blogTL;DR: A percolation test measures how fast your ground absorbs water, which tells you whether a soakaway or septic tank drainage field will actually work on your site. You dig a small test hole, pre-soak it, then time how long the water takes to fall 150mm, and turn that into a Vp value. This guide covers the correct method, how to calculate your result, and what a passing figure looks like.
A percolation test tells you whether the ground on your property can absorb water quickly enough to run a soakaway or a septic tank drainage field. Get it wrong and you could install a system that floods within months, or fall foul of Environment Agency rules. Learning how to do a percolation test for a soakaway is the single most important step before any off-mains drainage work in Cornwall, where clay subsoils and shallow bedrock make ground conditions unpredictable from one plot to the next.
The test itself is simple. The details are where most people slip up: the wrong hole size, no pre-soak, or testing after a week of rain will all give you a figure you cannot rely on. This guide walks through the correct method, the exact Vp calculation, and how to read your result so you know whether your ground passes.
What Is a Percolation Test?
A percolation test is a field measurement of how fast water drains out of the soil at a set depth. It produces a single figure called the Vp value, measured in seconds per millimetre, which describes how quickly your ground absorbs water. That figure decides whether a soakaway or drainage field is viable and, if it is, how large it needs to be.
The test matters because ground varies enormously. Free-draining sand takes water almost instantly. Heavy clay can hold it for hours. If you size a system without testing, you are guessing, and a guess that turns out wrong means either a waterlogged garden or effluent surfacing where it should not.
The standard method comes from Approved Document H of the Building Regulations, backed by the drainage field code of practice BS 6297. Building Control and, for septic systems, the Environment Agency both expect a test carried out to this method. It is not optional paperwork. Without a valid result, a drainage field cannot be legally designed or signed off.
Soakaway or Drainage Field: Know the Difference First
A soakaway handles clean rainwater. A drainage field handles treated effluent from a septic tank. The two are tested and sized to different standards, and confusing them is the most common and most expensive mistake homeowners make. Before you test anything, be clear about which one you actually need.
Rainwater soakaways are designed to BRE Digest 365 and cope with large volumes over short bursts, such as a downpour draining off a roof. Septic tank drainage fields are designed to BS 6297 and deal with small, steady volumes that the soil has to treat, not just soak up. The word “soakaway” gets used loosely for both, which is where the trouble starts.
This distinction is written into law. Under the Environment Agency’s general binding rules for discharges to ground, effluent from a septic tank must go to a properly designed drainage field, and it has been illegal since 1 January 2020 for a septic tank to discharge directly to a ditch, stream or watercourse. You cannot use a rainwater soakaway, well or borehole to get rid of sewage effluent. If you are dealing with a septic system, you need a drainage field, and that is what the percolation test below is designed for.
If your existing system is already struggling, a CCTV drain survey is often the quickest way to confirm whether the fault sits in the tank, the pipework, or a failed drainage field before you commit to any groundwork.
What You Need Before You Start
A percolation test needs a spade, a tape measure, a stopwatch or phone timer, a large water container, and something to record your readings. The equipment is basic. The conditions you test in matter far more than the kit you use.
Do not test during abnormal weather. Approved Document H is explicit on this point: heavy rain, severe frost or drought all distort the result. If the ground is already saturated from a wet Cornish winter, your test will read slower than the system will actually perform, and if it is baked hard after a dry spell, it will read faster. Aim for settled, average conditions.
You also need to test in the right place and at the right depth. Dig your test hole where the drainage field or soakaway will actually go, not at a convenient spot near the house. Ground can change completely over a few metres, especially on Cornish plots where a band of clay or a shelf of shillet rock can sit right under one corner of a garden.
A few site checks worth doing first:
- Confirm the water table is not close to the surface. A drainage field should sit around a metre above the winter water table, which rules out low-lying and coastal plots with high groundwater.
- Check your distances. A drainage field must be at least 10 metres from a watercourse, 15 metres from a building, and 50 metres from any well, borehole or spring used for drinking water.
- Avoid steep slopes and any ground that already waterlogs, as neither will treat effluent properly.
How Do You Do a Percolation Test, Step by Step?
To do a percolation test, dig a 300mm square hole to the depth of your proposed drainage pipe, pre-soak it overnight, then refill it and time how long the water takes to fall 150mm. You repeat this across two or three holes to get a reliable average. The full sequence is below.
- Dig the trial hole. Excavate a hole 300mm square down to 300mm below the proposed invert level of the distribution pipe. For most systems that pipe sits 700mm to 1 metre down, so dig a wider access pit and keep the tested base a true 300mm square.
- Pre-soak the hole. Fill it with water to at least 300mm depth and leave it to drain away completely, ideally overnight. This wets the surrounding soil so the test reflects long-term behaviour, not a dry-ground first fill that drains unrealistically fast.
- Refill to 300mm. The morning after the pre-soak, fill the tested section back up to 300mm of water.
- Time the 150mm fall. Record how long, in seconds, the water level takes to drop from 75% full (225mm) down to 25% full (75mm). That is a 150mm fall. Write it down.
- Repeat and spread out. Run the timed fall at least three times, and dig at least two or three separate trial holes along the line of the proposed field. Soil varies, so one reading is never enough. The council’s own infiltration testing guidance sets out the same trial-hole approach for drainage design.
One practical note: the first timed run is often the fastest, because the soil is still taking up water. That is why the pre-soak and the repeats matter. If your readings vary by more than about 50%, dig further holes and test again until the figures settle.
