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What Is a Soakaway? — Drainage Explained
A plain-English guide to soakaways: BRE 365 sizing, construction methods, when they are required, and UK regulations.
Definition
A soakaway is an underground drainage structure that collects surface water or rainwater and allows it to percolate slowly into the surrounding soil. It is the preferred method of surface water disposal under the UK Building Regulations drainage hierarchy (Approved Document H), which requires surface water to be dealt with on-site where practicable before considering connection to a sewer.
When Is It Used?
Soakaways are used for rainwater drainage from roofs, driveways, and patios. They are required when building a new property or extension where mains surface water drainage is not available. Under the 2010 Building Regulations hierarchy, you must first consider infiltration (soakaways), then watercourse discharge, and only connect to a public sewer as a last resort. Soakaways are also commonly installed for permeable paving systems and garden drainage.
Key Facts
- Sizing must be calculated using the BRE 365 percolation test method
- Minimum 5 metres from any building or road
- Modern crate soakaways (e.g., Polypipe Polystorm) provide 95% void ratio vs 30% for rubble
- Must be wrapped in geotextile membrane to prevent silt ingress
- Minimum depth of 1 metre below the invert of the incoming pipe
- Not suitable for foul water (sewage) - only surface water or rainwater
- Clay soils typically fail the percolation test, requiring alternative drainage
- Typical domestic soakaway volume is 1-3 m³ depending on roof area and soil type
Related Calculators
Use the Soakaway Sizing Calculator to size your soakaway, or the Rainwater Harvesting Calculator for alternative water management. The Drainage Fall Calculator helps with pipe gradients to the soakaway.
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How We Calculate This
Frequently Asked Questions
The BRE 365 percolation test involves digging a test pit (typically 300 x 300 mm), filling it with water three times to saturate the soil, then timing how long it takes for the water level to drop from 75% to 25% full. The soil infiltration rate (f) in m/s is calculated from this data, which is then used with the rainfall data for your area to determine the required soakaway volume.
Building Regulations Approved Document H requires a soakaway to be at least 5 metres from any building or road, and it should not be located within the zone of influence of any foundation. This distance prevents water from undermining foundations or causing subsidence. On clay soils, greater distances may be required.
Clay soils have very poor percolation rates, and soakaways are generally unsuitable for heavy clay. The BRE 365 test will confirm whether the infiltration rate is adequate. If the percolation test fails (typically if the soil infiltration rate is below 1 x 10⁻⁶ m/s), you will need an alternative drainage solution such as connecting to a surface water sewer or using an attenuation tank with controlled discharge.
If you are connecting new rainwater drainage to a soakaway (e.g., from a new extension or patio), you will need Building Regulations approval under Part H. You must demonstrate that the soakaway has been sized correctly using the BRE 365 method and that it is positioned at least 5 metres from any building.
A well-constructed soakaway using modern crate systems can last 50 years or more. Traditional rubble-filled soakaways tend to silt up over time and may need replacing after 15-25 years. Regular maintenance, including keeping gutters and gullies clear of debris, helps extend the lifespan significantly.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.