Drainage fields and soakaways on the Isle of Man
A drainage field lets water from a septic tank or treatment plant soak safely into the ground, and a soakaway does the same for rainwater. Both depend on your ground, so it starts with a proper test.
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- Percolation tests and design
- New fields and replacements
- Failed soakaways investigated
In short
- Drainage field
- Perforated pipes laid in gravel trenches that spread water from a septic tank or treatment plant through the soil, which cleans it naturally.
- Soakaway
- An underground pit or crate system that lets clean rainwater from roofs and drives soak into the ground.
- Percolation test
- Measures how quickly water soaks into your ground. It decides whether a field or soakaway will work, and how big it needs to be.
- On the Island
- Where the mains isn't available, DEFA prefers the water to soak fully into the ground. If it all soaks away, no discharge licence is needed. If some can reach a stream or ditch, a licence is needed.
- What it costs
- Your first consultation is free, and we give you a free estimate before any work starts.
How a drainage field works
- Water leaves the septic tank or treatment plant.
- A distribution chamber splits the flow evenly between the trenches.
- Each trench holds a perforated pipe laid in gravel, so the water spreads along its length.
- Undisturbed ground between the trenches gives the soil room to absorb and treat the water.
- The holes in the pipe face down, and the gravel lets water seep out on all sides and down into the soil.
- A membrane keeps topsoil out of the gravel, so the trench doesn't clog.
Drainage field or soakaway?
People often use the words to mean the same thing, and DEFA's rules use "soakaway" for both. On this website, a drainage field is the one for a septic tank or treatment plant, and a soakaway is the one for rainwater.
| Drainage field | Rainwater soakaway | |
|---|---|---|
| What goes in | Water from a septic tank or treatment plant | Rainwater from roofs, drives and patios |
| How it's built | A network of perforated pipes in gravel trenches | A pit filled with stone, or plastic crates wrapped in fabric |
| What it does | Spreads the water out so the soil can finish treating it | Stores rainwater and lets it soak away gradually |
| Must never take | Rainwater, which floods it | Foul water from toilets, sinks or appliances |
| Approval | Building Regulations approval, and planning permission when new | Building Regulations approval. A new soakaway may also need planning permission, so check first |
What a percolation test involves
Before a drainage field or soakaway can be designed, we need to know how your ground behaves. The guidance used on the Island, Approved Document H, sets out how.
Trial hole
A trial hole shows what the ground is made of and how high the water table rises. A field won't work if groundwater comes up close to the pipes, and in winter it is usually higher.
Test holes
Smaller holes are dug at the depth the pipes would sit, filled with water and left to soak.
Timing
Once the ground is soaked, the holes are filled again and we time how long the water takes to drop. The test is repeated to get a fair average.
Result
The result tells us whether the ground is suitable and how big the field needs to be.
Island ground varies a lot
Most of the Island sits on the old "Manx Slates" (mudstone and siltstone) with a covering of glacial material, while the northern plain and parts of the south have deeper sand, gravel and clay. Ground can change over short distances, so every site needs its own test.
Why too fast is a problem as well as too slow
Ground that drains too slowly can't take the water, so it backs up and surfaces. Ground that drains too quickly lets the water through before the soil has cleaned it, which risks polluting groundwater. The test shows whether your ground sits in the range that works.
When the ground won't drain
There are still options. Depending on the site, that might be a differently laid or larger field, a treatment plant with a partial soakaway, or, as a last resort in DEFA's order of preference, a licensed discharge from a treatment plant to a watercourse (a stream, river or ditch).
Signs a drainage field or soakaway is failing
- Wet, spongy or boggy ground over the field, even in dry weather
- Strips of grass that are greener or grow faster than the rest
- Smells around the tank or field
- Toilets and drains slow throughout the house
- Water sitting high in the septic tank's outlet chamber
- For rainwater soakaways: puddling, and gullies or downpipes overflowing after rain
Why they fail
- Solids carried over from a tank that hasn't been emptied, which clog the gravel and soil
- Fats, wipes and chemicals going into the system
- Rainwater connected to the tank, which overloads the field
- Roots growing into the pipes
- Compacted ground from vehicles, sheds or paving over the field
- A field that was too small, or built without testing the ground
- A high water table, especially in winter
Can a failed drainage field be fixed?
Sometimes. We start by finding out why it failed. That means checking the tank, looking at the pipes with a camera, and seeing what the ground is doing. If the pipes are simply blocked, cleaning them and dealing with the cause, such as an overdue tank, can be enough.
If the soil itself has clogged, it usually needs a new field in fresh ground, designed from a new percolation test. If the site can't take a new field, a treatment plant may be the better long-term answer. We'll explain what we find and what each option involves.
Drainage field and soakaway FAQs
How far does a drainage field need to be from my house?
The guidance used on the Island, Approved Document H, says a drainage field should be at least 15 metres from any building, 10 metres from any watercourse or permeable drain (such as a land drain), and 50 metres from any well or borehole used for water. These are guidance figures and Building Control has the final say, so we check them against your site.
Do I need a DEFA licence for my drainage field?
If all the water soaks into the ground, DEFA calls it a "full soakaway" and you don't need a discharge licence. If some of it can reach a stream or ditch, for example in wet weather, it's a "partial soakaway" and you do. You'll still need Building Regulations approval, and planning permission for a new system.
Can I park on or build over a drainage field?
No. Parking, sheds or paving compact the ground and stop it absorbing water, and they make the field hard to reach. Keep the area as lawn or planting with shallow roots.
Can rainwater go into my septic tank?
No. Rainwater should be kept out of septic tanks and treatment plants. It floods the system, washes solids into the drainage field and can cause it to fail. Rainwater needs its own soakaway or drain.
How long does a drainage field last?
A well-designed field that's looked after can last for many years. The biggest factor is keeping solids out of it, which means emptying the tank when it's due and not putting wipes, fat or chemicals down the drains.
Related pages
Wet ground over your drainage field?
Tell us what you're seeing. We'll help you work out whether it's the field, the tank or something else.