How Do You Calculate the Vp Value?
The Vp value is the average time in seconds for the water to fall, divided by 150. You add up your timed falls, divide by the number of tests to get the average time, then divide that average by 150mm. The result is your percolation rate in seconds per millimetre.
Here is a worked example. Say your averaged fall time across the holes came to 2,700 seconds. Divide 2,700 by 150 and you get a Vp of 18 seconds per millimetre. That single figure is what your whole drainage field design will be built on.
Keep every reading you take. Building Control and the Environment Agency can ask to see the raw data behind your Vp figure, and a bare number with no supporting readings will not satisfy them. Record the date, the weather, each hole, and each timed fall.
What Is a Good Percolation Test Result?
For a septic tank drainage field, a good percolation test result is an average Vp value between 15 and 100 seconds per millimetre. Below 15, water drains too fast and the soil cannot treat the effluent before it reaches groundwater. Above 100, it drains too slowly and the system will back up and waterlog.
Both limits exist to protect the environment, and both come from BS 6297 and Approved Document H. A Vp inside that band means your ground can, in principle, support a standard drainage field. The exact figure then feeds the sizing calculation, where a higher Vp means a larger field and more trench length.
What happens if your result falls outside the range?
- Vp below 15 (too fast): The ground needs help slowing the flow. A common fix is a layer of washed sand beneath the distribution pipes to add treatment, or an engineered solution such as a drainage mound.
- Vp above 100 (too slow): A standard drainage field will not cope. Options include a package treatment plant discharging to a watercourse where permitted, or a different disposal route altogether.
Cornish ground fails this test more often than many expect. Heavy clay soils, common across mid and north Cornwall, frequently push the Vp above 100, while fractured granite and shillet can drain too fast or unevenly. A poor result is not the end of the road, but it does mean the design needs an engineer rather than a standard field.
When to Bring in a Professional
A percolation test looks straightforward, and for a simple, well-drained plot a competent homeowner can carry it out. The problems come when the result is borderline, the ground is mixed, or the test has to stand up to Building Control and Environment Agency scrutiny. That is when getting it wrong becomes costly.
An inaccurate test has real consequences. Size a field on a flattering figure and it fails within a year or two, surfacing effluent and leaving you with a rebuild. Where a system is already failing, symptoms like slow drainage, foul odours or blocked drains often point to a drainage field that has stopped percolating, and that needs proper diagnosis rather than another guess.
Clear Stream Drainage Solutions carries out percolation testing and full drainage field assessments across Cornwall, with no call-out fee and a fixed-price quote before any work begins. Our engineers handle the testing, the interpretation, and the design, and back all repair work with a 5-Year Guarantee. If your test result points to a failing or non-compliant system, we can also advise on septic tank maintenance and the compliant options open to you.
Getting Your Percolation Test Right
A percolation test is the foundation of any soakaway or drainage field, and the whole design stands or falls on that one Vp figure. Get the method right and you protect yourself from an expensive failure down the line.
Three things to hold onto:
- Test in the right place, at the right depth, in settled weather, and always pre-soak first.
- A passing Vp for a septic drainage field sits between 15 and 100 seconds per millimetre.
- A soakaway is for rainwater, a drainage field is for effluent, and the two are not interchangeable.
If you would rather have the test done properly and signed off without the guesswork, our Cornwall team can help. Speak to Clear Stream about a percolation test and drainage assessment, or call our team on 01872 222555 for a fixed-price quote with no call-out fee.
Frequently Asked Questions
Do I Need a Percolation Test for a Soakaway?
Yes, in almost all cases. A percolation test is required to design a septic tank drainage field to BS 6297, and it is used to size a rainwater soakaway to BRE Digest 365. Building Control expects a valid test result before signing off an off-mains drainage system, and the Environment Agency’s general binding rules assume a properly designed drainage field for any septic discharge to ground.
How Long Does a Percolation Test Take?
Plan for at least two days. The pre-soak needs to drain away over several hours, ideally overnight, before you take any timed readings. The timed falls themselves can take anywhere from a few minutes to several hours each, depending on how fast your ground drains, and you repeat them across two or three holes.
What Is a Good Vp Value for a Drainage Field?
An average Vp value between 15 and 100 seconds per millimetre is acceptable for a standard septic tank drainage field. Below 15 the ground drains too fast to treat the effluent safely. Above 100 it drains too slowly and the field will waterlog. A result outside that range means you need an engineered solution rather than a standard field.
Can I Do a Percolation Test Myself?
You can carry out the test yourself if the plot is simple and free-draining, provided you follow the correct method and keep full records. Many people prefer a professional to run it, because Building Control and the Environment Agency need results they can rely on, and a flawed test leads to a system that fails. Borderline or mixed ground is best left to an engineer.
What Happens if My Ground Fails the Percolation Test?
A failing result does not always rule out a system, but it changes the design. If water drains too fast, a sand layer or drainage mound can add treatment. If it drains too slowly, a package treatment plant discharging to a watercourse, where permitted, may be the answer. Either way, the design needs a drainage engineer to keep it compliant.
Is a Soakaway the Same as a Septic Tank Drainage Field?
No. A soakaway disperses clean rainwater and is designed to BRE Digest 365. A drainage field disperses treated effluent from a septic tank and is designed to BS 6297. The two are not interchangeable, and you cannot legally dispose of sewage effluent through a rainwater soakaway.
