Soakaway vs drainage field — what's the difference?

In everyday use people say them interchangeably, but for a septic tank the correct term is drainage field: a network of perforated pipes in trenches designed and sized specifically to treat septic tank effluent. A generic 'soakaway' — like a crate-based system for rainwater — isn't built or sized to handle sewage effluent and isn't a compliant substitute.

Why has my drainage field failed?

Almost always because it's been overloaded — most often by an overdue septic tank letting solids carry over into the field and clog the soil, but also by age, poor original design, or a high water table. Once the soil clogs, effluent stops soaking away and starts surfacing instead.

Can I use a soakaway crate instead of a drainage field?

No. Soakaway crates are designed for surface water (rainwater run-off), not partly-treated sewage effluent. A septic tank's discharge must go through a proper drainage field built to BS 6297, sized from a percolation test — not a rainwater soakaway.

How big does a drainage field need to be?

It depends on how well your soil drains, found from a percolation test, and how much wastewater the property produces. Poorly draining soil needs a bigger field for the same property; a percolation test is what tells an installer the right size.

Septic tank soakaways & drainage fields explained

A drainage field is the network of perforated pipes, buried in shallow trenches, that a septic tank’s liquid flows into after the tank has settled out the solids — letting the surrounding soil finish treating the wastewater before it re-enters the ground. It’s what makes a septic tank legally compliant: under the General Binding Rules, a septic tank can only discharge to the ground through a properly built drainage field. It cannot discharge straight to a stream or ditch.

How a drainage field works

Liquid leaving the septic tank is still only partly treated. It’s distributed along a network of perforated pipes laid in gravel-filled trenches, where it seeps out gradually. As it passes through the surrounding soil, naturally occurring bacteria complete the treatment before the water rejoins the water table. No electricity, no moving parts — just the tank, the pipework and the soil doing the work.

The percolation test

Before a drainage field is installed, the ground needs a percolation test — a simple test of how quickly water drains through the soil. It determines how big the field needs to be: free-draining soil needs less pipework for the same property; slow, clay-heavy soil needs a bigger field to cope with the same volume. Skipping this step is a common reason new or replacement fields fail early.

Drainage field vs a soakaway crate

It’s a common mix-up: a soakaway in the general sense — often a crate or rubble-filled pit used to disperse rainwater run-off from a roof or driveway — is not the same thing as a septic tank drainage field, and isn’t a compliant way to discharge septic tank effluent. A drainage field for sewage has to be built to BS 6297, sized from a percolation test, and kept the right distance from watercourses, boreholes and buildings. A rainwater soakaway is designed for clean surface water, not partly-treated sewage, and using one for septic tank effluent is both non-compliant and likely to fail quickly.

Signs your drainage field has failed

Boggy or unusually lush, green patches over the field, even in dry weather.
A persistent smell of sewage outside, especially near the field.
Slow-draining sinks, baths and toilets across the whole house.
Sewage surfacing on the ground above the pipework — treat this as urgent.

A failing field is very often the knock-on effect of an overdue tank — see signs your septic tank needs emptying and how often to empty. Keeping the tank emptied on schedule is the single best way to protect the field, because it stops solids carrying over and clogging the soil.

Distances and standards

A compliant drainage field must be built to BS 6297, discharge no more than 2 m³/day, and sit outside a groundwater Source Protection Zone 1 — along with keeping proper distances from watercourses, boreholes, buildings and boundaries. See our General Binding Rules guide for the full conditions.

Replacing a failed drainage field

Once a field has clogged, it usually needs replacing rather than repairing — the soil itself has lost its ability to treat effluent. A replacement is sized from a fresh percolation test, so it’s built for what your actual ground can take. See soakaways & drainage fields for how we approach it, or get an indicative figure from the cost calculator.

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Sources: GOV.UK — General binding rules: small sewage discharge to the ground · GOV.UK — Approved Document H: drainage and waste disposal · BGS Geology Viewer