: side-wall area up to half the effective depth, times the infiltration rate f, times the storm duration. The base is ignored because it clogs [14].
- Storage S = I − O, worked out for durations from 5 minutes to 24 hours; the largest S is the design volume [12].
- The storm has a 10-year return period, and the soakaway must drain from full to half full within 24 hours [4].
- Gravel or rubble fill counts as 30% voids [15]; plastic crates up to 95.5%, by one manufacturer’s claim [16].
- BRE suggests adding 30% for climate change, but “There is no statutory requirement to take account of climate change” [17].
Worked example: a 100 m² roof in London and Glasgow
Example calculation with BRE 365 rainfall: roof 100 m², crate soakaway 2 × 2 × 1 m (4 m³, 3.8 m³ of water at 95% voids), side-wall area to half depth 4 m².
- London, f = 1 × 10⁻⁵ m/s: the critical storm lasts about 6 hours with 38.5 mm of rain, so S = 3.85 − 0.86 ≈ 3.0 m³. Crates are enough; the same pit filled with gravel would hold only 1.2 m³.
- Glasgow, same soil: the critical storm lasts about 10 hours and S ≈ 3.5 m³, because long rain is heavier in the west.
- Slow soil, f = 1 × 10⁻⁶ m/s: London needs 4.8 m³ and Glasgow 6.2 m³, and the pit would not half-empty within 24 hours. That soil is unsuitable, so move down the order to a watercourse or sewer [1].
- A garden room roof of 25 m²: 25 × 0.010 = 0.25 m³ of rain in 5 minutes [11].
Design rainfall for UK locations
BRE 365 takes the 5-year, 60-minute rainfall (M5-60) as 20 mm everywhere in the UK and lets only the ratio r (0.12–0.45, from a Defra map) vary by region; the 10-year growth factor is 1.19–1.24 in England and Wales and 1.13–1.20 in Scotland and Northern Ireland [21]. The Environment Agency groups the country into 8 rainfall zones [22], and the modern FEH22 data give values for any point through a paid web service [10].
The r values below come from drainage calculations submitted to councils: practice, not an official map. The 10-year depths (M10) are our calculation with the BRE 365 tables and M5-60 = 20 mm.
- South and east (r ≈ 0.40–0.44): short cloudbursts are the danger, so the critical storm is short.
- West and north (r ≈ 0.30): fewer cloudbursts, but a day of rain brings 66 mm against 52 mm in London, so slow soils need a bigger soakaway.
- Sheffield and Rochdale values are low-confidence. We found no evidenced values for Birmingham, Leeds, Liverpool, Newcastle, Edinburgh, Cardiff or Bristol itself; ask your drainage designer for FEH data.
| Location | r | M10, 15 min | M10, 60 min | M10, 6 h | M10, 24 h |
|---|---|---|---|---|---|
| London (Merton) [23] | 0.412 | 15.7 mm (63 mm/h) | 24.8 mm | 38.5 mm | 51.8 mm |
| Hertfordshire (Welwyn) [24] | 0.44 | 15.9 mm (64 mm/h) | 24.8 mm | 37.3 mm | 49.2 mm |
| Oxfordshire (Cherwell) [25] | 0.405 | 15.6 mm (63 mm/h) | 24.8 mm | 38.7 mm | 52.4 mm |
| Sheffield, textbook example [26] | 0.40 | 15.6 mm (62 mm/h) | 24.8 mm | 38.9 mm | 52.9 mm |
| North Somerset, near Bristol [27] | 0.35 | 15.2 mm (61 mm/h) | 24.8 mm | 40.9 mm | 58.1 mm |
| East Sussex (Hastings) [28] | 0.35 | 15.2 mm (61 mm/h) | 24.8 mm | 40.9 mm | 58.1 mm |
| Greater Manchester (Rochdale) [29] | 0.30 | 14.5 mm (58 mm/h) | 24.8 mm | 42.8 mm | 66.0 mm |
| Glasgow [30] | 0.30 | 14.1 mm (56 mm/h) | 23.8 mm | 41.8 mm | 65.6 mm |
Frequently asked questions
How far should a soakaway be from a house in the UK?
At least 5 m from any building or road in England [3], and 5 m from buildings and the boundary in Scotland [5].
Can I connect my downpipes to my septic tank?
No. Rainwater must not go into a septic tank or cesspool [2].
How big should a soakaway be for a 100 m² roof?
In our BRE 365 example for London on soil with f = 1 × 10⁻⁵ m/s, about 3 m³ of water storage: a 4 m³ crate soakaway, or a much larger gravel pit, because gravel holds only 30% of its volume [12] [15].
Are crates better than gravel for a soakaway?
Crates hold far more water: up to 95.5% voids by one manufacturer’s claim [16], against 30% for granular fill in BRE’s examples [15].
How much does a soakaway cost?
About £850 installed for a typical house (approximate, as of September 2026) [20].
Do I have to add 30% for climate change?
BRE suggests it, but it is not a statutory requirement [31].
