Dampness in building may occur due to bad design, faulty construction and use of poor quality of materials. Dampness not only affects the life of the building but also creates unhygienic conditions of the important items of work in the construction of a building.
The treatment given to prevent leakage of water from roof is generally termed as water proofing whereas the treatment given to keep the walls, floors and basement dry is termed as damp proofing.
Defects Caused by Dampness in Building
The various defects caused by dampness to building may be summarized as under:
- It causes efflorescence which may ultimately result in disintegration of bricks, stones, tiles etc.
- It may result in softening and crumbling of plaster.
- It may cause bleaching and flaking of paint with the formation of coloured patches.
- It may result in the warping, buckling and rotting of timber.
- It may lead to the corrosion of metals.
- It may cause deterioration to electrical fittings.
- It promotes growth of termites.
- It creates unhealthy living conditions for the occupants.
Causes of Dampness in Buildings
Absorption of moisture by the building materials is one of the chief causes of dampness. On account of granular nature of materials, moisture finds an easy access through the voids and this aided by capillary action assists the moisture to travel in different directions.
Thus, either on account of faulty design of structure or bad workmanship or by use of defective structures or by use of defective materials, moisture may find its way on the interior of the building either through the wall, floor or roof.
Sources of Dampness in Building
The important sources of dampness may be summarized as below:
- Dampness rising through the foundation walling. Moisture from wet ground may rise well above the ground level on account of capillary action.
- Splashing rain water which rebounds after hitting the wall surface may also cause dampness.
- Penetration of rain water through unprotected tops of walls, parapet, compound walls, etc may cause dampness.
- In case of sloped roofs, rain water may percolate through defective roof covering. In addition faulty eaves course and eave or valley gutters may allow the rain water to descend through the top supporting wall and cause dampness.
- In case of flat roofs, inadequate roof slopes, improper rainwater pipe connections, and defective junction between roof slab and parapet wall may prove to be the source of dampness.
Methods of Preventing Dampness in Buildings
Following methods are generally adopted to prevent the defect of dampness in a structure:
- Membrane damp proofing
- Integral damp proofing
- Surface treatment
- Cavity wall construction
1. Membrane Damp Proofing
This consists in providing layers of membrane of water repellant material between the source of dampness and the part of the structure adjacent to it. This type of layer is commonly known as damp proof course (DPC) and it may comprise of materials like bituminous felts, mastic, asphalt, plastic or polythene sheets, cement concrete, etc.
Depending upon the source of dampness, DPC may be provided horizontally or vertically in floors, walls, etc. Provision of DPC in basement is normally termed as tanking.
General principles to be observed while laying damp proof course are:
- The DPC should cover full thickness of walls excluding rendering.
- The mortar bed upon which the DPC is to be laid should be made level, even and free from projections. Uneven base is likely to cause damage to DPC.
- When a horizontal DPC is to be continued up a vertical face a cement concrete fillet 75mm in radius should be provided at the junction prior to the treatment.
- Each DPC should be placed in correct relation to other DPC so as to ensure complete and continuous barrier to the passage of water from floors, walls or roof.
2. Integral Damp Proofing
This consists in adding certain water proofing compounds with the concrete mix to increase its impermeability. Such compounds are available in market in powdered as well as in liquid forms.
The compounds made from clay, sand or lime (chalk, fuller’s earth, etc) help to fill the voids in concrete and make it waterproof.
Another form of compounds like alkaline silicates, aluminium sulfate, calcium chlorides, etc react chemically when mixed with concrete to produce water proof concrete.
Pudlo, Imperno, Siks, etc. are some of the many commercially made preparation of water proofing compounds commonly used. The quantity of water proofing compounds to be added to cement depends upon manufacturers’ recommendations.
In general, one kg of water proofing compound is added with one bag of cement to render the mortar or concrete waterproof.
3. Surface Treatment
As described earlier, the moisture finds its way through the pores of materials used in finishing. In order to check the entry of the moisture into the pores, they must be filled up.
Surface treatment consists in filling up the pores of the surfaces subjected to dampness. The use of water repellant metallic soaps such as calcium and aluminium oleates and stearates is such effective in protecting the building against the ravages of heavy rain. Bituminous solution, cement coating, transparent coatings, paints, varnishes fall under this category.
In addition to other surface treatment given to walls, the one economically used is lime cement plaster. The walls plastered with cement, lime and sand in proportion of 1:3:6 is found to serve the purpose of preventing dampness in wall due to rain effectively.
This consists in depositing an impervious layer of rich cement mortar over the surface to be waterproofed. The operation is carried out by use of a machine known as cement gun.
The assembly broadly consists of a machine having arrangements for mixing materials and a compressor for forcing the mixture under pressure through a 50mm diameter flexible hose pipe. The hose pipe has nozzle at its free end to which water is supplied under pressure through a separate connection.
The surface to be treated is first thoroughly cleaned of dirt, dust, grease or loose particles and wetted properly. Cement and sand (or fine aggregates) usually taken in proportion of 1:3 to 1:4 are then fed into the machine. This mixture is finally shot on the prepared surface under a pressure of 2 to 3 kg per square cm by holding the nozzle of the cement gun at the distance of 75 to 90 cm from the working surface.
The quantity of water in the mix can be controlled by means of regulating valve provided in the water supply hose attachment. Since the material is applied under pressure, it ensures dense compaction and better adhesion of the rich cement mortar and hence treated surface becomes waterproof.
5. Cavity Wall Construction
This consists in shielding the main wall of the building by an outer skin wall leaving a cavity in between the two. The cavity prevents the moisture from traveling from the outer to the inner wall.