The Ultimate Soakaway Guide

This page is all about soakaways, what they are, where they’re located, alternative options for drainage, regulations regarding rainwater and much more including a detailed look at soakaway prices.

For the avoidance of doubt, this guide is for rainwater soakaways, i.e. water from your roof, driveway or garden. Foul water from your toilet and sinks should never be directed into a rainwater soakaway, and a “foul water soakaway” is something entirely different.


This guide is over 1500 words long, so we have broken it down into digestible sections:

What is a Soakaway?

A soakaway is a hole dug into the ground and filled with plastic crates; this creates a void where rainwater can be stored until it seeps into the ground.

The “old school” method of filling a hole with rubble or other waste material is now obsolete, and you should only consider it for tiny areas that receive very little rainfall. Plastic crates are the best way to build an efficient soakaway that will last for decades.

Soakaways are designed to cope with large amounts of surface and roof rainwater, without them our homes and neighbourhoods would experience flash floods whenever it rained.

The rainwater from roof gutters, patios and driveways should ideally be directed towards a soakaway within the boundary of the property. If that isn’t possible, there are other options.

Soakaway crate

Plastic soakaway crate system

Regulations in a Nutshell

The following is just an overview of the critical points and is not intended to replace the official regulations document.

  • Should be located at least 5 metres from the wall of any building
  • At least 2.5 metres from the boundary
  • The water table should not reach the bottom of the pit at any time of the year
  • Should not be close to another soakaway or in a location where the water may become contaminated
  • The soil should be percolation tested (not much point in putting a soakaway in clay soil that won’t let the water drain away)
  • The size/volume of the soakaway should be based on a drainage calculation (this is relatively easy once a drainage percolation test is completed and you know the surface area where the water is coming from)

How to Quickly Test if a Soakaway Will Work

In the old days, a test pit was dug, and water poured into it and visually monitored to see if it drained away.

The percolation test is still the best way to check the soil in your garden, but there is a calculation you can use which is more reliable than trusting your eyeballs.

You should dig a 300 square millimetre hole 300mm below the lowest point of the proposed drainage pipe.

This hole should be filled with water, and the time it takes for the water to drain away measured. This will provide you with a figure that can be used to calculate the required water storage volume for the soakaway.

The figure you want is the number of seconds it takes for one millimetre of rainwater to drain away.

You can find details about the exact calculation you need to carry out by reading this pdf.

Soil percolation test

Test the soil before you invest in an expensive soakaway

Simplified Way to Calculate the Size of a Soakaway

While we recommend your ground-worker conducts a soil percolation test and creates a set of calculations, there is an alternative method that will give you a rough idea of the soakaway size you’ll need:

If your soil type is not too heavy/clay, you can assume that one crate (1000mm L x 500mm W x 400mm H) will be sufficient for ten sq metres of driveway or roof covering etc.

Five crates are equal to 1 cubic metre, and one cubic metre is suitable for 50 square metres of surface area.

So get your tape measure out and calculate the total surface area in square metres.

List of Materials Needed to Build a Soakaway

Here’s a list of materials needed when constructing a basic rainwater soakaway:

  • Soakaway crates (£200 per cubic metre)
  • Membrane felt (£15)
  • Underground pipe (roughly £6 per metre inc fittings)
  • Gravel (£variable)
  • Gully, drain head or other devices (see here) to stop leaves from entering the pipe (£variable)
Soakaway in a deep hole

Plastic crates wrapped in a membrane to prevent soil blocking the system. Image from Watlington Drain Renovations

Cost to Build a Simple Small Soakaway

The price guide below is for a one cubic metre soakaway located under a lawn or flowerbed, we’ve made some assumptions, and there are other costs to consider.

Assumptions:

  • a single pipe leading to a one cubic metre soakaway
  • location: in the garden, under the lawn
  • easy access to the garden
  • price excludes other landscaping work such as patio laying, tarmacking the surface etc

Materials – £325.00

Labour 2 people for one day – £300.00

VAT – £125.00

Total from – £750.00

We feel that the minimum price for a small garden rainwater soakaway is £750, but you’ll probably pay more, so keep reading.

Other Costs to Consider

If your soakaway is located under a patio or driveway then the cost to lift and re-lay the material could be more than the soakaway itself. The price depends on what type of surface you have; blocks, concrete, tarmac, patio slabs etc.

Mini digger hire isn’t included in our price guide. If you need to dig deep, it will cost you more. How deep will depend on your garden, elevated locations are an issue for soakaway construction as you’ll need to dig deep.

Ease of access is also a key issue. If you don’t have direct access to the location of the soakaway, then your tradespeople may have to go through the house or via a neighbour’s property. This will add time and cost to the project.

If you have several different surface areas connecting to the same soakaway (roof gutter, conservatory, patio, driveway etc.) then more piping and man hours will be needed.

If your soakaway is located under a driveway, it will require reinforced crates that cost more.

Waste disposal isn’t cheap, so if the excavated material can’t be left on site, it will need to be taken away at a cost.

For garden surface drainage, large-scale trench excavation and pipe installation can be time-consuming and costly.

Price Table

Check out our price table below, all prices include VAT and are estimates for the installation of a domestic rainwater soakaway:

Project:Price:
Basic 1 cubic square metre soakaway construction£750.00
More complex soakaway construction with additional depth, pipework, size or reinforcement£1500.00
Difficult, unusual or larger project with complex pipework, difficult access, deep excavation through hardened materials (concrete, garden walls etc), disposal of waste material and reinforced materials£3000.00+

Alternatives to a Soakaway

As a general rule of thumb, if you can install a soakaway for your driveway or roof rainwater etc. then you should.

There are occasions when it’s impossible to create a soakaway that works sufficiently:

  • Your garden soil is made of thick clay, and the water doesn’t drain away
  • The water table is high, and you can’t build a soakaway that doesn’t fill with groundwater
  • You cannot build one within the constraints of the regulations (within 5 metres of a building or 2.5 metres from a boundary etc.)

If a soakaway is impossible to achieve, you can discharge the water into a local watercourse if you live next to one.

If that isn’t possible, you can discharge the rainwater into a sewer, although you will need to notify the water company and they will charge you a yearly fee for this.

Here’s a quote from the building regulations and can be found on the official Planning Portal website:

Where it is impractical to use infiltration (eg. because of nearby foundations, impermeable or contaminated ground, or high groundwater), it is preferable to discharge it to a watercourse or, failing this, to a surface water sewer or, as a last resort, to a combined sewer. Surface water must not be discharged into a foul drain or sewer.

In the United Kingdom, we have many different water companies, and they all have slightly different procedures you’ll need to follow.

Here is a quote from Seven Trent:

We will only accept surface water (rain water) into a public sewer if there are no alternative options. If there are both foul and surface water sewer in the vicinity which can be used then you must make 2 separate connections. You’re responsible for providing supporting evidence that there are no alternatives when applying for a combined connection.

The most common option for surface water drainage is a soakaway. If this is not feasible please provide us with evidence that this is the case. This needs to be either:  An extract from the ground investigation report, results from a percolation test or an email/letter from building control.

Water companies will charge you a yearly fee for discharging rainwater into their sewer system. This is to cover the cost of treating the water and increasing their storage capacity, which is vital during and after heavy periods of rainfall.

Southern Water currently charges a very reasonable fee of £24 per year. Other companies’ prices may vary so do check with your local provider.

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We hope you found our guide to rainwater soakaways insightful.

We’ve teamed up with a service provider that can supply you with a competitive quotation for a soakaway.

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