Sources
- Approved Document H (Building Regulations 2010, England), 2015 edition — H3, Requirement H3(3); H3 para 3.2–3.4 (UK) · regulation · checked 2026-09-29 · source
- Approved Document H (Building Regulations 2010, England), 2015 edition — H3, H3 para 0.6 (UK) · prohibition · checked 2026-09-29 · source
- Approved Document H (Building Regulations 2010, England), 2015 edition — H3, H3 para 3.25 (UK) · regulation · checked 2026-09-29 · source
- BRE Digest 365 Soakaway design (2016), design storm (UK) · recommendation · checked 2026-09-29 · source
- Scottish Building Standards Technical Handbook — Domestic (April 2026), 3.6.3, 3.6.5 (UK) · regulation · checked 2026-09-29 · ? · source
- British Water Code of Practice — Flows and Loads 4 (2013) (UK) · recommendation · checked 2026-09-29 · source
- Natural Resources Wales — Running and maintaining a septic tank or small sewage plant, Maintaining a septic tank; Maintenance records (UK) · recommendation · checked 2026-09-29 · ? · source
- Approved Document H (Building Regulations 2010, England), 2015 edition — H2, H2 para 1.18 (UK) · regulation · checked 2026-09-29 · source
- Environment Agency General Binding Rules: small sewage discharges (England), Rules that apply to all discharges — If you use a non-standard system (such as a well, borehole and soakaway) (UK) · prohibition · checked 2026-09-29 · source
- City of Edinburgh Council — Flood Risk Assessment and Surface Water Management Plan guidance (v3, August 2025), 4.1.4, 4.2.1, 5.6 (UK) · regulation · checked 2026-09-29 · ? · source
- Approved Document H (Building Regulations 2010, England), 2015 edition — H3, H3 para 3.26–3.27, 3.30 (UK) · regulation · checked 2026-09-29 · source
- BRE Digest 365 Soakaway design (2016 revision), 3 Design — design procedure, equation of volumes (UK) · recommendation · checked 2026-09-29 · source
- BRE Digest 365 Soakaway design (2016) (UK) · recommendation · checked 2026-09-29 · source
- BRE Digest 365 Soakaway design (2016), 3.3.1–3.3.3 (UK) · recommendation · checked 2026-09-29 · source
- BRE Digest 365 Soakaway design (2016), 6, Example A (UK) · recommendation · checked 2026-09-29 · source
- gclproducts.co.uk (UK) · common practice (not a rule) · checked 2026-09-29 · source
- BRE Digest 365 Soakaway design (2016 revision), 2.1; 3.4 Climate change; Example A; Time of emptying (UK) · recommendation · checked 2026-09-29 · source
- Approved Document H (Building Regulations 2010, England), 2015 edition — H3, H3 para 3.28–3.29 (UK) · regulation · checked 2026-09-29 · source
- Scottish Building Standards Technical Handbook — Domestic (April 2026), 3.6.5 Soakaway single dwellings and extensions (UK) · regulation · checked 2026-09-29 · ? · source
- MyJobQuote — Soakaway Installation Cost Guide (UK Prices 2026), Average cost (UK) · price · checked 2026-09-29 · ✓ · source
- BRE Digest 365 Soakaway design (2016 revision) (UK) · recommendation · checked 2026-09-29 · source
- Environment Agency / Defra — Rainfall runoff management for developments (SC030219), Revision E, 4.2; Appendix 1, Figure A2.1; Appendix 3 (UK) · recommendation · checked 2026-09-29 · source
- London Borough of Merton — Surface Water Drainage Strategy 4003/SWDS, New Malden (May 2019), MicroDrainage Source Control (UK) · common practice (not a rule) · checked 2026-09-29 · ? · source
- Welwyn Hatfield BC planning — Calculations for drainage, 16 School Lane, Welwyn (Tedds), Design rainfall (UK) · common practice (not a rule) · checked 2026-09-29 · ? · source
- Cherwell DC planning — Upper Heyford, Catchment Area 1, Modified Rational Method (20.09.2010), User Input Data (M5_60, Ratio R from WINDES) (UK) · common practice (not a rule) · checked 2026-09-29 · ? · source
- Wikipedia — Flood Studies Report (worked example), Worked example (Sheffield, 6 h, 10 years) (UK) · common practice (not a rule) · checked 2026-09-29 · ? · source
- n-somerset.gov.uk, Rainfall Details (UK) · common practice (not a rule) · checked 2026-09-29 · ? · source
- East Sussex CC — Bexhill to Hastings Link Road, Preliminary Drainage Design, Appendix E1 (May 2006), WinDes Source Control, Calculate Storage Requirement using Windes Source Control (UK) · common practice (not a rule) · checked 2026-09-29 · ? · source
- T J Booth Associates (Rochdale) — drainage calculations, Tedds Wallingford Procedure (planning.org.uk, 14.10.2025), Design rainfall — Location of catchment area: Other (UK) · common practice (not a rule) · checked 2026-09-29 · ? · source
- docs.planning.org.uk, Design rainfall intensity — Location of catchment area: Glasgow (UK) · common practice (not a rule) · checked 2026-09-29 · ? · source
- BRE Digest 365 Soakaway design (2016), 3.4 (UK) · recommendation · checked 2026-09-29 · source
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