Drains: A Dirty Business

Not an exciting subject I know but if you are planning to extend your property, drains running nearby could potentially have an effect on what and how you build your project.

Older properties and in particular terraced properties,  will, in general, have drains that run parallel to the rear external wall. When planning to extend these type of properties or in fact any property that has drains running adjacent to it, the type and position of these drains can cause a headache.

Since October 2011 the Sewerage companies have adopted certain sewers and now we have two different types of drains and sewers.

A private drain is one that serves only one property, for example in the case of a detached house. If the property is a semi-detached or a terraced house and the drain is at the head of the drain run and a neighbour’s drain doesn’t connect into it then it is a private drain.

A public sewer is a drain that serves more than one property, sewerage companies have adopted these private sewers and it is their responsibility to maintain these pipes. A lateral pipe is one that lies outside the boundary.

If you are planning to build within 3m of a public sewer then the sewerage company will need to be consulted and a Build Over Agreement obtained. There will be a fee payable to obtain a Build Over Agreement which seems to vary from company to company. It is still the responsibility of the Local Authority Building Control Department to inspect the construction.

Although in certain circumstances it may be feasible to alter a design so that a new extension isn’t built over a public sewer, in practice it is more than likely that an extension will want to be built over it. One thing to bear in mind that it is not possible to have an inspection chamber within the building, and it may need to be repositioned.

There are certain regulations that need to be observed, such as how the pipe will be built through any new external wall. The sewer will need to be protected in order that any loads from the building are not transferred to the pipe which will result in damage.

Older drains were constructed with pipes made from clay with rigid joints that were jointed with a strong mortar and bedded and surrounded in concrete. It has to be acknowledged that this was a skilled job to form watertight joints.

Modern pipes are made from a very strong and durable plastic with joints that are slid onto each end of the pipe. This pipes can be up to 6m in length.  Clay pipes can also be used but still, use a plastic joint. These pipes are laid onto a bed of pea shingle and are known as a flexible system. The idea is to allow the pipes to accommodate a small amount of movement as necessary without fracture.

All pipes are laid with a fall or gradient so that gravity aids the removal of waste. At each change of direction, gradient and generally where pipes meet together, an Inspection Chamber or Manhole needs to be formed. These are normally rectangle or round and will have a cover and frame to allow for inspection and rodding. Older Inspection Chambers would have been built of Engineering Bricks from a concrete base, many finished internally with a strong render.


Brick built inspection Chamber

More modern chambers are made from plastic with a preformed base that includes a number of inlets to allow for easy connection to pipes. The chamber walls are made from plastic that connect to each other with a rubber insert to form a watertight seal. They are quicker and cheaper to install than a traditional brick built chamber. The plastic chambers are generally laid on a bed of pea shingle to form a completely flexible system.

When it comes to building anywhere near drains which is likely to happen, then an early survey is recommended. Drains can clearly have an impact on a project both in practical terms and also in monetary terms.