Soakaway problems: why they fail and how to fix them

A soakaway is a buried void — traditionally a rubble-filled pit, more often now a crated plastic chamber — that takes surface water from roofs and hard standing and lets it disperse slowly into the surrounding ground. It exists because there is nowhere else for the water to go: no surface water sewer to connect to, or a sewer that won't accept any more. Rural and edge-of-village property across Northamptonshire and the surrounding counties relies on them heavily.

A soakaway only works if the ground around it can actually absorb water at the rate the roof delivers it. That single condition explains almost every failure we get called to.

The classic symptom is water standing above or around the soakaway after rain, sometimes for days. Downpipes that back up and overflow at the gully, a gully that gurgles or brims during a downpour, and a patch of ground that stays permanently boggy through winter but dries in summer are all the same story. So is damp appearing at the base of a wall on the side of the house where the downpipe discharges.

The seasonal pattern is diagnostic. A soakaway that copes fine in summer and fails from November onwards is not blocked — it's saturated. The surrounding soil is already full of water, so there is nowhere for the soakaway's contents to go. A soakaway that fails year-round, including in dry spells, is more likely to be silted, crushed or blocked at the inlet.

Clay soil is the most common cause in this region. Heavy clay has very low permeability, and a soakaway installed in it will discharge slowly at the best of times and not at all when the ground is wet. Many were dug anyway, without a percolation test, because it was the quickest option at the time.

Silt is the second cause. Roof water carries grit, moss, leaf debris and sediment, and over ten or twenty years that material fills the voids in a rubble pit or blinds the geotextile wrapping a crate system. The chamber is still there, it just can't take water in. Undersizing is the third: an extension, a conservatory or a resurfaced driveway adds roof and hard standing area to a soakaway that was sized for the original house. Add a high winter water table — common near the Nene and Ouse valleys — and a system that was always marginal stops working entirely.

Then there is the physical damage category: crushed crates under a driveway that was later used for parking, a collapsed rubble pit, tree roots colonising the void, or an inlet pipe that has silted or fractured before the water even reaches the chamber.

Start at the inlet. A camera survey of the pipe running from the gully to the soakaway will show immediately whether water is even reaching the chamber — a silted or collapsed inlet is a much cheaper fix than a failed soakaway and it's more common than people expect. If the pipe is clear and the chamber is full, the ground is the problem.

A percolation test tells you what the soil can actually do. Dig a test pit near the soakaway, fill it with water and measure how fast the level drops. It's the standard method used to size a new soakaway, and it will tell you honestly whether a replacement in the same location has any chance of working. Doing this before spending money is what stops people installing a second soakaway that fails exactly like the first one.

If the inlet is silted, jetting the run and cleaning the gully restores the system for a fraction of the cost of anything else, and should always be ruled in or out first. If the soakaway itself is silted but the ground is workable, excavating and rebuilding with modern crates wrapped in geotextile, plus a silt trap or catchpit on the inlet, will give a long service life — the catchpit is what stops it silting up again.

If the soil is genuinely unsuitable, a bigger hole in the same clay is not the answer. The options are relocating the soakaway to more permeable ground, increasing capacity substantially so it acts as attenuation and releases slowly, connecting to a surface water sewer or watercourse where one is available and permission can be obtained, or in some cases a pumped arrangement. Any of these should follow a percolation test, not precede one.

Building Regulations Part H sets the basic requirements: a soakaway should generally be at least five metres from a building, and it must not be positioned where it could affect foundations or a neighbouring property. Foul water must never be discharged to a surface water soakaway — that's both illegal and an environmental offence, and misconnections do turn up in older properties where an extension was plumbed in carelessly.

If you're on a septic tank rather than mains drainage, the drainage field is a separate system from a surface water soakaway and is covered by the general binding rules for small sewage discharges. The symptoms of a failed drainage field look similar, but the diagnosis, the rules and the fix are different — worth being clear which system is actually failing before work starts.