Rainwater soakaway design guidance

Soakaways are a traditional way of disposing of surface water from buildings remote from a suitable public sewer or watercourse. A soakaway must have capacity to store immediate run-off from roofs and hard surfaces and the water must then be able to disperse into the surrounding soil quickly enough for the soakaway to be able to cope with the next storm.

Soakaways are probably the most common form of surface water disposal and are usually suitable for areas less than 100m². Soakaways are generally formed from square or circular pits, filled with rubble or lined with dry jointed masonry or perforated concrete ring units. Soakaways serving larger areas are generally lined pits, trench type soakaways or constructed from specialist proprietary units. It should be expected that a domestic rubble filled soakaway may need to be renewed about every ten years.

For small soakaways serving 25m² or less, a design rainfall of 10mm in 5 minutes is quoted in the Building Regulation Approved Document H as being an appropriate worst case.

For soakaways serving areas greater than 25m² reference should be made to BS EN 752-4, or BRE Digest 365 Soakaway design. BRE Digest 365 is the most commonly used document.

The Building Regulations dictate an order of priority for the selection of surface water from buildings and these are:

  • an adequate soakaway or some other adequate filtration system, or where that is not reasonably practical,
  • a watercourse, or where that is not reasonably practical,
  • an appropriate sewer

A soakaway must always be the first choice but must not be used:

Within 5m of a building or road, 2.5m of a boundary or in an area of unstable land in ground where the water table reaches the bottom of the soakaway at any time of the year. near any drainage field, drainage mound or other soakaway so that the overall soakage capacity of the ground is exceeded and the effectiveness of any drainage field impaired; where the presence of any contamination in the runoff could result in pollution of groundwater source or resource.