How to install a drainage cube soak away

An in-depth guide on installing garden soak away areas using drainage crates.
Diagram of 2 drainage cubes underneath a lawn

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A garden soak away area is used to redirect water from where it’s not wanted. They essentially act as a place for the water to sit and soak away slowly leaving other areas of the garden dry.

Soakaway areas can be combined with types of channel drain and French drain to make up part of a larger garden drainage system.

Soakaways can be ideal for some moderate to bad garden drainage issues.

Its essential understand why your garden is flooded or waterlogged before you decide on a drainage solution. 

This article will walk you through an example of how to install a drainage cube soakaway area including all measurements, considerations and tricks of the trade. Installing drainage can be dangerous and cause damage to property if done incorrectly, be sure to fully understand what you are doing before you begin. 

Using our fictional example we’re going to:

  • Work out the positioning of the soakaway in a garden 
  • Work out how big the soakaway needs to be for the size of issue
  • Dig the correct holes 
  • Install the drainage 
  • Cover the area up again

What you will need for the job

Tools 

Personal protective equipment

Materials 

Not sure if a soak away is the best solution? Use our free drainage survey tool to see if there is a better solution for your garden drainage issues

Step by step guide to installing soakaway garden drainage

If other less labour intensive garden drainage options are not an option, installing drainage cube soakaway areas can be an effective way to improve your drainage. 

Step 1: Work out how much drainage capacity you need

To be effective, soakaway areas need to have enough water storage capacity. Let’s look at one way we could work out how much storage capacity our soakway would need. 

As you can see there is some waterlogging in the grass which leaves it boggy and unusable for a couple of days after it has rained. This is damaging the grass and threatens to damage the patio foundation too. 

To work out how much water needs to be drained first we need to work out the rough surface area.

Start by measuring the size of the waterlogged/pooled area a few hours after it has rained.

Measure what look like the widest points of the area then work out roughly what the surface area would be. This doesn’t need to be exact.

The tape measure above is a great dirt resistant, sturdy tape measure. Great for outdoor work.

Surface area

Average diameter: 3.17m 

I will treat this as a circle to simplify the calculation so google tells me the surface area of a circle with a 3.17m diameter is 

Surface area: 7.89m2

Depth

Next measure the depth of any surface water and add on 80mm for water stored in the top layer of turf. If you want to be more thorough you can account for more depth. It’s better to have too much drainage than too little. 

I am adding 80mm to the depth of the water to account for water stored in the soil 

Centre point depth: (30mm + 50mm) 80mm

Edges: (5mm + 50mm) 55mm

The centre has much less area than the outer, shallower areas so i will offset my average for this: 

((Centre depth x 1) + (edge depth x 3)) / 4

Average depth = 61.25mm (0.061m)

Volume

Surface area (7.89m) x Average depth (0.061m)

Total volume of water to be removed: 0.48 m3 

Note: Leaving the area to drain for a few hours after raining will give a more accurate indication of the drainage issue

Step 2: Work out how big the soakaway needs to be

The soakaway will be built from drainage cubes, most of which have a capacity of 200L or 0.2 cubic meters of water (see image below).

We recommend Polystorm soak away crates due to their extremely sturdy design.

The gravel above and around the drainage cubes has a storage capacity of around 30% which means it can hold 30% of its volume in water in the gaps between the stones.

Typically a drainage crate like this one will have 50mm of gravel around it so work out the volume of this gravel then multiply it by 0.3 to get the estimated capacity of the gravel.

In our example it looks like we will need 2 or 3 drainage crates, so let’s work out if we can get away with 2. I have the assistance of a computer program but it’s easy enough to work this out. 

Work out the surface area of the gravel 

Storage volume of gravel

Volume of the gravel: (surface area of gravel x 0.05) x 0.3 = 0.23m3 

Pro tip: Most gravel is bought by weight rather than volume. Multiply the volume of gravel by 1.6 to get weight in tons for ordering

Storage volume of gravel = 0.07m3

Total storage volume of soak away with 2 crates: 0.47m3

This is close enough that an additional crate may not be worth the effort and expense however if you want to be sure it will work then it could still be a good idea to have additional drainage capacity. 

Note: Over time the capacity of soakaway crates diminishes because they fill with sediment so it can be a good idea to have additional capacity if it is practical. 

Step 3: Choose the right location for the soakaway

Ideally you want your soakaway to have maximum effect, require the least work and cause as little damage as possible to its surroundings. To achieve this you need to take 2 main factors into account: 

  1. Where would it be the most effective
  2. Where will it cause the least disruption during and after installation

The centre of the cross is roughly the centre of mass of the drainage issue. Thankfully our 1.1m x 1.1m soak away fits nicely in this area so not much consideration needs to be given to surrounding features. 

Your situation may be different so it is still essential to understand the impact that excavation can have on surrounding features. 

Digging can weaken and collapse the surrounding ground which can cause damage to surrounding features.

Digging close to buildings also increases the risk of subsidence which can damage foundations and cause larger issues. We would recommend that excavations be no more than 0.8m depth and are at least 4m away from any house. If you need to dig closer to the house then consult a professional. 

Note: Make sure to follow safe digging guidelines at all times when excavating ground for garden drainage

Step 4: Installing the drainage crates 

You have worked out the location and planned the materials, now it’s time to install. This should be done on a dry day if possible. 

Using a good quality spade, carefully cut the top turf into strips, and create turf rolls which can be placed to the side then laid back down afterwards.

Mark out the correct sized hole  (1.1m x 1.1m for our example)  which will contain your drainage crates and the surrounding gravel. 

Depth

The image above shows our recommended depth and layout. 

Using a spade and a wheelbarrow, remove the soil or clay. Leave aside any good quality soil to refill the subsurface layer afterwards. 

We recommend getting a skip or someone with a tipper van to help dispose of the waste.

Geotextile

Wrap the drainage crates in geotextile fabric. This stops them filling with large sediment and prolongs their life span. 

Using 20mm limestone gravel or the equivalent, fill the bottom of the pit to 50mm and level off. Place in the wrapped drainage crates then fill any remaining space around the sides with gravel. 

Make sure to prod it and stamp it down to fill any gaps. Gaps could lead to sinkage in the future. 

Cover the crate with a further 50mm of gravel. 

Finally, cover up the surface with at least 100mm of soil or sub base then lay your turf or alternative surface back on top

Done properly, there will be no visible signs that any work was carried out. 

Conclusion

And there you have it, a fully installed soak away to take care of your garden drainage issues. If you are unsure which drainage solution is right for you then you should try our free drainage calculator tool which gives personalised recommendations based on your situation. 

If you are based in the North west and want professionally installed garden drainage then get in touch for a quick online estimate. 

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