CLEARMASTERS
The short version
Why do drainage fields fail?
Most drainage fields fail because the soil gets clogged or flooded. Solids and fat escaping from a tank that isn’t emptied often enough build a slimy layer that seals the ground, and rainwater, groundwater or extra use can give it more water than it can take. Caught early, emptying, jetting and cutting the water going in can bring a field back. Once the soil has sealed, it usually needs replacing.
Underground
How a drainage field works, and how it fails
Step through what happens in the trench as a field goes from healthy to failed.
Liquid from the septic tank runs along perforated pipes, trickles through the gravel and soaks into the soil. Bacteria in the soil finish the treatment before the water reaches the water table.
If the tank isn’t emptied often enough, or fat and solids get through, a black, slimy clogging layer builds up where the gravel meets the soil. Water soaks away more and more slowly.
Wet weather, a high water table, rainwater connected to the system or a leaking tap fill the ground. The trench stays wet, and you may notice extra-green grass over the pipes.
The trench fills up and liquid rises to the surface as soggy, smelly patches, or backs up the pipes to the tank and the house. Untreated effluent on the surface is pollution, so it needs dealing with quickly.
The soil does the treating
A septic tank only settles out the solids. The liquid that leaves it still carries bacteria and nutrients, and it’s the soil in the drainage field that filters and treats it. That’s why the ground matters as much as the tank.
A thin layer is normal
Every field grows a thin film of bacteria where the gravel meets the soil, and in a healthy field it stays in balance. Trouble starts when solids, fat and too much water let it grow thick enough to seal the ground.
Symptom checker
Is my drainage field failing?
Tick the signs you’ve noticed to see how serious it’s likely to be and what usually causes them.
What we find
Why drainage fields and soakaways fail
Usually it’s more than one of these at once.
Tank not emptied often enough
Sludge and scum build up until they spill out with the liquid and coat the soil. This is the most common cause of early failure we see.
Fat, wipes and chemicals
Fat seals the soil, wipes block the pipes, and heavy use of bleach upsets the bacteria that keep the clogging layer in check.
Rainwater and groundwater
Gutters or drives connected to the tank, a high winter water table or a leaking tap all flood the field with water it was never sized for.
More use than it was designed for
Extra bedrooms, a home office or a holiday let add people, and more people means more water.
Driven over or built on
Cars, patios, sheds and even trampolines compact the soil and crush the pipes, so the water has nowhere to go.
Tree roots
Roots seek out the moisture in a drainage field and grow into the pipes, blocking them from inside.
Unsuitable ground
Heavy clay drains too slowly and some sands too fast. A field that was never tested properly may have been struggling from the start.
Old soakaways and collapsed pipes
Many older homes have a single soakaway pit, not a proper drainage field. Over decades these silt up, and old clay pipes crack and sink.
Fat, wipes and chemicals are covered in detail in our guide to what not to flush with a septic tank.
Repair or replace?
How to fix a failing drainage field
The right answer depends on whether the problem is in the pipes, the tank or the soil itself. That’s why we always find the cause first.
Empty the tank and check the outlet
Quick first stepTakes the pressure off straight away and lets us see whether the outlet, the baffles and the tank itself are working.
CCTV survey of the pipes
Finds the causeA camera through the distribution pipes shows roots, collapses, silt and blockages, so you only fix what’s actually wrong.
Jet the distribution pipes
Often helpsHigh-pressure jetting clears silt, roots and slime from the pipes. It can bring a struggling field back if the soil itself is still sound.
Take the extra water out
Often helpsDisconnect gutters and drives, fix leaking taps and toilets, and spread out washing. Less water gives the soil a chance to recover.
Rest the field, or add a second one
SometimesSome sites have room for a second field, so the two can be used in turns and the resting one recovers.
Replace the drainage field
When it has failedA failed field usually needs replacing in fresh ground, after a percolation test, with building control approval.
Upgrade to a treatment plant
Cleaner effluentCleaner water from a treatment plant is kinder to the soil, needs a smaller field and, in some cases, can go to a ditch or stream instead.
Build a drainage mound
High water tableWhere the ground is too wet, a raised mound of sand and gravel gives the effluent enough depth of soil above the water table.
Testing the ground
What is a percolation test?
Before a new drainage field goes in, the ground has to be tested to show it can take the water. Here’s how it’s done.
Dig a trial hole
A deep hole shows the soil layers and the highest water table, which should stay well below the pipes all year round.
