How do I calculate the size of a soakaway required for my extension?

There are generally two types of soakaway, either a crate system or rubble filled.  Below is information on how to calculate the size required for each.



Where ground has good soakage (such as sand/gravel) then the size/volume of a crate type soakaway can be calculated as follows:

Volume = Roof area being drained x (50mm rainfall rate per hr/3000).

For example, a 50m2 roof area would require a crate soakaway of

50m2x(50/3000) = 0.84m3



These soakaways are normally filled with clean brick/block/concrete pieces no larger than 150mm.  It can be assumed that the voids left between the rubble equates to about 30%.

To calculate the size/volume of a rubble filled soakaway, the same calculation can be used but the result divided by 0.3.

For example, 50m2 roof area requires a rubble filled soakaway of (50m2x(50/3000))/0.3 = 2.8m3



All soakaways must be sited at least 5m from any building or road and preferably at least 2.5m from any boundary.


For larger areas to be drained or where ground soakage is not good, a soakage test and more detailed calculations should be carried out and further guidance can be found in Approved Document H and BRE Digest 365

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