How does a soakaway work, and do I need one?

A soakaway is not a hole that water disappears into. It is a temporary store, sized so that it can hold a storm's worth of water and release it into the surrounding ground slowly enough that the ground can take it. That means two things decide whether one is viable: how fast your soil absorbs water, and how much water you are asking it to take. Both are measurable before you dig anything.

Safety

Step by step

1. Establish that you actually need one.A soakaway is the answer where surface water has nowhere else lawful to go — no surface water drain, no watercourse, and a foul drain that must not receive rainwater. If there is an existing surface water drain with capacity, connecting to it is usually simpler.
2. Do a percolation test before anything else.Dig a hole about 300mm square and 300mm deep at the intended location, fill it with water and let it drain to wet the sides. Fill it again and time how long the water takes to fall from 75% to 25% full. Repeat three times and average. Fast drainage means the ground will work; water still sitting there the next day means it will not.
3. Understand the clay problem honestly.On heavy clay a soakaway fills and stays full. It becomes a pond with a lid, it backs up into whatever drains into it, and in winter it never empties. No amount of extra crates fixes ground that does not absorb. On clay the answer is a drain to a lawful outfall, or a much larger attenuation system designed by someone who does this professionally.
4. Keep it well away from buildings.Water discharged into the ground close to a foundation can soften the bearing soil and, on shrinkable clay, cause heave. Guidance places soakaways a substantial distance from any building — commonly at least five metres — and away from boundaries, retaining walls, other soakaways and any drainage field. Confirm the distances with building control for your situation.
5. Size it against the area it drains.The volume needed depends on the impermeable area feeding it, the rainfall intensity for your location and the soil's infiltration rate from the percolation test. This is a real calculation, not a rule of thumb, and getting it wrong produces a soakaway that overflows every heavy storm.
6. Build it as a wrapped void, not a hole full of soil.Either modular plastic crates or clean, single-size stone — 20 to 40mm, no fines — with the whole void wrapped in a permeable geotextile so silt cannot migrate in and clog it. The geotextile is what gives a soakaway a long life.
7. Get the water into it cleanly.The inlet pipe should enter above the stored water level, and there should be a silt trap or a catchpit upstream so leaves and grit are caught somewhere you can clean rather than inside the soakaway where you cannot.
8. Provide an inspection point and an overflow route.An access chamber over or beside the soakaway lets you check whether it is emptying. And think about where water goes when the soakaway is full in an exceptional storm — somewhere harmless, not against the house.
9. Check whether the work is notifiable.Drainage for new buildings and new hard surfaces falls under building regulations, and a soakaway serving them is part of that. Where a new front garden hard surface relies on drainage to a permeable area, the planning position depends on it too. Check both with your local authority before building.

Tips

• Do the percolation test in winter if you can, or at least when the ground is not baked dry. A summer test on cracked clay gives a flattering and completely misleading result.
• Crates are far quicker to install than stone and give much more void per cubic metre excavated, which means a smaller hole for the same storage.
• Photograph the soakaway and measure its position from two fixed points before backfilling. Nobody ever remembers where it is ten years later.
• A rainwater butt on every downpipe reduces the peak flow reaching a soakaway and is the cheapest thing you can do to help it cope.

Common mistakes

• Digging a hole, filling it with rubble and calling it a soakaway — Rubble contains fines that fill the voids, so it stores very little water and clogs quickly. Clean single-size stone or crates, wrapped in geotextile, is what makes it work.
• Building one close to the house — You are deliberately introducing water into the ground near a foundation. On shrinkable clay this can cause structural movement, and the distance requirements exist for that reason.
• Connecting a soakaway's overflow to a foul drain — Rainwater into a foul sewer overloads it, causes pollution incidents downstream and is an offence. If the soakaway cannot cope, the answer is a bigger or different system, not a foul connection.
• Skipping the percolation test — It is a couple of hours of work and it is the only thing that tells you whether the whole plan is viable. Building first and testing by observation costs a great deal more.

If it doesn't work

Questions people ask