Soakaway Calculator

Soakaway size is calculated using BRE Digest 365 methodology. The required storage void volume equals the catchment area multiplied by the design rainfall depth, with a 10% safety factor applied. For a 50 m² roof draining to a 1-in-10 year storm (20 mm depth): 50 × 0.020 × 1.1 = 1.1 m³ of void storage. The excavation volume depends on the soakaway type — plastic crates achieve around 95% void ratio, rubble fill around 33%, and concrete rings 100% of the internal ring volume.

Under UK Building Regulations (Approved Document H) a soakaway must be at least 5 metres from any building and at least 2.5 metres from any boundary. This prevents moisture from saturating the ground near foundations. In practice, many builders aim for 5–10 metres where space allows, and always position the soakaway downhill or away from the building's drainage direction.

Carry out a BRE 365 percolation test. Dig a 300 mm cube test pit at the soakaway depth, fill it with water and allow to drain twice to saturate the soil. Then fill again and time how long it takes for the water level to drop from 75% full to 25% full. Divide this time in seconds by 150 mm to get Vp in seconds per millimetre. Vp under 100 is generally suitable. Clay soils typically exceed Vp 200 and are not suitable — soakaway drainage will not work.

Plastic modular crates achieve around 95% void ratio, meaning almost all the excavated volume stores water. A rubble-fill soakaway achieves only around 33% void ratio, so you need to excavate roughly three times the volume for the same storage capacity. Crates cost more per unit but typically save significantly on excavation, haulage and disposal — making them more economical in most residential projects. Rubble fill is sometimes preferred on sites with limited access for specialist materials.

Soakaways for surface water drainage from a single dwelling do not normally require planning permission as they fall under permitted development. However, you should notify building control — particularly if connected to new drainage works from an extension or new build — and comply with Building Regulations Approved Document H. If the soakaway serves more than one property, is in a flood zone, or is in a conservation area or listed building setting, check with your local planning authority first.

No. Soakaways are for surface water (rainwater from roofs, drives and patios) only. Foul drainage from toilets, sinks and bathrooms must connect to a public sewer or an approved treatment system such as a septic tank or package treatment plant. Discharging foul water into a soakaway is illegal under the Water Resources Act 1991 and can cause serious ground contamination and pollution of watercourses.

BRE Digest 365 requires Vp to be less than 100 seconds per millimetre (s/mm) for a soakaway to be feasible. Vp under 12 s/mm (gravel or chalk) indicates very rapid drainage. Vp of 12–60 s/mm (sand, sandy loam) is well-suited. Vp of 60–100 s/mm (sandy clay) is marginal and the soakaway may drain slowly — check the estimated drainage time in the results and ensure it empties within 24 hours. Vp above 100 s/mm (clay) means a soakaway will not work and an alternative drainage strategy is needed.

DEFRA guidance and the updated SuDS manual recommend applying a climate change uplift to drainage volumes. For most domestic projects in England, 20% is the standard allowance. This increases to 30% for sensitive locations or sites in high-risk flood zones, and 40% where the drainage infrastructure has a design life beyond 2080. Some local planning authorities now require the climate change uplift to be included in drainage calculations submitted with planning applications. Building control for straightforward permitted development soakaways may not enforce it, but applying 20% is good practice and adds minimal cost to the design.

A trench soakaway distributes water along its length, giving a larger wetted surface area than a compact pit — which can be an advantage in soils with moderate percolation rates. They work best in long narrow gardens where a square pit isn't practical, on sloping sites where the trench can follow a contour, when excavation access is limited to a mini-digger that can only travel in one direction, or where the water table is relatively high and a shallow-but-long trench is safer than a deep pit. The trade-off is that trenches require a longer run of perforated pipe and geotextile membrane, and trenches over about 30 metres should include inspection access points for future maintenance.