How to build a soakaway for garden drainage.

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What is a soakaway/drainage pit?

Soakaways or rainwater drainage pits are part of a garden drainage system. It's a large hole that collects water from a French drain system, allowing the water to soak-away into the ground.

This article describes how to construct a Soakaway for draining water away from a domestic garden and is best read together with my other drainage related pages linked from here. lawn and garden drainage 

Diagram of a Soakaway system
Diagram of a Soakaway system

Soakaways also known as drainage pits are a simple way of collecting and dispersing surface and storm water when connection to the domestic drain system is impractical. However, Soakaways rarely work when dug out in heavy clay. Clay soils do not allow water to pass through. Instead, the water collects rather like a pond.  Most gardens in London, Surrey, parts of Berkshire and surrounding counties are built on clay.

Other options. If you can't connect into the flood water system and the garden soil is mostly clay, another option to disperse water is to install a pump and sump system.

Protecting property foundations. To avoid water collecting in the soak away leaking into and damaging the foundations, The soakaway  should be situated at least 5m away from any building.

The size of the soak away will need to be worked out in advance. Generally a soakaway built on non clay soil to take and disperse water from a gravel French drain system draining a 120 m2 waterlogged garden will not need to be  1m square and 1.5m deep.  I have included a formula below to calculate the required volume of a soak away constructed to take roof water and  garden drainage.

Positioning the soak away. The Soakaway should be sunk in the lowest part of the area being drained, with the French drains sloping toward the Soakaway. I have included a. Drainage guide  lower down this page.


Scroll on down for; Best position to build a soakaway, How to build a soakaway, Calculating the size.



Tools and materials to dig out and build a soakaway

What's the best position to build a soakaway

  • If the soakaway is being used as a standalone drainage system it should always be positioned in the lowest part of the area being drained, so that water will flow into it and at least five meters from any property to avoid water soaking into the foundations.
  • If the soakaway is part of a French drain system, again it should be positioned at the lowest point, lower than the drains so that water from the French drains will flow into it. This will avoid having to use pumps to transfer the water from the trenches into the soakaway.   The fall along the entire length needs to be just enough to keep the water flowing.   Again, position the soakaway at least five meters from any property.

How to build a soakaway

  • To construct a basic stand alone soak away or one to use as a dispersal point for a French drain system where the water will gradually seep away into the surrounding soil.
  • Dig a large hole in the ground, line the sides with heavy-duty landscaping fabric, to keep the earth from over time clogging up the gravel. (fabric available above).
  • Next dig the trenches that are discharging into the soak-away ensuring that there is an adequate fall along the entire length, including the side drains if you are using a herringbone pattern. This page on my website explains how garden drainage works and is installed.
  • Connect the trenches to flow into the hole. Tidy up the soak away and trenches by removing any loose soil that has fallen in.
  • Fill with large chunky clean rubble or 20mm gravel to about 5" from the top.
  • Wrap the fabric over the top and shovel in topsoil, slightly overfilling to allow for settlement.
  • You're done, leave the earth to settle for two or three weeks and turf or sow grass seed over the top.

If you are using the pre wrapped soak away crates featured above in place of gravel, line the bottom of the hole with gravel to get it level and fit the crates, back fill any gaps around the edge with gravel and cover with earth.

Calculating the size of a soakaway.

The types of soakaway I have described here can also be used to disperse roof water on non-clay soil. Getting the size of the hole correct to be able to handle the volume of water is important.

This formula is generally used to calculate the volume of the soakaway and should be suitable for most situations.

  • C= (AxR)/3

Where C is the capacity or volume of the soakaway in m3
A is the lawn area or roof of the building in m2
R is the rainfall expressed in m/h (metres / hour)

For the rainfall a standard 50mm per hour is usually used which equates to 0.05m/h

Using this formula we can quickly calculate the size of the soakaway that needs to be installed. Say we have an area of 60 m2 and we us the standard rainfall of 0.05 m/h the calculation is as follows: C=(60x0.05)/3- C=(3)/3-C=1 m3

Step by step DIY guide to install French drain garden drainage draining into a soakaway.

Garden and lawn drainage D.I.Y. guide

  • Installing a French drain system
  • Marking out and digging trenches
  • Finding lowest spot for positioning Soak away
  • Materials and Quantities (how much gravel)
  • Telephone back-up 
  • Simple easy to follow steps

D.I.Y. garden drainage page.



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Converted to $ and €  when purchased outside the U.K.

This is an easy to follow basic guide to help install a french gravel drain.

Our gardens can help combat global warming by catching more carbon than they release. 


create a rain garden and help the environment

This article will explain

  • What a rain garden is and what it's used for.
  • Where to site it.
  • Suitable plants to plant in a residential rain garden.
  • How it is different from a bog garden.
  • How to build/create a rain garden.