What This Requirement Covers
Soakaways are the preferred method of rainwater disposal in the UK where ground conditions permit infiltration. The design must demonstrate that the soakaway can store and infiltrate the rainfall from the contributing roof and paved areas without overflowing. BRE Digest 365 (Soakaway Design) is the standard design method.
Key Requirements
Percolation Test Method (BRE 365)
- Excavate a test pit to the proposed soakaway depth (minimum 1 metre below finished ground level)
- Fill the pit with water to a depth of at least 300 mm
- Allow the water to drain away completely (this saturates the surrounding soil)
- Refill the pit to 300 mm depth
- Record the time for the water to drain from 75% full to 25% full (i.e., from 225 mm to 75 mm depth)
- Repeat the test three times and use the average result
- Calculate the soil infiltration rate
- in m/s from the results
Soakaway Sizing
The soakaway must be sized to store the design rainfall event
- Design storm: 1-in-100-year return period rainfall, plus a 40% climate change allowance (or as specified by the local planning authority)
- The storage volume is the difference between the rainfall inflow and the infiltration outflow during the design storm
- Common soakaway types: Crate systems (modular plastic crates), rubble-filled pits, concrete ring systems
- Crate systems have a high void ratio (typically 95%) and are the most space-efficient
Location Requirements
- Minimum 5 metres from any building
- Minimum 2.5 metres from any boundary
- Not in contaminated ground or within the zone of influence of a foundation
- The base of the soakaway must be above the winter groundwater table
Practical Compliance Tips
- Carry out the percolation test before committing to a soakaway design; if the soil does not infiltrate, an alternative drainage method is needed
- Sandy and gravelly soils have high infiltration rates; clay soils have low rates and may not be suitable
- Use the full BRE 365 method (not just a single test fill) for an accurate infiltration rate
- Size the soakaway for the 1-in-100-year storm plus climate change; undersized soakaways flood during heavy rain
- Provide an overflow route in case the design storm is exceeded
- Keep the percolation test results and soakaway design calculations for Building Control and the building file
- Crate soakaways are lighter and easier to install than rubble-filled pits; they also have better long-term performance