Soakaway Size Calculator (UK Drainage)

Soakaway Design Guide (UK BRE 365)

What a Soakaway Is

A soakaway is an underground structure that collects surface water (from roofs, driveways, patios) and allows it to soak gradually into the surrounding soil. It manages rainwater on-site rather than discharging to the sewer or watercourse. When you need one: new buildings/extensions where surface water can't connect to a public sewer. Driveways and paving (especially since SuDS requirements). Where the local water authority requires on-site disposal. Increasingly required under Sustainable Drain

Percolation Testing

The soil's ability to absorb water (percolation rate) is critical. The BRE 365 percolation test: dig a test pit (typically 300mm square, 300mm deep, in the proposed soakaway location). Fill with water, let it drain completely (pre-soak). Refill to 300mm depth. Measure time for water to fall from 75% to 25% full (i.e. the middle 50%). Repeat 3 times, take the average. Calculate Vp (seconds per mm): Vp = time (seconds) / (effective depth drop in mm). Soil type guide (rough Vp values): gravel/sand:

Sizing the Soakaway

BRE 365 method (simplified): the soakaway must store the inflow from a design storm minus what soaks away during the storm. Storage volume needed depends on: drained area (A) — bigger roof/paving = more water. Design rainfall — UK uses specific return-period storms (e.g. 1-in-10-year). Soil infiltration rate (from percolation test). The required storage volume is calculated for several storm durations, and the worst case (largest required volume) governs the design. Construction void ratios: pla

Construction and Rules

Key requirements (BRE 365 / Part H): minimum distance from buildings: 5 metres (to avoid affecting foundations). Distance from boundaries: typically 2.5m+. Below the water table: soakaways don't work if the ground is already saturated — check seasonal water table. Depth: typically 1-3m, within permeable soil, above the water table. Inlet: surface water pipe from gutters/drains. Silt trap/catchpit: recommended before the soakaway to prevent silt clogging it over time. Geotextile membrane: wraps c