Groundworks & Foundations: Drainage

ABOVE: The Alpha sewerage treatment plant from Klargester being installed. klargester.com

Designing a drain system is relatively simple. You conventionally start with the destin - ation – be it a public sewer, a septic tank or a soakaway of some sort – and then you work out how to get everything there in the simplest, most efficient manner. The manufacturers have customer service depart ments which will design your drainage scheme and specify fittings, if you can provide them with some essentials such as the site plan and the depths to which you are working (known as invert levels). A 100mm drain (the standard) is going to be adequate for all situations where there are less than five WCs. There is no set depth for drains, but less than 600mm and you may have to cover the pipe with paving slabs, deeper than 1,200mm and you will have problems working. The optimum fall is 1:80, which is equivalent to 250mm on a 20m run. If you have to cope with gradients much steeper than this then stepped falls will have to be designed into the system.

Main Drains

The vast majority of new and existing homes are located close enough to a public drain to make connection relatively easy, if not exactly cheap. The first thing to work out when assessing a new site is just where the public drains are, and how easy it will be to connect to them. The local water companies hold records of public sewers and it’s possible to consult these.

At the initial survey stage, try to ascertain just where the public drains are and how complex and expensive it will be to connect with them. If you are building on land below the public drains, or they are a long distance (say more than 30m), you may be able to pump into them using a small bore pipe. If connection is too expensive (more than £10,000) or impractical, you will have to look at on-site solutions.

Off-Mains Drains

There are a number of companies catering to this market and the products available are surprisingly diverse, from a relatively simple septic tank with an overflow, to electrically driven treatment plants of various descrip tions, right through to natural reed beds, which can be a great feature if you have enough garden space. Look to pay between £3,000 and £8,000 to provide off-mains drainage.

Remember that each site is different. Some sites have planning conditions attached which specify what sort of drainage should be used. If not, then your building inspector will advise as to your choices. Generally, septic tanks are the simplest and cheapest option, but you may not be allowed to let the liquids run off into the ground.

The Natural Solution - A natural reed bed filtering system can be a great solution to off-mains drainage if you have enough garden space. They purify the waste without any electric pumps or motors, although on the grim side, they need a settlement tank that requires an annual ‘de-sludging’.

"They wanted me to pay £400,000!"

Pepita Collins was shocked at the quote she received to connect to mains drainage

Pepita Collins’ newly renovated coastal Devonshire cottage is still off mains, despite several attempts to connect. “There was no mains water or sewage drainage and so raw sewage drained straight into the harbour. I asked South West Water for help and was told it would cost me £400,000 to have them installed! I discovered some legislation which means they have to cover the costs, but their budget is currently exhausted, so I am still waiting for mains water and sewerage…”... Read more about this coastal cottage renovation

Further reading:

• Groundworks & Foundations: Excavations
• Groundworks & Foundations: Foundations
• Groundworks & Foundations: Ground Floors
• Groundworks & Foundations: Rainwater Management

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