If you’re having trouble calculating your soakaway size or feeling a bit confused, this calculator will make it clear in just a few simple steps. Estimate the required size of soakaway crates or rubble pits for your extension, driveway, or roof drainage. It is compliant with BRE Digest 365 methodologies.
Calculation Results
Includes base, sides, top and 20% overlap.
How the Soakaway Calculator Works
Quick Estimate Method
This mode uses a standard rule of thumb suitable for small residential extensions or patios where a full BRE 365 test hasn't been performed. It assumes a typical rainfall event storage requirement.
The Formula: Volume = Catchment Area × 0.02 (approx 20mm storage). If you select Rubble fill, we divide the result by 0.30 to account for the stones taking up 70% of the space.
Detailed Method (BRE 365)
This calculates the balance between water coming in (Inflow) and water soaking into the ground (Outflow) during a storm. This is the method required for Building Regulations.
- Inflow: Rain hitting your roof/driveway.
- Outflow: Water soaking out through the sides of the pit (the base is usually ignored in design to be conservative/prevent silt clogging).
- Net Storage: The difference between Inflow and Outflow is the volume you need to store in crates or gravel.
Inputs Explained
Catchment Area
The footprint of the roof or paved area draining into the soakaway. For a house extension, calculate Length × Width.
Runoff Coefficient
Not all rain runs off. Pitched roofs run off almost 100% (1.0). Permeable paving or gravel driveways might only run off 50% (0.5), reducing the soakaway size needed.
Infiltration Rate
This is the speed water soaks into your soil, measured in metres per second. You find this by digging a trial pit and performing a percolation test.
Sand: Fast (10⁻⁴) | Loam: Medium (10⁻⁵) | Clay: Slow/Blocked (10⁻⁶ or lower).
Installation & Siting Checklist (UK)
- Distance: Must be at least 5m away from building foundations and 2.5m from boundaries.
- Depth: Pipes usually enter 500-600mm below ground. The soakaway sits below this.
- Silt Trap: Always install a pre-filter or catchpit gully before the soakaway to prevent silt entering the crates.
- Water Table: The base of the soakaway should be at least 1m above the groundwater level.
Frequently Asked Questions
Do I need a geotextile membrane?
Yes. You must wrap the entire outer face of the crate structure (or line the trench for rubble) with non-woven geotextile. This lets water out but stops soil washing in and clogging the void.
Can I use a soakaway in clay soil?
Often, no. Heavy clay has very low infiltration rates. If the water doesn't drain away within 24 hours, the soakaway will fail during the next storm. In clay, you may need an attenuation tank that releases water slowly to a sewer or watercourse (with permission).
What size crates should I use?
The most common domestic crate is approx 1m × 0.5m × 0.4m (0.2m³), or 1.2m long versions. Our calculator estimates the count based on the standard 0.2m³ volume per crate.
Why is the base area excluded?
Standard design practice (BRE 365) assumes the bottom of the soakaway will eventually silt up, rendering it impermeable. We only count the side walls for infiltration to be safe.
Disclaimer
This tool is for estimation and guidance purposes only. Soil conditions vary significantly. Drainage design should comply with Building Regulations Approved Document H and local planning requirements. Always verify results with a qualified drainage engineer or groundworker before ordering materials.