Soakaway and infiltration testing report
Infiltration testing to BRE 365 to find out whether soakaways will work on your site, with soakaway sizing.
About this service
A soakaway only works if the ground can absorb water quickly enough. Guessing is risky: an undersized soakaway floods the garden in the first big storm, and one in the wrong place can soften the ground under your foundations. Infiltration testing gives real figures. We dig trial pits at the proposed soakaway locations, fill them with water and measure how fast it drains, following the method in BRE Digest 365, which Building Control and councils widely accept.
From the measured rate we calculate the size of soakaway needed for your roof or paved area and advise on where it can go. We also tell you plainly when soakaways are not viable. Much of London sits on London Clay, which drains so slowly that a soakaway can take days to empty. Other areas, particularly on river terrace gravels, can drain very well. Testing is the only reliable way to know which you have.
Infiltration results feed straight into a surface water drainage strategy or a SuDS strategy, and are often requested to discharge a drainage planning condition. If testing shows infiltration will not work, the same strategy can switch to storage and a controlled outfall without starting again. Either way, the money spent on testing is rarely wasted, because the result settles a question that would otherwise come back at Building Control or condition stage.
When you need it
- You want to use soakaways for rainwater
- Building Control or the council needs infiltration test results
What's included
- Trial pit infiltration tests
- Calculation of infiltration rates
- Soakaway sizing and location advice
How it works
- Plan the test locationsWe review the site, drainage proposals and any known services, then choose trial pit positions that represent where soakaways would actually go, a safe distance from buildings.
- Excavate the trial pitsPits are dug to the depth of the proposed soakaway. We record the soils exposed, note any groundwater and confirm the pit dimensions needed for the calculations.
- Run the infiltration testsEach pit is filled with water and the fall in level is timed. The test is repeated, ideally three times, because the ground drains more slowly once wetted.
- Calculate and sizeWe calculate the infiltration rate from the results, then size soakaways for the contributing area using the design storm, and confirm whether infiltration is viable at all.
- ReportWe issue a report with logs, photographs, test data, calculations and recommendations on soakaway type, size and position, ready for Building Control or the council.
What you receive
Why soakaways often fail on London Clay
Clay is made of very fine particles with tiny pore spaces, so water moves through it slowly. In a test pit on London Clay, the water level may barely drop over several hours, and the test cannot be completed in the usual way. That is a valid result in itself. It tells the council infiltration is not practical, which is often exactly the evidence needed to justify storage and a controlled discharge to the sewer instead.
Position is just as important as size. Building Regulations guidance expects soakaways to be kept a minimum distance from buildings, and they should not discharge into ground that could affect neighbouring foundations, basements or retaining walls. Wetting clay near shallow footings can also cause problems. Our guide to London Clay and foundations explains why clay soils react to changes in moisture.
Frequently asked questions
What is a BRE 365 soakaway test?
It is the standard method in the UK for measuring how quickly water drains into the ground. A trial pit is dug to the depth of the proposed soakaway, filled with water and the drop in level is timed, usually three times in succession. The results give an infiltration rate, which is used to size the soakaway. Building Control and councils widely accept it.
How far should a soakaway be from a house?
Building Regulations guidance sets a minimum distance between a soakaway and a building, generally 5 metres, to avoid softening the ground under foundations. Greater distances may be sensible on clay, near basements or where the ground slopes towards a building. Your Building Control officer will confirm what applies to your project, and we take this into account when advising on location.
Do soakaways work in London Clay?
Usually not well. London Clay has very low permeability, so water drains slowly and a soakaway can stay full for long periods. Testing is still worthwhile, because the clay is sometimes overlain by sand or gravel, and the result provides the evidence the council needs either way. Where infiltration fails, rainwater is normally stored on site and released slowly to a sewer.
Does Building Control need a soakaway test?
Building Control often asks for evidence that a new soakaway will work, especially on new houses and larger extensions, and a percolation or infiltration test is the usual way to provide it. Councils may also require test results to discharge a planning condition. Requirements vary, so ask your officer early. Our testing starts from £600 + VAT.