A soakaway is an underground structure - a pit filled with gravel, broken stone, or proprietary plastic attenuation crates - that collects surface water runoff from roofs, driveways, and paved areas and allows it to drain slowly into the surrounding soil. Soakaways avoid the need to connect to a public surface water sewer and reduce peak runoff to watercourses, lowering flood risk downstream.

How far from a building does a soakaway need to be?

Approved Document H requires soakaways to be located at least 5 metres from any building or road, and at least 2.5 metres from any boundary. This separation prevents saturated ground from undermining foundations and avoids nuisance to neighbours. The soakaway must also be in ground that will accept the design flow - a percolation test (BRE Digest 365) must confirm the soil has adequate infiltration capacity before a soakaway is designed.

How is a soakaway sized?

Soakaway size is calculated using BRE Digest 365 (Soakaway Design). The method uses the results of on-site percolation tests to determine how quickly water infiltrates the soil, combined with the design rainfall intensity for the site (from the FEH rainfall data) for a 1-in-10 year storm of the appropriate duration. The volume of the soakaway void must be sufficient to store the design runoff volume while the soil absorbs it. Proprietary crate systems have a much higher void ratio than rubble-filled pits and require significantly less excavation.

Can I connect foul drainage to a soakaway?

No - soakaways may only receive surface water (rainwater from roofs, driveways, and paved areas). Foul water (from WCs, sinks, baths, and washing machines) must go to a foul sewer, septic tank, or sewage treatment plant. Connecting foul drainage to a soakaway is illegal, causes ground contamination, and will be rejected by Building Control. Surface water and foul drainage must always be kept in separate systems.

Drainage & Utilities noun

Soakaway

/ ˈsəʊkəweɪ /

Also known as: soakaway pit, infiltration device, soakaway crate system

Definition

A soakaway is an underground structure - a pit filled with clean angular gravel, broken stone, or proprietary plastic attenuation crates - that collects surface water runoff from roofs and paved areas and allows it to infiltrate gradually into the surrounding soil. It is the preferred method of surface water disposal for new dwellings under Building Regulations where soil conditions allow, avoiding connection to a public sewer and reducing downstream flood risk.

In practice

Before designing a soakaway, on-site percolation tests must be carried out to confirm the soil will accept water at an adequate rate - clay soils and waterlogged ground are generally unsuitable. The soakaway must be located at least 5m from buildings and roads and 2.5m from boundaries. Traditional rubble-filled soakaways require large excavations; modern plastic geocellular crate systems (such as Polystorm or Hydro-Blok) provide a much higher void ratio in a smaller volume and are now the standard approach.

The inlet pipe from the roof drainage system discharges into the soakaway, typically via a rodding eye or inspection chamber upstream to allow jetting and clearing of blockages. The soakaway is wrapped in geotextile membrane to prevent fine soil particles from migrating into the void and clogging it over time. Inspection access should be provided to allow the condition and performance of the soakaway to be monitored.

Building Regulations

Relevant PartPart H - Drainage and waste disposal

Minimum distance from buildings5 m

Minimum distance from boundary2.5 m

Design standardBRE Digest 365 - Soakaway Design

Approved Document H sets out the requirements for surface water drainage. Soakaways must be designed to the BRE Digest 365 method using site percolation test results and local rainfall data. Building Control will require confirmation that percolation tests have been carried out and that the soakaway has been sized to accept the 1-in-10 year design storm. Connection to a public surface water sewer or watercourse requires consent from the relevant authority.

Full Building Regulations guidance

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