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Damp Proofing

By: Chris Hogan MSc - Updated: 12 Aug 2010 | comments*Discuss
Property Renovation Renovate Roof Walls

Keeping water out is of paramount importance in any property in the British Isles, what with the weather the way it is.

Penetrating Damp

This is water that seeps in through the fabric of the walls and is fairly simple to identify and remedy. It's more prevalent in older houses that don't have a cavity in the outside walls. If there's a cavity, as in most houses built since around 1930, even if water gets in past the brick exterior wall, it will be difficult for it to get across the cavity to the inner wall.

Keeping your roof, guttering and window frames in good order will stop the water from getting in through the brickwork in any case, so it's a good idea to get into the habit of checking everything annually at least. If you have penetrating damp in an area where there's no leak from a gutter or other obvious cause, the brickwork may need re-pointing. This entails scraping out and reapplying the mortar in between the brick as the profile of that mortar plays an important part in helping rainwater to run off the bricks, rather than sit in the gaps and soak in.

Rising Damp

The more difficult type of damp to deal with is that which comes up from the ground, known as rising damp. This is normally dealt with by a layer of waterproofing a couple of courses up from the ground, known as a damp-proof course (DPC). Water is drawn up into the bricks that touch the ground by capillary action and the idea is that the brick below the DPC are allowed to get wet while the non-permeable layer of the DPC stops those above it getting damp.

DPC's can be made from many materials, as long as they are durable and waterproof. In the past slate, lead, a layer of bitumen or a course of engineering bricks (much denser than ordinary bricks) would have been used. These days it's more likely to be a stiff sheet of plastic across both skins of the exterior wall, occasionally a separate strip for each skin.

Installing a DPC

Many older buildings will not have a DPC at all, although there are options for fitting them. In brick, chemical solutions such as silicone can be introduced into the wall through a series of holes at regular intervals. Capillary action then draws this solution into the nearby bricks and hardens to form a non-permeable layer. This is often done from the inside of the property so it's necessary to remove the skirting boards, then re-fit them and re-decorate.

This method can be used to repair DPC’s, which have deteriorated in certain areas, but it cannot be used for stonewalls. They cannot be drilled in the same way, but there are electrical systems, which can be used. These rely on a series of probes inserted into the wall, which are then electrified to drive moisture from the area.

Damp-proof Membranes

In a modern build the DPC will often be connected to a damp-proof membrane or DPM that is under the concrete screed of the ground floor. In this way a complete waterproof barrier is set up right the way across the property a couple of inches above the ground.

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