Percolation tests explained

A percolation test measures how quickly your soil accepts water. Trial holes are dug in the area where the drainage field would go, soaked to represent normal working conditions, then filled with water, and the time it takes the level to drop a set amount is recorded and averaged. That figure, the percolation value, feeds directly into how large the drainage field must be. The test is carried out to the British Standard BS 6297, and it produces a written result you keep.

The logic is simple: a drainage field works by letting soil absorb and treat the liquid from your tank. Fast-draining soil can do that over a smaller area; slow soil needs a bigger field to handle the same volume; and soil that barely drains at all cannot do it safely no matter how big the field.

The percolation result determines two things that govern the whole job: whether a conventional drainage field is legally permitted on your site at all, and if so, how big it has to be. Because the size of the field is the biggest single variable in the cost of an installation, no honest price can be put on the work before the test is done. This is why a contractor quoting a drainage field over the phone, without a test result, is guessing at the number that matters most.

If the test fails, meaning the soil drains too slowly (or, occasionally, so fast it would not treat the effluent), the General Binding Rules do not allow a conventional drainage field. That is not the end of the road, but it does change the plan. The recognised alternatives are a sewage treatment plant discharging to a watercourse, which treats the waste to a far higher standard, or an engineered solution such as a mound system where the ground is built up to create the drainage the natural soil cannot provide.

A failed test is a common outcome, not an unusual one, and it is better discovered at the test stage than after a field has been built and fails in its first wet winter.

Shropshire is exactly the kind of place where you cannot assume the result. The glacial clays across the north of the county drain slowly and fail percolation often; the thin soils over rock in the border hills and along Wenlock Edge can lack the depth a field needs regardless of how the top layer drains; and two fields a mile apart, or even two corners of the same plot, can give opposite results. A test on the neighbour’s land tells you nothing reliable about yours.

Timing matters too. A test carried out in a dry summer can flatter a site that is saturated by February, so it is worth knowing when a test was done and what allowance has been made for the winter water table, which sits high across much of the county.