Percolation tests explained.

How the test is done.

The method is in the guidance to Building Regulations Part H and in BS 6297.

1. A trial hole, at least 1m square and 2m deep or 1.5m below the proposed pipe, checks the groundwater level.
2. A 300mm square test hole is dug to 300mm below the proposed pipe level.
3. It is filled to 300mm with water and left to soak away overnight.
4. Next day it is refilled to 300mm, and the time for the water to fall from 75% to 25% full, a drop of 150mm, is recorded.
5. The test is run at least three times in at least two holes, and the results averaged.

Working out Vp.

Vp is the average time in seconds for the water to fall 1mm. You get it by dividing the time for the 150mm fall by 150. So a 3,000 second fall gives a Vp of 20.

• Part H guidance says drainage fields should only be used where the average Vp is between 12 and 100.
• BS 6297 and the Environment Agency's general binding rules use a minimum of 15.

Below the range, effluent drains away too fast to be treated. Above it, the ground drains too slowly and the field can flood. The Vp result is also used, with the number of people served, to size the drainage field.

Other things to know.

• Do not test in abnormal weather, such as heavy rain, severe frost or drought. Summer results should be checked against likely winter groundwater levels.
• Groundwater must not rise to within 1m of the drainage pipe.
• Under the Environment Agency's general binding rules, small discharges to ground are limited to 2 cubic metres a day, and the rules cannot be used if a public sewer is within 30m of the property boundary.

Need rainwater drainage instead? That is a soakaway test.