People use four or five different names for what is often the same job, and one genuine distinction gets lost in the middle of it. Here is the plain version.
The ones that mean the same thing
Porosity test, percolation test, soakaway test, soil infiltration test and perc test are, for surface water, all the same thing. If a builder says porosity test and an engineer says BRE 365 percolation test, they want the same result: a measured infiltration rate to size a soakaway.
You will not be caught out by using the wrong one. Ask for a percolation test and everybody will know what you mean.
The distinction that does matter
The real difference is not the name, it is what the water is.
- Surface water — rain off a roof, a drive, a yard. Tested to BRE Digest 365. The result sizes a soakaway.
- Foul water — treated effluent from a septic tank or package treatment plant. Tested to BS 6297, which Part H of the Building Regulations refers to. The result sizes a drainage field.
Different procedures, different calculations, different numbers. They are not interchangeable. If your planning condition or building control officer asks for one, the other will not satisfy it — and finding that out after the visit means paying for a second one.
How to tell which you need
Follow the water. If you are dealing with rainwater and a soakaway, it is BRE 365. If you are dealing with a septic tank, a treatment plant or a drainage field, it is BS 6297. If your site has no mains foul connection and you are also building something with a roof, it is very likely both.
A note on ground investigation
A percolation test is not a ground investigation. It measures one thing: how fast water leaves a hole. It does not tell you the bearing capacity of the soil for foundations, and it is not a contamination survey. If a condition asks for a site investigation or geotechnical report, that is a different piece of work by a different specialist.
Confusing the two is common, and it is worth checking the condition wording carefully before booking anybody.
Still not sure?
Send us the condition wording and the postcode. We will tell you which test it is asking for — and if it is asking for something we do not do, we will say so rather than take the booking.