How to install a garden soakaway

Inspired by a helpful YouTube guide. We based this walk-through on the popular UK video "Build a Better Soakaway in a Day!" from the Skill Builder channel, which covers the whole process clearly — including the crate system that has largely replaced rubble fill. Worth watching before you hire a mini-digger; the section on testing drainage first will save you a wasted day.

1. Test whether a soakaway will work in this spot

Soakaways rely on the soil absorbing water. They do not work in heavy clay. Before digging anything, do a simple percolation test: dig a small hole about 300mm square and 300mm deep, fill it with water, and watch how long it takes to drain away.

If the water is gone within an hour, you have reasonable drainage. If it sits there for half a day, a soakaway is not going to solve your problem and you need a different approach — a French drain to a ditch, or a connection to the surface water sewer.

Also check position. Building Regulations require a soakaway to be at least 5 metres from any building and 2.5 metres from a boundary. Do not site one under a drive or patio where you cannot get back to it.

2. Mark out and dig the pit

For a typical domestic downpipe — one drainpipe from a roof — a pit of around 1.8 metres square and 1.2 metres deep is a reasonable starting point. Larger roofs need bigger pits; there are online calculators that will work it out from roof area and local rainfall figures.

Hire a mini-digger if the ground is hard or the hole is big. That said, a spade and a good afternoon works fine for a smaller pit. Keep the spoil well clear of the edge. Pile it on a tarp so you can move it easily.

If you hit groundwater before you reach the target depth, stop. You do not want the soakaway sitting in a water table.

3. Choose your fill material

There are two main options. Rubble fill — broken brick and hardcore — is the old way. It works, but it leaves relatively little void space for water to occupy, so the pit needs to be large.

Plastic soakaway crates are better. They are lightweight polypropylene structures that stack together to fill the pit with very high void ratio — around 95 percent empty space versus roughly 30 percent for rubble. You need a much smaller excavation for the same storage volume. They cost more than rubble, but the digger hire you save usually covers it.

4. Line the pit with geotextile membrane

Before filling, line the entire pit — sides and base — with geotextile landscape fabric. This is the critical step that most failed soakaways skipped. The fabric lets water pass through into the soil but stops silt migrating back in and clogging the voids over time.

Cut the fabric generously. You need enough to fold over the top of the fill once it is in place. Overlap joins by at least 300mm and secure with tent pegs temporarily while you work.

5. Install the inlet pipe

The inlet pipe runs from the downpipe or surface water channel into the top of the soakaway. Use 110mm perforated or slotted drainage pipe — the standard orange or grey underground drainage size.

The pipe should enter the crate stack or rubble fill near the top, not the bottom. This gives the water somewhere to go as the pit fills from the bottom up. Lay the pipe on a slight fall towards the soakaway — around 1:40 is plenty.

If you are connecting a downpipe, use a standard 110mm underground drainage adapter. Fit a rodding eye near the entry point so you can clear any blockage later without digging.

6. Backfill and reinstate the surface

Fold the geotextile membrane over the top of the crates or rubble. Lap the edges so the fill is completely wrapped. Then add a layer of pea gravel or fine gravel — about 150mm — over the membrane to protect it.

Finally, backfill with the excavated topsoil. Compact it lightly in layers. Leave it proud of the surrounding surface by 50mm or so to allow for settlement. Turf or seed over once it has settled.

Do not build over the soakaway with a hard surface. It needs to remain accessible, and vegetation above it helps with evapotranspiration, which actually improves performance.

When to call a handyman

Call Richard if the percolation test shows poor drainage and you need advice on alternatives, if the run from the downpipe is long or needs digging under a path, or if the soakaway is close to the house foundations and needs careful positioning. A job done properly in a day is always cheaper than fixing one done wrong.