How to build a soakaway: Step-by-step

Knowing how to build a soakaway will potentially help homeowners protect their homes from surface water flooding, as well as being a sustainable drainage option.

Surface water flooding is becoming a common problem in the UK and by adding a soakaway to your garden or under your driveway, you can potentially solve waterlogging and naturally drain excess rainwater back into the ground.

The UK was on average 1% wetter between 2009 and 2018, than between 1981 and 2010, according to the Met Office. And it was 5% wetter than between 1961 and 1990 — so installing drainage systems to our homes is becoming increasingly important.

How to build a soakaway: Step-by-step

A soakaway is in its most basic form a hole dug into the ground, which is then filled with rubble or — more ideally these days — specialist soakaway crates. This allows surface water additional space to percolate back into the earth in a measured way.

A soakaway's construction therefore involves digging a large hole in the ground and stacking soakaway or attenuation crates within it. The crates feature voids, allowing large volumes of water sufficient surface area to seep back into the ground. However, there are a few additional checks you need to make before getting started.

We asked property and drainage expert Gregory Smith at building jobs website PriceYourJob.co.uk for a step-by-step guide on installing a soakaway. We also have a more in-depth explainer in our article what is a soakaway if you are looking for more information before you learn how to build one.

1.       Check your soil is permeable

Before you start, you first need to ensure that the ground beneath is permeable, which will require a soil percolation test. There's no point installing a soakaway if your soil won't allow water to naturally seep into it so this test is essential before building this kind of rainwater drainage system. You can read more about what is a percolation test in our guide.

2.       Calculate your soakaway size

Make sure it’s adequate for the size of your house and grounds, otherwise it will be overwhelmed.

3.       Select a soakaway filter

Adding a filter to your soakaway will stop silt and dirt clogging up your soakaway. You can buy these off-the-shelf to sit between your drainage pipe and soakaway crates.

4.       Dig your hole and level the bottom

When using a crate system, be sure to level the bottom of your hole and layer it with shingle or sharp sand that’s compacted down. This will give the crates a solid base to sit on.

5.       Buy your crates

Crates come in different strengths. So, if you’re installing your soakaway beneath a driveway, check the loadbearing strength before buying.

6.       Use geotextile to line your crates

Always line crate systems with a geotextile. It needs to be the non-woven type as this creates a barrier to stop soil getting in and allows water to filter out.

7.       Ensure a snug fit

Make sure your geotextile is wrapped around the crates and pipe entry point, so they don’t clog up. Use overlap tape to be sure of a snug fit.

8.       Don't spill your soil

Whether you’re using crates or stones for your soakaway pit, be careful not to spill soil inside your geotextile lining as it won’t work effectively.

9.       Fill your soakaway

Fill around the sides and top of crate soakaways with a good, compact layer of sand or gravel, before backfilling with soil. This helps to prevent movement.

How deep should a soakaway be?

A driveway soakaway pit needs to be set deeper than a lawn soakaway, so you may need to dig down at least 1.5m.

The depth of your soakaway depends on where it’s going to sit. The bigger your roof and the greater your driveway’s surface area, the more rainwater you’ll have to deal with.

If you need a large and deep soakaway, you may find hiring a mini digger makes it easier.

What is the rule of thumb for soakaway?

The soakaway should be at the lowest point in the rainwater drainage system. The width and depth of the hole is determined by the size of the crates you will be using.

When calculating the size of the area that needs excavating, remember to allow extra width, length and depth for a base, side, and backfill. And remember, if it is under an area of traffic, such as a driveway, it will need to be deeper than if it is in an untouched area.

According to CNC Building Control, a partnership of five local authority building control departments in the east of England, an effective way of calculating the size of a crate soakaway where the ground has good soakage (such as sand or gravel) is:

Volume = Roof area being drained x (50mm rainfall rate per hr/3000).

For example, a 50m2 roof area would require a crate soakaway of

50m2x(50/3000) = 0.84m3

To calculate the size/volume of a rubble-filled soakaway, the same calculation can be used but the result divided by 0.3. For example, 50m2 roof area requires a rubble-filled soakaway of (50m2x(50/3000))/0.3 = 2.8m3

If a soakaway isn't far enough from the home, or isn't draining correctly, this could cause issues for the home's foundations. And bear in mind soakaways aren't suitable for poor draining soil.

Gregory Smith

Gregory Smith has worked in the property and construction industry for over 20 years.

He currently continues to work on domestic building projects while also providing expert comments in the property and construction field for PriceYourJob.

He has worked with PriceYourJob for the past four years and has been published in a range of publications