Dampness is the presence of hygroscopic or gravitational moisture. It gives rise to unhygienic conditions apart from reduction in strength of the structural components of the building. Damp proofing in construction is a type of moisture control applied to building walls and floors to prevent moisture from passing into the interior spaces. Damp problems are one of the most frequent problems encountered in homes.
Damp proofing is defined by the American Society for Testing and Materials (ASTM) as a material that resists the passage of water with no hydro-static pressure and waterproof as a treatment that resists the passage of water under pressure. Generally damp proofing keeps moisture out of a building where vapor barriers keep interior moisture from getting into walls. Moisture resistance is not necessarily absolute: it is usually defined by a specific test method, limits, and engineering tolerances.
Causes of dampness
- Moisture rising up the walls from the ground
- Rain seepage from the roofs
- Rain beating heavily against the external walls
- Condensation of moisture
- Poor drainage conditions
- Imperfect orientation of walls
- Imperfect roof slope
- Defective construction
- Absorption of water from water pipes
Methods of damp proofing
Damp proofing is accomplished several ways including
- Adamp-proof course (DPC) is a barrier in a masonry wall designed to resist moisture rising through the structure by capillary action such as through a phenomenon known as rising damp. The damp proof course may be horizontal or vertical. A DPC layer is usually laid below all masonry walls, regardless if the wall is a load bearing wall or a partition wall.
- Adamp-proof membrane (DPM) is a membrane material applied to prevent moisture transmission. A common example is polyethylene sheeting laid under a concrete slab to prevent the concrete from gaining moisture through capillary action. A DPM may be used for the DPC.
- Integral damp proofingin concrete involves adding materials to the concrete mix to make the concrete itself impermeable.
- Surface coatingwith thin water proof materials for resistance to non-pressurized moisture such as rain water or a coating of cement sprayed on such as shotcrete which can resist water under pressure.
- Cavity wall construction, such asrain screen construction, is where the interior walls are separated from the exterior walls by a cavity.
- Pressure groutingcracks and joints in masonry materials.
Materials used for damp proofing
- Hot bitumen – Highly flexible material placed on the bedding of concrete and mortar and applied in hot condition.
- Mastic asphalt – Semi rigid material which is quite durable and completely impervious.
- Bituminous and asphaltic felts – This is a flexible material which is available in rolls and after laying the laps should be sealed with bitumen.
- Metal sheets – Sheets of aluminium, lead and copper can be used to DPC
- Combination of sheets and bituminous felts
- Bricks
- Stones
- Mortar
- Cement concrete
- Plastic sheets