Top Ten Rainwater Soakaway Questions and Answers

If you need to install a garden soakaway, you’ve probably got a few questions that require answers.

On this page, we’ll answer the top ten soakaway questions.

Before you read any further, we must point out that this page refers to rainwater soakaways and not foul water storage systems.

1 – What is a Soakaway?

A soakaway is a large hole dug into the ground usually around 5-8 metres away from a property or in the middle of a garden. Rainwater from roof gutters, conservatories, driveways, patios and gardens is diverted into it where it then seeps into the ground. The purpose of a soakaway is to temporarily store rainwater that would otherwise cause a flash flood during heavy rainfall.

2 – How Far Away From the House Must it be Located?

The regulations state that a rainwater soakaway must be located at least five metres from the wall of a building and at least two and a half metres from a boundary. This is to prevent subsidence of the wall and to stop rainwater on your property flowing into your neighbour’s garden.

Rainwater soakaway diagram

3 – How Deep Should a Soakaway be?

There is no minimum or maximum depth. The amount of water the soakaway can hold and how quickly (or slowly) the water seeps away into the ground are the critical factors, not vertical depth or horizontal spread. You should locate the soakaway at a low point in the garden, so water runs down to it (water doesn’t like to flow uphill). The water table should never reach the soakaway at any time of the year so careful planning is required if you live in a low-level area near the water table.

4 – What Materials Should I Use to Fill the Void?

In the old days, builders would fill the hole with rubble. This isn’t the most efficient option as it takes up a lot of space and leaves very little room for the water. Today, tradespeople construct modern soakaways with plastic crates that are wrapped in a felt membrane. This method creates a 90% void capacity, and the external layer keeps out roots and soil that could otherwise block the soakaway.

Soakaway crate

Plastic soakaway crate system

5 – Can a Soakaway be Used in Clay Soil?

That depends on how quickly or slowly water seeps through the clay. If the soil consists of very thick clay, then a soakaway may not work. If the ground is only light in clay contents, then a slightly larger soakaway may suffice. The only way to be sure is to dig a test hole, measure how quickly the water filters through the soil and then complete a set of calculations that include the size of the surface catchment area. Read this guide for more information.

Soil percolation test

Test the soil before you invest in an expensive soakaway

6 – How Much Does it Cost to Build a Soakaway?

Many factors will affect the price a tradesperson quotes you for a new soakaway:

  • Location, whether it’s near a driveway, patio or other hardened material
  • Depth and width of the hole and the number of crates needed
  • Distance from the building
  • How many surfaces discharge into the soakaway (roof gutters, patios, driveways, conservatories etc.)
  • The cost to replace or reset patio slabs, driveway blocks, tarmac or concrete etc. after the installation
  • Waste disposal cost if your tradesperson cannot leave the excavated material on site

A simple, small and basic soakaway will be from £750, but most will cost between £1000 and £1500. For more complex large installations, expect to pay upwards of £3000.

As prices vary so much, you should obviously get at least three quotes.

7 – Can Rainwater be Diverted into a Sewer?

In general, foul water and rainwater should be kept separate with foul water sent via pipework to a water treatment plant and surface rainwater diverted either to a soakaway or a watercourse such as a ditch or a stream.

There are limited cases when you can divert the rainwater from your property into a sewer:

  • You cannot build a soakaway within the constraints of the regulations (i.e. at least 5 metres from a building or 2.5 metres from boundary etc.)
  • A soakaway won’t work because the soil is clay based or the water table is so high that water won’t drain away
  • You cannot send the water to a watercourse as there isn’t one near your property or it’s not a viable option

Here is a statement from Trent Water:

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.

As there are dozens of different water companies in the UK, we suggest you first contact your local one and ask them about their policies for rainwater.

All companies charge a fee to cover the extra cost of treating any rainwater that enters their system.

Southern Water currently charges £24 per year which they add to your bill.

8 – Can a Rainwater Butt or Harvesting Storage Tank be Used as an Alternative?

You’ve probably seen storage tanks like this in garden centres:

These plastic tanks are eco-friendly as you can store rainwater from your roof and use later it to water the garden.

Unfortunately, in the winter they fill up very quickly, and this is the time of year you’re least likely to water your garden.

You should ensure excess rainwater is diverted from the tank to a soakaway or other drainage system.

A standalone water tank not connected to a drainage system will soon overflow, cause mould, damp and stains to any nearby walls, patio slabs or hard concrete standing etc.

These tanks are not a realistic or acceptable alternative for a soakaway and would not meet current Building Regulations for sustainable drainage.

Here’s a photo of a correctly installed water butt, located next to a rainwater drain:

 

9 – Do I Need to Build a New Soakaway When Laying a New Driveway?

Current building regulations state that rainwater falling on a new driveway must not discharge onto a public footpath or road.

There are a few solutions, and a soakaway is only one of them.

Also, existing soakaways might be able to cope with the rainwater so no, it would not be accurate to say that you must construct a new soakaway.

Check out our guide to driveway regulations here.

10 – Where Can I find Official Documents and Regulations?

The Planning Portal is the best place to start. It contains information taken directly from the Building Regulations.

Need More Information?

Explore the ultimate guide to soakaways here.