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What is Damp Proofing?

By Patrick Lynch
Updated May 17, 2024
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Damp proofing is a barrier placed in walls or floors to prevent moisture from passing through them into interior space and thus causing rising damp. A damp-proof course or damp-proof membrane can be added to walls and floors for this purpose. Damp proofing materials consist of plastic sheets, copper, or concrete. Cavity walls require a course on both sides, with the inner wall materials placed below ground level. It is also possible to use a single plastic sheet which has been cut at an angle to fit into both cavity walls.

One method for stopping rising damp is by creating a damp-proof course (DPC) which is a horizontal barrier in a wall. A solid floor is protected in a similar manner by a damp-proof membrane (DPM). This resistance to moisture is calculated by engineering tolerances and various types of tests.

Rising damp is a regular occurrence, and damp proofing is required to prevent it. Common construction materials such as bricks and mortar are porous and allow damp-carrying nitrates and chlorides to rise from the ground. These ground salts suck moisture out of the atmosphere which can cause damp walls in humid conditions.

Modern building standards in most countries require structures to have damp proofing when they are being constructed. This can come in the form of plastic strips or engineering slate. There are a variety of other materials used to prevent damp proofing. These include flexible materials such as copper, rigid materials such as cement concrete, or rough layers of sand under the floor.

Damp proofing for walls usually consists of an extremely thin layer of plastic placed between a pair of blocks. A DPM usually consists of a polythene sheet placed beneath a floor slab. It is also possible to make a more formidable barrier by welding pieces of DPC or DPM together.

Cavity walls usually have a DPC on both outer and inner walls. A DPC is normally placed 6-8 inches (15-20) cm above the ground. This ensures that water splashing from puddles does not affect the wall above the level of the DPC. As the bottom of the wall can be saturated, the inner wall’s DPC is constructed below ground level.

Another alternative is a single sheet of angled plastic. This sheet should be able to fit across the cavity and slot into both walls. If this single sheet method is used, weep vents must be created to ensure that rainwater drains from the cavities. Failure to install these vents could lead to rising damp.

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