Soakaway vs French Drain — UK Drainage Guide

A soakaway is a pit (filled with crates or rubble) that collects water and allows it to soak into the ground. A French drain is a trench with a perforated pipe in gravel that collects and diverts water along its length, discharging to a soakaway, watercourse or drain. A soakaway is the endpoint; a French drain is the transport system.

A soakaway for surface water (gutters, driveways) does not usually need Building Regulations approval unless it is connected to a new drainage system as part of an extension. A soakaway must be at least 5m from a building and not in clay soil. Building Control may require a percolation test (BRE 365) to confirm the ground can absorb water.

Poorly or not at all. Clay has a very low percolation rate — water takes days to drain away. A BRE 365 percolation test will confirm whether a soakaway is viable. If the soil is heavy clay, you will need an alternative: a French drain discharging to a watercourse, a surface water sewer connection (with water company permission), or a rainwater harvesting system.

Soakaway size depends on the roof/surface area draining to it and the soil percolation rate. A typical domestic soakaway for a 3-bed house (80-100 m² roof) is about 1m x 1m x 1m using soakaway crates (storage volume ~0.8 m³). Our soakaway sizing calculator can give you an exact size based on BRE 365 test results.

A well-installed French drain with geotextile membrane wrapping the gravel lasts 15-25 years. Without membrane, the gravel clogs with silt much faster (5-10 years). The perforated pipe itself lasts 30+ years. Flushing the pipe every few years extends the life. If the gravel becomes clogged, the trench needs re-digging.