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The basics
What is a drainage field?
A drainage field is the last stage of an off-mains sewage system. Liquid from your septic tank or treatment plant flows into a network of perforated pipes laid in gravel trenches, seeps out and soaks slowly into the soil, where bacteria finish treating it.
Solids escaping from a tank that hasn’t been emptied are a common cause of failed drainage fields, so regular septic tank emptying helps protect the new one. If you’re upgrading, we can also fit a sewage treatment plant.
We do this work across Surrey, West Sussex, East Sussex, Kent, Hampshire and London, with tankers and engineers in each area most days.
Drainage fields don’t last forever. Solids escaping from a neglected septic tank clog the gravel and soil, and once the ground can’t take any more, the field has to be replaced.
From above
How a drainage field is laid out
A typical layout, seen from above. Hover over or tap the numbers to see each part.
- 1Septic tank or treatment plantSettles and treats the wastewater before it goes to the drainage field.
- 2Distribution chamberSplits the flow evenly between the pipes, and is a handy place to sample the effluent.
- 3Perforated pipes in gravel trenchesThe liquid seeps out of the pipes into the gravel and soaks slowly into the soil, where bacteria finish treating it.
- 4WatercourseA drainage field must be at least 10 metres from any watercourse or permeable drain, and 15 metres from any building.
Percolation test calculator
Is your ground suitable?
Before a drainage field can be designed, a percolation test shows how quickly water soaks into your ground. The result, called Vp, is the average number of seconds it takes the water to drop 1 mm.
How the test is done: dig a hole 300 mm square, 300 mm below where the pipes will go. Fill it with at least 300 mm of water and leave it to drain overnight. Then fill it again and time how long the water takes to drop from 75% to 25% full, a drop of 150 mm. Repeat at least three times in at least two holes.
Enter your times to see your Vp, whether the ground is likely to be suitable and roughly how big the drainage field would need to be.
Enter the time, in seconds, the water took to drop from 75% to 25% full (a 150 mm drop) in each test. Use at least three results.
A guide only. It uses the Building Regulations method (area = people × Vp × 0.25 for a septic tank) and the BS 6297 figure of 0.2 for a treatment plant. A drainage field must be designed to BS 6297, so talk to us before you dig.
Replacements
Why do drainage fields fail?
Most drainage fields fail because solids get into them. When a septic tank isn’t emptied often enough, sludge escapes through the outlet and clogs the gravel and the soil around the pipes.
Others fail because the ground was never suitable, the field was too small for the property, rainwater is getting in, or tree roots and vehicles have damaged the pipes. Most last many years, but none last forever.
Signs of a failing drainage field
- Wet, boggy or smelly ground over the field
- Unusually green grass over the pipes
- The septic tank or plant filling up or backing up
- Slow drains throughout the house
- Sewage on the surface
On the day
How we install a drainage field
Site assessment and testing
Trial holes and percolation tests to check the ground and the water table.
Design
A drainage field sized and laid out for your property and ground, to BS 6297.
Excavation and installation
Trenches dug, gravel laid and perforated pipes installed to the right levels.
Connection and testing
Connected to your septic tank or treatment plant, tested and backfilled.
Why Clearmasters
Why choose Clearmasters?
We test, design and install drainage fields, and we empty and service the tanks and plants in front of them, so we can make sure your new field lasts.
Ask us a questionAlways on hand
Planned servicing at a time that suits you, and 24/7 call-outs when something goes wrong.
Straight answers
We explain what we found in plain English, and give you honest advice on what needs doing now and what can wait.
Trained engineers
Our engineers hold current confined space entry certificates and work on treatment plants, pump stations and drainage every day.
We look after what’s upstream
Regular emptying of your septic tank protects your new drainage field.
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Questions answered
Drainage field FAQs
What is a percolation test and why is it needed?
It measures how quickly water soaks into your ground. The result, called Vp, decides whether a drainage field is suitable and how big it needs to be.
What Vp result is acceptable?
BS 6297, which the Environment Agency’s rules require drainage fields to be designed to, uses a range of 15 to 100. The Building Regulations guidance allows 12 to 100. Outside that range, a standard drainage field isn’t suitable.
How big does a drainage field need to be?
The Building Regulations method multiplies the number of people by the Vp and by 0.25 for a septic tank, to give the area of trench floor needed. BS 6297 uses 0.2 for treatment plant effluent. Try the calculator above for a rough idea.
Can a drainage field go in the same place as the old one?
Sometimes, but the old ground is often clogged, so a new field is usually laid in fresh ground. We’ll test and advise.
Can I use a soakaway instead?
Not for sewage. A soakaway is designed for rainwater, and one installed after December 2007 doesn’t meet the rules for septic tank effluent, which needs a proper drainage field.
How long does a percolation test take?
The holes need filling and leaving overnight first, so it usually takes two days.
Got a different question? Call 03333 233566 or send us a message. You can also read the rules for discharging to ground on GOV.UK.
Related services
Fully accredited & compliant
We're independently vetted and certified so you can trust who you're letting onto your site.
SafeContractor
Health & safety pre-qualification accreditation, verifying our processes meet recognised safety standards.
RISQS
Railway Industry Supplier Qualification Scheme — approved to work safely within the rail industry supply chain.
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Utilities Vetting Database membership, confirming we meet the utilities sector's supply chain standards.
Trading Standards Approved
Vetted and approved under the local authority's trusted trader scheme for honesty and fair dealing.
NADC Member
Members of the National Association of Drainage Contractors, the UK trade body for the drainage industry.
Registered Waste Carrier
Registered with the Environment Agency, so your waste is handled and disposed of fully within the law.