Soakaway Size Calculator (UK Drainage)

Soakaway Design Guide (UK BRE 365)

What is a Soakaway?

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 Drainage Systems (SuDS) guidance, which favours managing rainwater on-site rather than sending it straight to the combined sewer.

What's the key thing to understand about 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: drains very quickly. Clay: drains very slowly and may not be suitable for a soakaway at all without significant oversizing.

What do I need to know about 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: plastic crate systems typically offer around 95% void space for water storage, compared to roughly 30% for a traditional rubble-filled soakaway of the same size.

What's the key thing to understand about 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 completely around the soakaway structure to prevent surrounding soil particles migrating in and clogging the void space over time.