Soakaway Crate Calculator — Crate Count, Membrane & Excavation

Soakaway crates are modular plastic units that stack together to form an underground chamber for stormwater infiltration. They have a very high void ratio (typically 95%) compared to traditional rubble-filled soakaways (30%), meaning you need far less excavation for the same storage volume. They are wrapped in geotextile membrane to prevent silt ingress.

The required soakaway volume is determined by a BRE 365 percolation test. This involves digging a test pit, filling it with water, and measuring how quickly it drains. The soil infiltration rate is then used with the impermeable area and design rainfall to calculate the storage volume needed. Your drainage engineer will provide this figure. For landscaping projects, also see our Soakaway Sizing Calculator for garden and driveway soakaways.

A non-woven geotextile membrane (typically Terram 1000 or equivalent) is wrapped around the assembled crate stack to prevent silt and soil from entering and clogging the system. All joints should overlap by at least 300mm. Some installations use an impermeable membrane on top to prevent surface water entering directly.

The base of the soakaway should be at least 1 metre above the highest recorded water table level. The top must be at least 500mm below finished ground level. Soakaways must be at least 5 metres from any building and at least 2.5 metres from a boundary. These are Building Regulations Part H requirements.

Yes, crate systems are now the preferred method for new soakaways. They offer 95% void ratio vs 30% for rubble-fill, are quicker to install, easier to calculate accurately, and take up less space. They are accepted by Building Control and meet SuDS requirements when properly designed.