Soakaways Explained: How They Work, When They Fail and the Legal Bits

Reviewed by the DCI Drainage team · Last updated September 2026

Soakaways are the drainage you cannot see doing the job you never think about. Thousands of properties across Cornwall and Devon rely on one to get rid of roof and surface water, and plenty of owners only discover they have one when it stops working. This guide explains what a soakaway actually is, how it works, why they fail, what the law says, and how to work out whether the boggy patch on your lawn is a soakaway problem or something else.

What a soakaway is and how it works

A soakaway is a deliberately built void in the ground that receives water from a pipe and holds it while it seeps out into the surrounding soil. That is the whole trick: instead of sending rainwater to a sewer, or letting it run off wherever gravity takes it, the soakaway spreads the load into the ground gradually, where it does no harm. The pipe from your gutters or gully drops the water in; the ground does the rest.

For a soakaway to work, two things must stay true. The void has to remain open, so water has somewhere to sit while it disperses. And the surrounding soil has to keep absorbing, which depends on the ground: free-draining soils take water readily, while heavy clay barely takes it at all, which is why soakaways in clay ground are either large, carefully designed or a bad idea from day one. When either the void or the soil stops doing its half of the job, the soakaway fails, and the water goes back to doing what water does: finding the lowest point, which is often your lawn, your drive or the base of your wall.

The types you will find in this part of the world

The old rubble pit

The traditional soakaway, and the one under most older gardens in Cornwall and Devon, is simply a big hole filled with rubble, brick and stone, with the drain pipe run into the top and the whole thing buried. They work, sometimes for decades, but the voids between the rubble are small and silt up steadily, and nobody alive usually knows exactly where the pit is or how big it was.

Modern crate soakaways

The current standard is a set of strong plastic crates, wrapped in a permeable geotextile membrane and buried. Crates are almost all void, so a modern soakaway stores far more water for its size than a rubble pit, and the membrane keeps soil and silt out of the storage. Installed correctly, with a silt trap on the inlet, they have a long service life.

Drainage fields for septic tanks

Some rural properties have a different animal: a drainage field, a network of perforated pipes in gravel trenches that disperses the treated liquid from a septic tank. People often call this a soakaway too, but the rules around it are stricter because the water is effluent rather than rain. If your soakaway is connected to a septic tank rather than your gutters, the legal section below matters to you doubly.

Why soakaways fail

Soakaways fail slowly, then suddenly. The common causes:

• Silt. Every shower of rain washes fine sediment off your roof and paving into the soakaway. Over years it fills the voids in a rubble pit or blinds the membrane of a crate system, especially where no silt trap was fitted.
• Leaves and debris. Autumn's contribution. Leaf litter breaks down into exactly the sort of sludge that seals a soakaway floor.
• Blocked or broken feed pipes. Often the soakaway itself is fine and the pipe leading to it has blocked, cracked or been crushed. From the surface the symptoms are identical.
• Compacted or sealed ground. The soil around the soakaway can become blinded with fines, so it stops absorbing even though the void is clear. Driving vehicles over a soakaway compacts the ground and speeds this up.
• Tree roots. Roots hunt water, and a soakaway is a reliable source. They enter through pipe joints and crate walls and gradually choke the system.
• Undersizing. A soakaway built for a small roof, then asked to serve an extension, a conservatory and a paved drive, was never going to keep up. More hard surface means more water, and the original pit may simply be too small for the modern property.

Signs of a failed or blocked soakaway

Because the soakaway is invisible, you diagnose it by what the water does when it stops working:

• Gullies or downpipe shoes overflowing during ordinary rain, not just storms
• Water ponding on the lawn or drive, roughly over or downhill of the soakaway
• A patch of ground that stays boggy for days after everywhere else has dried
• Damp appearing on walls near the soakaway or along the feed pipe run
• In drainage-field systems, sewage smells or lush wet ground over the field

A note on that damp point, because it is the expensive one. A failed soakaway close to the house keeps the ground beside your footings saturated, and saturated ground next to masonry eventually shows up indoors. It quietly masquerades as a damp-proofing problem, and it is why current guidance keeps new soakaways at least five metres from any building.

The legal bits

The rules depend entirely on what the soakaway handles.

Two situations bring the legal side to the surface: house sales, when a buyer's survey asks how surface and foul water actually leave the property, and flooding disputes, when a failed soakaway sends water into a neighbour's garden. In both, knowing what you have and showing it works beats finding out under pressure.

Diagnosing it properly

A waterlogged garden has several possible causes: a failed soakaway, a blocked feed pipe, a collapsed drain leaking into the ground, a high water table, or simply heavy ground and a wet winter. They all look like each other from the lawn, and they carry very different price tags, so the diagnosis matters more than the digging.

The professional approach runs in order of least disruption. First, trace the pipe runs and find the soakaway, which is often the hardest step on an older property. Second, put a camera through the feed pipes: a CCTV drain survey shows whether the pipe is blocked, broken or root-bound, and confirms exactly where the run goes, all without lifting a spade. Third, test the soakaway itself by introducing water and watching how it copes. Only then does anyone talk about excavation.

The fixes follow the findings. A blocked feed pipe is usually cleared with jetting. A broken pipe is a targeted drain repair at the point of failure rather than a trench across the garden. A genuinely silted or undersized soakaway is rebuilt, normally with modern crates, a silt trap and the right capacity for today's roof and paving, sited properly away from the building. None of that is guesswork when a camera has already shown you the problem.

Frequently asked questions