The Importance of Damp Proofing When Extending
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 ProblemsRising 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 DampRising 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 dampPenetrating 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.