What Is a Soakaway? — Drainage Explained

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 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.