Drainage: What Can Go Wrong?

An adequate, free flowing drainage system will be a vital aspect of your build. Hidden away underground, if it goes wrong, it can be costly and extremely disruptive to put right.

Designing a good drainage system is more involved than you might think. The most common option is to use a gravity-feed system. This will lead to a public sewer for foul waste and a soakaway for surface water. If you can’t easily use these approaches on your project, there are other routes.

The key is to ensure the design is properly prepared before you start works, as changing anything will be difficult and costly. Here’s what you need to know:

1. Consider the lie of the land & the position of the house

There are strict parameters regarding the fall (gradient) of surface and foul water drainage systems. If you can’t achieve the minimum gradient, it may not be possible to install a gravity system and some form of pumped setup might be required for the foul waste. There are a number of options, often labelled as packaged pumping stations, on the market.

More from Self-Build Zone

Fundamentally, the sewage is stored in a container and regularly pumped uphill to the public sewer. This kind of installation will have ongoing maintenance requirements and associated running costs.

2. House size & final outfall point

The size of the house and nature of the surrounding landscaping will have a bearing on the position of the rainwater downpipes, gullies and extent of the underground drainage system needed. Likewise, the total number of toilets, showers, baths and basins will impact on the specification of foul drainage.

3. The right pipe for the right job in the right location

The pipe’s position will influence the material, depth, bed and surround. At depth, a plastic pipe with a pea shingle bed and surround may be perfectly acceptable – but that will need to change closer to the surface.

A pipe laid under a drive or road at a shallow depth may need to be of clay or even cast iron, with a concrete bed and surround, to prevent it being crushed. Ground levels must be considered, too. Surface water should fall away from a house to a suitably sized pipe, with rainwater goods dropping to gullies.

4. Is the site suitable for a surface water soakaway?

Normally foul and surface water are separate systems. If ground conditions don’t permit a soakaway or there isn’t enough room on the site (a traditional soakaway normally needs to be at least 5m away from any structure), it may be possible to direct surface water into the foul system.

Agreement for this must be obtained from the relevant water authority. Surface water can, in some cases, be redirected to a water course. This may require a discharge licence.

5. Mains sewer alternatives

If there is no public sewer then you may be faced with treating and disposing of your own waste. There are a number of packaged systems on the market designed for this purpose. The setup’s location on site and how any effluent is discharged are key in identifying the right product. Once it has been treated, the water can be discharged to a soakaway, water course (subject to agreement) or something more complex, such as a reed bed.

6. Do you want to reuse surface or grey water?

It’s now more common to install storage tanks and piped systems to collect and redistribute surface or grey water to improve a project’s environmental credentials. As with all drainage systems, the positioning and installation of the water harvesting tank must be properly thought through. Remember, grey water must be treated before it is reused. Any excess is normally passed to a soakaway.