Soakaway Design Calculator
Estimate attenuation crates and volume for surface water drainage.
Roof area + paved areas draining to soakaway
Storage Capacity
SuDS Design: Managing Surface Water
Attenuating the Storm
Sustainable Drainage Systems (SuDS) are now a mandatory requirement for many UK developments. Instead of sending rainwater into the overstretched sewer network, a soakaway allows the water to return naturally to the water table.
Our **soakaway calculator** provides a baseline estimate for attenuation crates based on the catchment area and standard UK storm intensities.
Design Constants
Crate Installation
Soakaway crates must be wrapped in a high-quality **geotextile membrane**. This allows water to pass through into the crates but prevents soil and silt from washing in and clogging the system.
The crates should be laid on a 100mm bed of sand or pea shingle and covered with at least 500mm of soil for non-traffic areas (or 900mm+ for driveways).
The Clay Problem
If you have heavy clay soil, a standard soakaway will act like a swimming pool, holding water rather than draining it. In these cases, you may need a 'partial' soakaway or an alternative SuDS solution like a rain garden.
Percolation Test (Vp)
A percolation test involves digging a pit, filling it with water, and timing how long it takes to drain. The resulting **Vp value** (seconds per mm) is the only accurate way to size a structural soakaway.
Pro Tip: Silt Traps
Always install a **Silt Trap** or a leaf filter before the water enters the soakaway. This catches debris from the gutters and prevents your crates from filling with mud over time, which is impossible to clean.
Soakaway & SuDS FAQ
Technical Disclaimer
This calculator is for estimation only. Structural soakaway design for planning applications must be performed in accordance with BRE 365 standards by a qualified civil engineer or drainage specialist.