Soakaways are a long established way of dealing with rainfall. They are essentially a pit in the ground into which you run your rainwater drainage. They are also used, less frequently, to dispose of the effluent from septic tanks, where they are more commonly known as leaching fields.
The overall idea is much the same — a way of disposing of surplus water slowly, by directing it away from the house or hard standing and letting it seep into the ground. This both mitigates against flooding and stops damage to the house and its foundations.
Solutions for surface water runoff aren't optional either — it's covered by Part H of the Building Regulations for new homes. Get to know how soakaways work with this helpful guide.
What are the Pros and Cons of Soakaways
Soakaways are an efficient drainage system for surface water run-off, with little to no drawbacks if installed correctly.
The benefits 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
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 potential drawbacks include:
- Soakaways can get blocked by dirt and leaves when not fitted with a silt filter
- If 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 some material which will have enough voids in it to absorb large volumes of water and yet strong enough to be covered over so that you wouldn’t know it was there.
Before you start, you first need to ensure that the ground beneath is permeable, which will require a percolation test. A soakaway has to actually to let water soak away if it is to be successful.
Instructions on how to do a more accurate soil percolation test and for installing soakaway crates can be found at www.drainagepipe.co.uk.
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.
(MORE: What you Need to Know About Rainwater Harvesting)
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 of course 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. Careful installers may decide to install a silt trap just upstream from the soakaway to prevent the soakaway filling up with dirt over time.
How to Conduct a Percolation Test 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 very well.
On the other hand, if the water sits in the hole without draining, then you will have to explore further. First consider 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 its far enough from the structure to prevent damage — 5 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
Soakaways for Driveways
Current planning guidance allows you to pave over your front garden provided you to make some allowance for the surface water run-off. Porous surfacing, such as gravel, permeable paving or porous alphalt is the best driveway material for reducing water run-off entirely.
Alternatively, the surface must be set to a fall that collects rainwater and diverts it to a soakaway or a water course. What is not acceptable is to let the rainwater drain into the roadway.
An awful lot of rainwater falling onto properties still drains into public sewers which are owned by the ten 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, usually around £40 per annum.
You may also be able to claim a refund for some money you have previously paid for surface water drainage. If you qualify for an exemption you will receive it through a lower charge. The level of rebate can be found in your sewerage company’s charging scheme.
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.
Mark is the author of the ever-popular Housebuilder’s Bible and an experienced builder. He’s just finished his latest self build.
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