Percolation Testing

Can You Do A Percolation Test For Me?

Yes, we are happy to provide a percolation test service. We will carry out the tests and produce a signed report detailing the suitability of your proposed drainage site and the size of drain field you will require.

What Is A Percolation Test For?

Percolation tests are conducted for two main reasons:

1. To determine the suitability of a proposed drainage site

2. To calculate the size of drain field required in a given location

Why Do I Need A Drainage Field?

If you cannot connect into a foul sewer then the next best alternative is to allow treated effluent to soak away into a drain field.

If you choose to spread the discharge into land via a drainage field, you should check there is enough suitable land to do this. You will need a good depth of well-drained, well-aerated soil. Avoid steeply sloping sites and sites prone to waterlogging. It should be located away from watercourses, wells, boreholes and from any dwelling. You should always check any restrictions with us and your local planning authority. The exact area of land required for your drainage field will be determined by a percolation test. A competent person should do this. A discharge from a package sewage treatment plant requires a smaller drainage field for effective treatment than a discharge from a septic tank. Drainage mounds can provide an alternative to drainage fields in certain circumstances.

Construction of a Drainage Field

Drainage fields should be constructed as per the diagram below:

Drainfield illustration

Guidelines For The Percolation Test

The EA recommends the following methodology is used:

Environment Agency Guidelines

Appendix A: Percolation Test

Avoid carrying out this test in extreme weather conditions such as drought, frost and heavy rain.

  1. Excavate at least two (three in Northern Ireland) holes 300mm square to a depth 300mm below the proposed invert level (bottom of pipe) of the infiltration pipe and space them evenly along the proposed line of the sub-surface irrigation system
  2. Fill each hole with water to a depth of at least 300mm and allow to seep away overnight
  3. Next day, refill each hole with water to a depth of at least 300mm and observe the time in seconds for the water to seep away from 75% full to 25% full (i.e. a depth of 150mm). d) Divide this time by 150. This answer gives the average time in seconds (Vp) required for the water to drop 1mm

NOTE: The test should be carried out at least three times with at least two trial holes. The average figure from the tests should be taken. This is the percolation value Vp (in seconds).


The average figure for the percolation value (Vp) is obtained by summing all the values and dividing by the number of values used

  • Drainage field disposals should only be used when percolation tests indicate average values of Vp between 15 and 100 and the preliminary assessment of the trial hole tests has been favourable
  • The minimum value of 15 ensures that untreated effluent cannot percolate too rapidly into ground water
  • Where Vp is above the limit of 100 effective treatment is unlikely to take place in a drainage field as there will be inefficient soakage in this location which may lead to sewage ponding on the surface
  • domestic premises, the floor area of the drainage field (A in square metres) required may be calculated from:

A = p x Vp x 0.25 for septic tanks

A = p x Vp x 0.20 for package sewage treatment plants

Where; p is the number of people served by the tank (this should be the maximum number of people that could live in the house). Vp is the percolation value described above.

If in doubt, consult your professional advisor or local authority building control officer for advice.

Environment Agency
Pollution Prevention Guidelines 4, Appendix A PPG4