Damp Proof Course
What is a Damp Proof Course?
Simply put, a damp proof course, or DPC, is a layer of impermeable material that prevents water moving within a structure. This might be from the exterior to the interior of a building, from the ground up into the walls, or from any part of the structure to another. Modern DPCs should conform to BS 6398 and generally consist of flexible materials such as bitumen, polyethylene, sheet metal or polymers; or semi-rigid materials such as mastic asphalt. BS 6398 does allow for a rigid DPC of dense bricks alone (see BS EN 771-1 for the requirements of suitable dense bricks), but this is only effective against rising damp and will not provide damp proofing against moisture travelling downwards.
Where to fit Damp Proof Courses
The most familiar application of a DPC is one that is designed to prevent moisture rising up into the walls of a building (rising damp). Back in the day these would have been made out of slate or other similar material and laid on a mortar bed between courses of bricks. Nowadays these DPCs consist of a course of one of the above stated materials. Often a flexible or semi-flexible DPC will be laid on top of a couple of courses of dense bricks. The DPC should be fitted around 150mm – 200mm above exterior ground level. The exact measurement obviously depends on the circumstances and situation of the individual building and should take account of groundwater levels, drainage and possible splashing and pooling of groundwater. The DPC should be below the level of the internal floor if floorboards are fitted. If the building’s interior has a solid floor then the DPC should be level with this.
Damp proof courses are also required around openings (doors and windows) in cavity walls. These will be vertical at jambs and horizontal below window sills and door thresholds. Vertical DPCs need to be fitted with a slight projection from the brickwork to work efficiently, normally around 10mm from the front of the frame and 25mm into the cavity. Horizontal DPCs should never extend into the cavity as this can trap mortar dropping inside the cavity which can eventually bridge the cavity gap causing damp problems within the wall.
Structures capped with coping stones or other caps, such as retaining walls or chimney stacks may also require a damp proof course. In all of the above situations, separate lengths of DPC should be overlapped by around 100mm to ensure effective damp proofing. If the DPC is to prevent the downwards movement of water however, such as in a capped wall or chimney stack, they should be overlapped and bonded.
Damp proof courses are essential to maintaining the integrity of your building. Dampness can cause many problems, both cosmetic and structural. If any of the DPCs in your property have become damaged, or have been badly constructed from the start, seek the help of a damp proofing specialist who can supply effective remedial action.
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