What is a soakaway? An in-depth guide to what you need to know

Soakaways can be an ideal drainage solution to deal with rainfall that doesn't naturally drain from your property. It is basically a pit in the ground into which flows rainwater drainage. Less often, soakaways are also used to dispose of effluent from septic tanks, known as "leaching fields". 

Both work in the same way — removing excess water and moving it away from a structure or hard standing. This is essentially achieved by letting the water seep into the ground. Soakaways can prevent flooding and therefore stop any potential damage to a house and its foundations.

While this sounds great, soakaways aren't just an optional "nice to have" — they are often essential in a building project and covered by Part H of the Building Regulations for new homes. 

Here's what you need to know about these drainage systems, including what is a soakaway, all the pros and cons and whether your home is suitable for one to fix any rainwater drainage issues you might have.

What is a soakaway? 

Soakaways are a drainage system for surface rainwater run-off. Their construction 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. This needs to be constructed in a way that will be strong enough to support weight over the top of it once filled back in. 

Typically a soakaway can support a number of downpipes on your property. Bear in mind, however, the larger the surface area you collect rainfall from, the bigger the hole needs to be. A soakaway needs to be properly sized for your property to ensure it doesn't fail.

What are the pros and cons of soakaways?

While the pros of soakaways usually far outweigh the downsides, it's important to be aware of both.

The pros of soakaways include:

• Reducing pressure on overloaded mains drainage systems 
• Potential of bill rebate for surface drainage costs from your sewerage company
• Cost-effective to install, and can be retrofitted relatively easily
• It camouflages into your garden without any ugly manhole covers.

However, it's worth noting that while soakaways are low maintenance and reliable as the drainage is natural, if they go wrong for any reason, it could lead to potential issues. The cons of soakaways include: 

• Soakaways can get blocked by dirt and leaves when not fitted with a silt filter
• If a soakaway isn't far enough from the home, or isn't draining correctly, this could cause issues for the home's foundations 
• Soakaways aren't suitable for poor draining soil.

How do I build a soakaway?

Constructing a soakaway is a relatively simple affair. You dig a hole in the ground and then backfill it with material. 

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.

When it comes to constructing a soakaway, the traditional way was to dig a hole and fill it with broken brick and builder’s rubble. This is cheap to do, but it limits the actual volume available for the rainwater to collect. 

The modern option is to use dedicated plastic boxes, known as soakaway or attenuation crates. These leave virtually the whole space open to be filled with rainwater and are therefore much more effective. The crates can be assembled two or three deep and clipped together for strength, and in varying depths.

You then mark out the hole size on the ground, adding 150mm around each edge, get a digger on site to take the ground up, and dispose of the waste. 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. 

The hole is then lined with geotextile sheeting which allows water to drain through but prevents the void filling with soil. The crates are then placed inside the excavated hole, and then the geotextile is wrapped around the whole structure.

You then run 110mm drains from the rainwater downpipes and ground gulleys to the soakaway entrance, all laid to a fall. The hole edges are then backfilled with pea shingle or similar and the whole is covered over so that you wouldn’t know there is anything underneath. 

Installation typically involves the fitting of a silt trap just upstream from the soakaway to prevent the soakaway filling up with dirt over time.

How do I know if my garden is suitable for a soakaway?

There is a simple test you can undertake to establish whether the soil in your garden is potentially suitable for a soakaway.

Dig a hole in your garden measuring about 300mm square and 300mm depth. Pour a bucket of at least 10 litres of water into that hole. If the water has soaked away within about two hours then soakaways should work well in your garden. 

It is worth mentioning that certain soil types like heavy clay will be unlikely to allow sufficient water drainage for a soakaway, while sand, chalk and limestone based soils are likely to be suitable.

On the other hand, if the water sits in the hole without draining, you are likely to have to consider a different types of drainage or rainwater harvesting system. 

You can also do a deeper percolation test, going down to a depth of a metre. If the water still stands without draining away, then you will have to seek an alternative destination for your rainwater drainage.

How far should a soakaway be from the house?

You need to ensure it is far enough from the structure to prevent damage. Five metres is the accepted distance, but this also depends on the calculated size of the soakaway.

How big does a soakaway need to be?

The larger the surface area you collect rainfall from, the bigger the hole needs to be.

Assuming the ground is deemed suitable to take a soakaway, the next step is to calculate how much ground you need. Various rules of thumb exist to work this out and explicit guidance is available (if hard to understand) in Part H of the England and Wales Building Regulations. 

Generally, each 50m2 of surface area to collect rainfall (which may include areas of hardstanding as well as roofs) requires about 1 cubic metre of soakaway. So if you were collecting rainfall from an area of 200m2, which might include both roof and patio or driveway, then you would anticipate a 4m3 soakaway. 

However, be aware that there are moves afoot to make soakaways much bigger to help deal with the extended periods of heavy rainfall that we are now seeing more regularly.

What about surface water run-off from drives?

When installing a new driveway, especially if paving over garden, often one of the planning requirements is to ensure there is sufficient drainage and allowance for surface water run-off. 

You can use porous surfacing such as gravel, permeable paving or porous alphalt as the best driveway material for reducing water run-off entirely. Alternatively, if using a non-porous material you can divert surface run-off to a soakaway. 

The driveway surface must be set to a fall that collects rainwater and diverts it to a soakaway. What is not acceptable is to let the rainwater drain into the roadway.

A soakaway can also be constructed under a new drive if space is limited. However, construction will be more expensive as the crates will need to cope with the weight requirements. 

Does having a soakaway reduce your water bill?

Yes, if you have a soakaway you can put in a claim for a soakaway rebate with your water company. 

An awful lot of rainwater falling onto properties still drains into public sewers which are owned by the 10 water and sewerage companies in England & Wales. The companies are responsible for removing and processing this rainwater, which is surprisingly expensive. 

The companies collect around £1 billion each year to cover the costs of this service. If rainwater drains from your property into a public sewer, you will be charged for surface water drainage through your sewerage bill.

If however you manage your rainwater so that it drains to a soakaway on your property, you should be entitled to a surface water drainage rebate. This will usually manifest itself as you paying for drainage of 'foul water only' rather than foul and surface water.

If you qualify for an exemption, you can potentially also backdate it as far back as six years.

What are the alternatives to a soakaway?

The regulations allow for two alternative approaches. One is to run your rainwater into a water course, pond or stream. This only works if there is a water nearby which is very site specific. 

The other option is to run the rainwater into a surface drain, probably located on or under the road near to the site and quite likely to be the same drain network as your foul drains go into. 

This is a solution of last resort, especially as we have now identified rainwater in the foul drainage network as being a major cause of unwelcome overflow releases of sewage into our river systems, as the plants aren’t built to cope with sudden storm surges.