A damp proof membrane (DPM) is a durable waterproof sheet installed beneath the floor, in walls or a ceiling to keep moisture out. A DPM is always installed in conjunction with a damp proof course (DPC).
Moisture is a threat to the structure of any home or building. When damp enters into walls, wood floors or roofing it can weaken cement through capillary action and also lead to the development of mold. This moisture is kept out through the use of a DPM. These membranes are usually plastic sheeting, bitumen and butyl rubber. Other semi-rigid and rigid materials are used for the DPC, such as mastic asphalt, cement, bricks and stone. A layer of sand can also be placed on top of the membrane to help protect it and prevent damage as the DPC is installed over the DPM.
During installation, the contractor may use a continuous sheeting or sections of sheeting that are overlapped by 4 to 6 inches. The overlapped sheets are sealed together with double-sided mastic tape or a mastic strip compound to prevent any moisture from seeping through the barrier. It is possible to damage the moisture barrier during installation or later on during remodeling. Damaged barriers will allow water through and should be repaired with a fresh piece of DPM that overlaps at least 6 inches in all directions.