It is a fair question. A percolation test is, at heart, a hole in the ground filled with water and timed as it drains. So why pay someone to do it? The short answer is that you can dig a hole and watch it drain, but that is rarely enough for building control or a planning condition. Here is what a proper test involves, why a home test usually is not accepted, and what you can usefully check yourself.
What a proper percolation test involves
There are two tests, and which one you need depends on what is going into the ground.
For a surface water soakaway, the method is BRE Digest 365. A trial pit is dug where the soakaway is proposed, typically 0.3 to 1 m wide and 1 to 3 m long, with vertical sides and taken down to the depth of the proposed soakaway. The pit is filled with water and allowed to drain three times, on the same day or consecutive days. Each time, the fall from 75% to 25% of the effective depth is timed, and the soil infiltration rate is calculated from the volume of water and the surface area it drains through. The lowest of the three results is the one used for design.
For a septic tank or treatment plant drainage field, the method is set out in Approved Document H and BS 6297. A hole 300 mm square is dug 300 mm below the level of the proposed drainage pipes, filled and left to soak overnight, then refilled and timed as it falls from 75% to 25% full. That gives a percolation value, Vp. The test is carried out at least three times in at least two holes and the results averaged. Our foul infiltration testing page explains that test in more detail, and our BRE 365 soakaway testing page covers the soakaway test.
Why a DIY test usually is not accepted
Building control officers and planning authorities are not just asking whether water drains. They want evidence that the ground was tested properly, in the right place and at the right depth. In practice that means a report showing:
- the method followed, BRE 365 or BS 6297
- the size and depth of each pit or hole, and where it was dug
- every reading, for every fill
- the calculated soil infiltration rate or Vp
- photographs, and who carried out the test
One fill is not enough. Dry ground soaks up the first fill quickly, so a single fill makes the ground look far better than it is. That is why both methods repeat the test and why BRE 365 designs from the slowest result. Weather matters too: Approved Document H says the test should not be carried out in heavy rain, severe frost or drought.
A garden test with a bucket and a stopwatch cannot show any of this, so it is very unlikely to discharge a planning condition or satisfy building control. Every council has its own way of working, so if in doubt, ask your building control officer what they need before you dig.
What you can check yourself
There is something useful you can do. Dig a hole a spade or two deep where the soakaway or drainage field might go, fill it with water and look at it the next day. If the water has barely moved, the ground is probably heavy clay and a soakaway may not be viable, which is worth knowing before you plan around one. If it has drained away, that is encouraging but not proof, because one fill in dry ground always flatters the result.
Check for buried cables and pipes before you dig, and never climb into a deep excavation: trench sides can collapse without warning.
The risks of getting it wrong
A soakaway built in ground that does not drain fills up and stays full. Water then backs up, floods the garden or collects near the house, and the usual fix is to dig it out and start again with a new design. A drainage field that fails can leave effluent on the surface, which is a health and pollution problem as well as a nuisance. And if the planning condition is not discharged, the build can stall while you wait for a proper test anyway. A test at the start costs far less than putting any of that right; our guide to how much a percolation test costs explains what decides the fee.
When to call us
Call us if your planning permission has a drainage condition, if building control has asked for an infiltration or percolation test, or if you are planning a septic tank or treatment plant. We bring our own machinery, dig the pits, carry out the fills to BRE 365 or BS 6297 and give you a report written to the standard. If you are not sure which test you need, tell us what is proposed and we will tell you.
or call 01902 475653, Monday to Friday, 8am to 6pm.