Damp Proofing in a construction is a technique of preventing moisture from entering the interior of the structure to prevent weakening or decomposing of construction materials (i.e. weakening of bricks due to overwater absorption and oxidation of steel results in the development of cracks in walls). It is basically a technique of making the outer surface impermeable.
The requirements of ideal material for damp-proofing are as follows :
- Material should be durable
- Material should be steady and does not allow any movement in itself
- Material should be perfectly impervious
- Material should be capable of resisting safely the loads
- Material should be flexible so that it is capable of accommodating the structural movements without any fracture
- Material should be dimensionally stable
- Material should be reasonably cheap
- Material should be free from deliquescent salts like sulphates, chlorides and nitrates.
Types of materials used for damp-proofing are as follows :
- Hot Bitumen :
- This is a flexible material placed on the bedding of concrete or mortar.
- It should be applied with a minimum thickness of 3mm.
2. Mastic Asphalt :
- It is a semi-rigid material and forms an excellent impervious layer for damp-proofing.
- It is durable and completely impervious material.
- It can withstand very slight distortion.
- It is liable to squeeze out in very hot climates or under very heavy pressure.
3. Bituminous Felts :
- It is a flexible material.
- It is easy to lay and is available in rolls of normal wall width.
- It is laid on layer of cement mortar.
- It can accommodate slight movements.
- It is liable to squeeze out under heavy pressure and it offers little resistance to sliding.
4. Metal Sheets :
Lead, Copper and Aluminium are used as membranes of damp-proofing.
5. Combination of sheets and felts :
- A lead foil sandwiched between asphalt or bituminous felt, known as lead core
- It is economical, durable and efficient
6. Stones :
- Two courses of sound and dense stones such as granites, slates etc. laid in cement mortar with vertical breaking joints can work as an effective damp-proofing course
- Stones should extend for full width of walls
7. Bricks :
- Bricks absorbing water < 4.5% of dry weight can be used as damp-proof material
- Bricks are used where damp-proofing is to be done on existing wall
8. Mortar :
- It is used as damp-proofing by mixing 1 part of cement and 3 parts of sand by volume
- For plastering work, the water-proof mortar is prepared by mixing 1 part of cement, 2 parts of sand and pulverized alum at a rate of 120 N/m3 of sand.
9. Cement Concrete :
- A cement concrete of 1 : 2 : 4 is provided at plinth level
- Depth of layer varies from 40-150 mm.
10. Plastic sheets :
- Material is made of black polythene having thickness of about 0.5-1 mm
- Treatment is relatively cheap and not permanent