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The Importance of Damp Proofing When Extending

By: Chris Hogan MSc - Updated: 30 Mar 2017 | comments*Discuss
Extension Build Do-it-yourself Diy Shop

It's essential that extensions are damp proofed properly, as keeping water out is a big problem for property in the United Kingdom. There are two main issues with water and property, rising damp and penetrating damp.

The Two Main Problems

Rising damp is caused by water being drawn up from the ground through the walls by capillary action, and penetrating damp is where rainwater seeps through the walls. Both types will cause condensation problems in the rooms near the problem areas, such as mould and growths on the walls, damaging plaster and decoration, and in extreme cases damaging the fabric of the house. Unless the problems are that extreme, then the cause and the damage can be put right relatively easily,, although it is, of course, far better to design the extension so that this doesn't happen in the first place.

Rising Damp

Rising damp can often be seen in cheaply built lean-to extensions without any real way to prevent the water rising through the walls. They are cold, damp and smelly and can really only be used for storage because the cost of keeping them warm and driving the damp out will be astronomical. Worse than that, the damp may spread and cause problems for the rest of the property.

The main barrier to the water rising through the brickwork is the damp proof course (DPC), sometimes called a damp proof membrane. This is a layer of waterproof material, these days usually plastic but in the past made of slate, rubber or various other materials, that is laid on top of the second or third layer of bricks. The idea is that the bricks below the DPC are allowed to get wet but the DPC stops damp rising above it. The floor, inside the extension, should be above the DPC as well.

Penetrating damp

Penetrating damp in an existing property is usually caused by a fault in the guttering, fascias or downpipes that allows water to cascade down the side of the house. Of course, the side of a house gets rained on all the time, but when there's a constant soaking from a fault of this kind the wall never gets a chance to dry out and that's when the water starts to work it's way through the wall.

So when building an extension, it's important to make sure that the guttering system is properly installed, with a fall (gradient) that encourages water to travel along the guttering to the downpipes. Make sure windows, doors and any other fittings that are set into the walls have the appropriate profiles to encourage water to run off as well.

Drainpipes should be linked to a drain of sufficient capacity to deal with the amount of water that's likely to pass through it. Ideally it should join the existing house drainage system, but if that's not easy or would be very expensive, it can be linked to a soakaway.


A simple soakaway is a pit that is dug downhill from the downpipe, which is then filled with gravel or another material that leaves plenty of air gaps. When it rains, these gaps provide a place for the water from the drain to be stored temporarily before it then trickles away into the surrounding soil over time. There are many other types of soakaway and it's necessary to look at the type of soil that you have, the gradient, and the amount of water that the soakaway will have to deal with before deciding on the size, location and type. There are plenty of books and websites that can help you in this assessment and explain the calculations.

Prevention Better Than Cure

Most of this should be standard fare for a builder with any amount of training and experience, but if are doing it yourself, perhaps assembling a conservatory from a DIY store, then close attention to damp proofing is required to stop your home-built extension bringing you misery instead of pleasure.

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Dee - Your Question:
Lounge extension built 2015. Water ingress under French doors when we get rain. B.R. Won't sign off until resolved. What is the cause and how can we sort it???

Our Response:
We can't tell you what the course is unfortunately. The easiest way is to ask the builder to look at it. You could ask a building surveyor to give an expert opinion. If it proves to be the fault of the builder, they should offer to rectify it free of charge. If not, you may need the small claims court.
ExtensionBuild - 31-Mar-17 @ 12:35 PM
Lounge extension built 2015. Water ingress under French doors when we get rain. B.R. Won't sign off until resolved. What is the cause and how can we sort it???
Dee - 30-Mar-17 @ 5:49 PM
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