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Modifying the Drainage System for an Extension

By: Chris Hogan MSc - Updated: 5 Sep 2020 | comments*Discuss
Extension Drainage Drain Modifying

When building an extension to your home, altering your drainage will often be inevitable. Whether it’s just outside guttering to direct rainwater into underground drains, or adding new foul water pipes for a kitchen extension, successful modification of your drainage system is important.

Planning drainage modifications is vital

Making a plan of exactly how you want change your drainage system will help you to avoid some of the pitfalls that accompany these jobs. It is fairly simple to work out how your roof drainage and above-ground pipes link together, but underground drainage is more of a challenge.

Working out the underground drain layout for your home can be done in three ways. The first is to ask the council if they have any plans of the house and, if they do, use them as your reference. It is likely you will be charged a fee for this but, it will almost definitely be cheaper than employing a surveyor to work out how the drains lie beneath your home. The main disadvantage with using plans from the council is that they well may be from when the house was originally built, and so things could have changed.

Using a surveyor should give you current information regarding your drainage system, but it will be costly. Apart from approaching the council or a surveyor, the other option is to start lifting manhole covers and try to work things out for yourself. Unless you are pretty confident you can do this effectively it is best to go with one of the other two options.

Know the different drainage types

Making changes to underground drainage can be tricky. Your home will have two separate types of underground drainage, called surface water drainage and foul water drainage. Surface water drainage is simply gutters, pipes and drains that carry rain water away from your home and into the nearest canal or river.

Foul water drainage pipes take water from your kitchen sink, washing machine, bath, shower, and the stuff that is flushed down your toilet to the nearest sewer. These two systems are always kept separate (expect on very old properties), and it is only ever acceptable to let surface water run into a foul water drainage system, never the other way round.

Grey water – the third category

It’s worth mentioning here that there is growing interest in ‘grey’ water systems. These keep the used water from baths, sinks and appliances separate from toilets (for health reasons) and collect it in tanks. It is then filtered and pumped to storage tanks for use in the home, but in areas where clean water isn’t strictly required. For example, watering the garden, flushing toilets and washing cars.

The interest in grey water systems is growing because it reduces the consumption of expensive and resource intensive drinking water for tasks that don’t strictly need it. It also reduces the amount of water that drainage systems have to cope with and, be extension, reduces flood risk. These systems are in their early days through, and rare in the UK.

Planning and regulations

If you have to move a drain away from its original location it needs to be planned out thoroughly so that all problems are dealt with before work starts. Whether or not you need permission to make changes to your underground drainage system depends on the individual project, so you should seek professional advice in every case.

To find out exactly what you have to adhere to when altering your drainage system, refer to Part H of schedule 1 of the Building Regulations. It is likely that a building inspector will have to pay you a visit and check that everything meets the regulations set out.

Modifying gutters and drainpipes

If you only have to add to or change your roof drainage system when constructing your extension then you have the easiest type of modification. Try to find the same make of drainage system to the one currently being used at your home. This will make it much easier to link the new pieces to the existing pieces, and will also be more aesthetically pleasing than using a different make of drainage system.

The tools you will need to install gutters and drainpipes are a sturdy ladder, a spirit level, an electric drill and screwdriver, and some screws and wall plugs. If you are trying to join drainage pieces of different makes some silicone sealant may come in useful to fill in any small gaps.

Modifying underground drains

These may seem obvious, but there are two simple and essential rules you must remember when changing drains or adding new ones. Firstly, make sure that all drains run downhill. The downhill angle only has to be slight, but the angle must be continuous. Shoddy gutter installation will mean they become full and overflow instead of directing water to drains. With lots of rain they will also begin to sag and their lifespan will reduce dramatically.

Not paying attention to this rule when installing underground drains will lead to overflowing drains, nasty smells, and blockages, which leads onto the next rule. It’s essential that all drainage systems and their pipes can be unblocked. The way this is done is by locating inspection chambers at junctions in the drainage system and bends in the pipe work.

You also need to be aware of the channels that allow air into the system. You can see these in inspection chambers, they are smaller bore and higher up than the channels that take the water away. Don’t block them as air is needed to replace the water as it drains. If your modifications are drastic enough to need new major junctions and inspection chambers then it’s best to get expert assistance.

Get the drain experts in if at all in doubt

Modifying drainage systems is not always straightforward, but it is not necessarily the most difficult part of building an extension to your home. If in doubt about how you should change your drainage system to best suit an extension, get some advice from a qualified builder.

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[Add a Comment]
Our neighbour (we share a boundary wall) is building a large flat roof ground floor extension.He now has his foul drainage pipes on top of this roof to the end then it goes vertically down parallel to our shared fence which is next to our patio.Thus we see this large drain pipe as we sit outside.In otherwords, no underground sewage drainage from the back of the original house. Is this allowed?
Shosh - 5-Sep-20 @ 2:14 PM
I wish to extend my kitchen by a further 2 metres. Unfortunatley there is a drain and a rodding eye where I want o build. can these be extended?
Jojo - 27-Aug-20 @ 8:47 AM
I like to move a manhole from the far corned of my sitting room to outside the garden.Could your company help? Thx, Chris
chris - 8-May-19 @ 5:56 PM
Hi Would like to build an extension to the back of the house. I have a mains sewer in the way, can anybody tell me the breakdown in costs to have it relocated. The complete cost breakdown and procedure involved in moving the mains sewer please? I don't want it covered over with carpets or anything else. Thank you Reg
Reg - 16-Jul-17 @ 9:30 AM
Good advice. However, linkage to access point and junction planning/changes would make it better :-)
Roy - 24-Sep-16 @ 11:20 AM
The information on this page is useful as it allows a novice person like me stay on top of the builder especially when hes trying to cut corners and get the job done quicker,.....Thanks
MoPatel81 - 8-May-13 @ 2:20 PM
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