Technical guidance for soakaway design

Soakaways are traditionally square or circular pits filled with hardcore and are suitable in good draining ground such as sand or chalk. In less porous grounds, such as clay, hollow soakaways are more suitable. These are formed in various methods – perforated concrete rings, honey-combed brickwork or perforated crates.

Calculation of drained area

It is important to size your soakaway appropriately to ensure efficient storage and infiltration. The following notes provide guidance on how to calculate the correct size and number for general domestic scenarios. The siting of soakaways should be 5m from buildings and boundaries.

Type of surface Effective design area
Flat roof Plan area of relevant portion
Pitched roof at 30° Plan area x 1.29
Pitched roof at 45° Plan area x 1.50
Pitched roof at 60° Plan area x 1.87
Pitched roof over 70° or any wall Elevation area x 0.5

Design soakaway


  1. Soakaway capacity to hold maximum daily rainfall (taken as 40mm)
  2. Storage depth minimum 1.0m
  3. Soakaways to be honeycombed brickwork, hollow concrete rings, or storage cells

Volume required = design area x rainfall (0.040)

Example 1

  • Flat roof 3m x 6.5m = 19.5m2
  • 19.5 x 0.040 = 0.78m3
  • Using 1.2m diameter ring soakaway per 1m depth
  • IIr2 x 1.0 = II x 0.62 = 1.13m3
  • Number of soakaways = 0.78/1.13 = 0.7
  • Therefore provide 1 soakaway 1.2m diameter minimum 1m depth below drain invert.

Example 2

  • Flat roof 3m x 6.5m = 19.5m2
  • 19.5 x 0.040 = 0.78m3
  • Using typical pre-formed storage cells 1.0x0.5x0.4=0.2m3
  • No. of cells= 0.78/0.2 =3.9= 4no cells.
  • Therefore provide 1 soakaway 1.0m below invert of pipe 1.0mx1.0m with a minimum of 4 cells. See manufactures details for lining materials and backfill.
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