Dig test holes
At least two holes, 300 mm square, dug 300 mm below where the pipes will sit, spread across the proposed field.
Soak and time
Each hole is filled to soak the ground, then refilled and timed as the water drops from 75% to 25% full: a fall of 150 mm.
Work out the Vp
The time in seconds divided by 150 gives the seconds per millimetre. Tests are repeated and averaged to give the percolation value, the Vp.
What the result means
Building Regulations guidance says a drainage field is suitable where the average Vp is between 12 and 100 (the British Standard uses 15 to 100). Below that, the ground drains too fast to treat the effluent; above it, too slowly to take it away.
Testing at the wrong time
Tests shouldn’t be done in drought, frost or very heavy rain, because they give misleading results. A field designed on a dry summer test can struggle every winter.
Calculator
How big should a drainage field be?
Move the sliders to see roughly how much trench a house needs. It shows why slow-draining ground, or a bigger household, needs a much larger field.
Uses the Building Regulations formula: area = people × Vp × 0.25 for a septic tank, or × 0.2 for a treatment plant. A rough guide only: a real design needs a percolation test, a check on the water table and building control approval.
The rules in England
Drainage field rules and siting
Under the Environment Agency’s general binding rules, a septic tank must discharge to the ground through a drainage field, not to a ditch or stream. Drainage fields should meet the British Standard (BS 6297), and rainwater soakaways installed after 2007 don’t count.
A new or replacement drainage field also needs Building Regulations approval, and in some cases planning permission. Discharges to ground over 2 cubic metres a day need a permit or a sewer connection.
If you’re not sure whether yours is a proper drainage field or an old soakaway, a septic tank survey will tell you. Our guide to septic tanks, cesspits and treatment plants explains the wider rules.
Prevention
How to make your drainage field last
Do
- Empty the septic tank at least once a year
- Spread washing and dishwasher loads across the week
- Fix dripping taps and running toilets quickly
- Keep gutters, drives and pool water out of the system
- Know where your field is, and mark it
- Have the tank and field checked if anything changes
Don’t
- Drive, park or build over the field
- Plant trees or big shrubs nearby
- Pour fat, oil or strong chemicals down the drains
- Flush wipes or sanitary items
- Rely on additives to fix a problem
- Ignore soggy patches or smells
Questions answered
Drainage field FAQs
What is a drainage field?
A drainage field, also called a leach field or soakaway system, is a network of perforated pipes laid in gravel-filled trenches. It spreads the liquid from a septic tank or treatment plant through the soil, where natural bacteria finish treating it before it reaches the groundwater.
How do I know if my drainage field is failing?
Common signs are slow or gurgling drains, sewage smells in the garden, soggy ground or extra-green grass over the field, liquid in the tank sitting above the outlet pipe, and problems that get worse after rain. Sewage coming to the surface or backing up into the house means it has failed.
Can a failed drainage field be repaired?
Sometimes. If the problem is in the pipes, such as roots, silt or a collapse, jetting and repairs can restore it. If the soil itself has clogged after years of solids and fat, the field usually needs replacing in fresh ground. A CCTV survey and an inspection of the tank tell you which you are dealing with.
How long does a drainage field last?
A well-designed field that is looked after can last for decades. The biggest factors are emptying the tank at least once a year, keeping fat, wipes and rainwater out, and not driving or building over it. Fields that are overloaded or neglected can fail within a few years.
What is a percolation test?
A test that measures how quickly water soaks into the ground where a drainage field will go. Holes are dug, soaked and then timed as the water drops. The result, called the Vp, decides whether a drainage field is suitable and how big it needs to be.
How big does a drainage field need to be?
Building Regulations size it with a formula: the number of people the house is designed for, multiplied by the percolation value, multiplied by 0.25 for a septic tank or 0.2 for a treatment plant. The answer is the floor area of the trenches in square metres. Our calculator on this page gives a rough figure.
Is a soakaway the same as a drainage field?
Not quite. A soakaway is usually a single pit filled with rubble or a crate, often designed for rainwater. A drainage field spreads the effluent along trenches. Under the general binding rules in England, septic tanks must discharge to a proper drainage field, and rainwater soakaways installed after 2007 do not qualify.
Where can a drainage field go?
Building Regulations guidance says a drainage field should be at least 15 metres from any building, 10 metres from any watercourse and 50 metres from any well, borehole or spring used for drinking water, with the water table at least a metre below the pipes all year round.
Got a different question? Call 03333 233566 or send us a message.