What is a percolation test?  Why do you need a percolation test?

A percolation (perc) test determines the rate at which water can penetrate the ground.  It will assess the soil’s suitability and necessary size for a drainage field or any type of soakaway. Before constructing a drainage field or soakaway, you will need to perform a percolation test.  This will ensure that the soil can absorb wastewater or treated water without saturating the ground.

We at Mantair carry out percolation tests to guarantee the correct functioning of your drainage field before any construction takes place. During the test, we dig holes at the proposed site of the drainage field and fill them with water.  We then time how quickly the water is absorbed by the soil (absorption rate).  The test usually takes two days to complete.

What to consider

When calculating the size of a soakaway/drainage field, there are several factors you need to consider.

  • Firstly, the results of the percolation test.
  • Secondly, the number of bedrooms in the property.
  • Additionally, the type of effluent being discharged, such as septic tank or treated effluent.  It is worth noting that a soakaway / drainage field receiving the treated effluent from a sewage treatment plant requires 20% less plant area than that of a septic tank.


Furthermore, using the correct materials during construction is crucial for the operation and lifespan of a soakaway/drainage field.  It is recommended to use non-flexible perforated underground drainage pipework with slots or holes, along with a clean stone surround.  However, coiled land drainage pipework is unsuitable.

Distance requirements

Finally, there are minimum distance requirements for a soakaway/drainage field that must be observed:

  • 10 metres from a ditch or watercourse.
  • 50 metres from a water abstraction point.
  • 15 metres from any building or other soakaway network.
  • 2 metres from any boundaries.

Further information

Read the Environment Agency’s guide on how to assess the risks to groundwater for treated effluent discharges here.

If you require any further information, or assistance regarding a drainage field, or soakaway, please contact us.