A guide to drainage

It’s not the most exciting part of a self-build but it’s certainly one of the more important areas to get right. Self-build expert Mark Brinkley investigates the pros and cons of mains drainage versus on-site drainage

House drains are something we take for granted but their very existence is one of the basic underpinnings of a civilised society and something that took centuries to develop. City life without drains was not only very smelly, but also very unhealthy, and the growth of cities didn’t really become a possibility until a functioning drainage system was in place.

There are many regulations covering both foul water and rainwater or surface runoff and these can be gleaned from our Building Regulations. All good architects, designers, builders and groundworkers will be familiar with how this work is undertaken, but the ultimate responsibility of what waste goes where is always down to the client paying for the work.

What does a drainage system consist of?

Internally, there is a web of small waste pipes running from sinks, basins, showers and toilets. These all feed into a soil stack, which drops down into an underground pipe, which then, in theory, falls gently away from the house towards public sewers or an on-site tank depending on how the system is set up. Above-ground waste pipes, routinely referred to as soil and vent systems, are usually installed

by a plumber. The soil stack can be either internal or external and there may be more than one. Critically, they have to have some way of venting above roof level, in order to let any bad odours escape from the house. Each waste pipe feeding into the soil stack also has to have a trap built into it, usually a U-bend or an S-bend, which is designed to stop drain odours leaking out into the home.

Below ground, the pipes are referred to as drainage and are normally installed by the groundworkers who build the foundations of a house. Generally speaking, if your site is close to an existing mains drainage, and access is straightforward, it makes really good sense to feed your waste into it. It is usually a cheaper solution than specifying on-site drainage, which is the alternative. Sometimes you need to carry out investigations to determine what your best option is.

Who is in charge of the mains drainage system?

Normally a district’s water company will be responsible for its drains, but in any case, this is the kind of information that routinely turns up on a legal search when a property or plot changes hands and it is never usually hard to find out. Whatever company has responsibility, they will be who you pay your water and sewage bills to. Their permission will have to be sought if you wish to make a drain connection and they will likely charge a considerable fee giving it, but they are unlikely to carry out this work themselves.

How do I find out where the nearest drains are?

Your local water company will hold maps of drainage runs, which will show the depth of the drains. These maps aren’t always completely accurate but they usually give a good indication of the state of play. The main drains are usually run under the street and connections to them often involve digging roadworks to gain access, the kind of event the requires traffic control and the like. Such works are expensive but, in the great scheme of

things, they are still usually cheaper than the alternatives.

It is possible that you will have access to shared drains that collect foul waste from a group of homes before entering the main or public sewers in the road. This is typical on housing estates and for terraced homes. Finding out where these drains run can be a complicated undertaking and you may have to enlist the help of a drainage contractor with camera equipment to help build up your own map.

Alternatively, you might have to put in a long drain run across land you don’t own. In such a situation, you have to have permission from the owners and there may be a fee attached, and a hefty fee. Once the likely connection costs run over £15,000, it is time to start looking at onsite drainage options (see overleaf).

How far from the drains can you be to make a sensible connection?

As long as you are working with gravity, then there is nothing, in theory, to stop you running a 100mm foul waste pipe (the conventional diameter) as far as you want. Foul drains should be run at small fall: between 1 in 40 and 1 in 110. If the pipe is much steeper than this, solids in the waste will become separated from the liquids and the pipe will require frequent rodding to stop it getting blocked.

If the fall is steeper than 1 in 110, then you have to introduce an inspection cover for access at a point with a sudden drop of a metre or more.

Of course there is a cost to laying conventional drains as they have to be excavated, laid in pea shingle, and have access points. At distances of more than 50m, as well as road opening fees, the overall cost starts to become prohibitive, and on-site drainage becomes a viable option.

What if the main drains are located too high?

Having to work 'against gravity' introduces another complexity.the usual solution is to have a tank with macerator pump, often referred to as a lifting station. This