Definition of damp proofing

One of the essential requirements of a building is that it should be dry. 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 building adversely, but also creates unhygienic condition for the occupants. The treatment given to prevent leakage of water from roof is generally termed as waterproofing, where as the treatment given to keep the walls, floors and basement dry is termed as damp proofing.

            A damp proof course (DPC) is a physical barrier inserted into the fabric of a building to stop water passing from one place to another. This can be on a horizontal plane, stopping water rising up from the ground by being sucked up by the dry masonry above, or vertically to stop water passing from the outside of a building, though the masonry, to the inside. DPC's have taken many forms through the ages and one of the earliest forms was to use a layer of slate in the construction. Slate is still used but the less expensive plastic version (below right) is now more widely used.

 Horizontal dpc

Causes of Dampness:

    The dampness in building is a universal problem and the various causes, which are responsible for the entry of dampness in a structure, are as follow.

1)      Rising of moisture from ground

                     The ground on which the building is construction may be made of soil, which easily allows the water to pass. Usually the building material used for the foundations, absorb moisture by capillary action. Thus the dampness finds its way to the floor through the sub structure.

                                                 Rising of grond water level

2)      Action of rain

                      If the faces of wall, exposed to heavy showers of rain, are not suitable is protected, the become sources of dampness in the structure. Similarly the leaking root also permits the rainwater to enter a structure.

3)      Exposed of top wall

                      The parapet wall and compound wall also should be providing with a damp proof course on the exposed tops. Otherwise the dampness entering thought these exposed tops of such walls may lead to serious result.

4)      Condensation

                       The process of condensation takes place when warm humid air is cooled. This is due to the fact that cool air can contain less invisible water vapour than warm air. The moisture deposits on the walls, floors and ceiling. This is the main causes in badly designed kitchen.

            There are various causes of dampness as mention below

1. If the site located on a site, which cannot be easily drained off, the dampness will be interring in structure.

2. The orientation of a building is also an important factor, the wall obtaining less sunrise and heavy shower of rain are liable to become damp.

3. The new constructed walls remains damp for short duration.

4. Very flat slope of a roof may also lead the penetration of rain water which is temporary store on roof.

5. The dampness also caused due to bad workmanship in construction

Such as defective joints in the roofs, improper connection of wall.

Effect of dampness

The building material such as bricks, timbers, concrete etc, has moisture content, which is not harmful under normal condition. The rise in moisture content of these materials beyond the certain level from where it come visible or when it deterioration leads to the real dampness. If absolute terms, the moisture content of different materials may be same, but the acceptable limit differs from material to material. For instance, the presence of 10 per cent by weight in timber is not harmful. But the same level could saturate a brick or cause deterioration of plaster.

The structure is badly affected by dampness. The prominent effect of dampness is as follow.

1.      A damp building gives rise to breeding of mosquitoes and creates unhealthy condition for those who occupy it.

2.      The metals used in the construction of material are corroded.

3.      The decay of timber takes place rapidly due to dry-rot in a damp atmosphere.

4.      The unsightly patches are formed on the wall surface and ceiling.

5.      The materials used as floor covering are serious damaged.

6.      It results in softening and crumbing of the plaster.

7.      The materials used for wall decoration are damaged and it leads to difficult and costly repairs.

8.      The flooring get loosened because of reduction in the adhesion when moisture enters through the floor.

Methods of damp proofing

Following methods are used for prevent the defect of dampness in structure

1.      Membrane damp-proofing

2.      Integral damp-proofing

3.      Surface treatment

4.      Guniting

5.      Cavity wall construction

1.      Membrane damp-proofing  

                   This consists in proving layer or membrane of water repellent 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 and it may comprise of material like bituminous felts, mastic asphalt, silicon, epoxy, polymers, plastic or polythene sheets, cement concrete etc depending upon the source of dampness, d.p.c may be provided horizontally or vertically in floor, walls etc. provision of d.p.c in basement is normally term as tanking.

                                   Membrane damp proofing  

General Principles to be observed while laying  d.p.c are as under

1.      The d.p.c should cover full thickness of wall excluding rendering.

2.      The mortar bed upon which the d.p.c is to be laid should be made leveled, even and free projections. Uneven base is likely to cause damage to d.p.c.

3.      When a horizontal d.p.c is to be continued to a vertical face, a cement concrete fillet 75 mm in radius should be provided at the junction, prior to the treatment.

4.      Each d.p.c should be placed in correct relation to other d.p.c, so as to ensure a complete and continuous barrier to the passage of water from floors, walls or roofs.

2. Integral damp proofing

                    This consists in adding certain water-proofing compound with the concrete mix to increase its impermeability. Such compounds are available in market in powdered as well as liquid form. The compounds made from clay, sand or lime help to fill the voids in concrete and make it water proof.

                      Another form of compound like alkaline silicate, aluminum sulphates, calcium chloride etc. react chemically when mixed in concrete to produce water proof concrete.

             Pudlo, permo, impermo etc are some of the many commercially made preparations of water-proofing compound commonly used. The quantity of water proofing compound to be added to cement depends upon the manufacture recommendations. In general, one kg of water proofing compound is added with one bag of cement to render the mortar or concrete water-proofing.

3. Surface treatment

 The moisture finds its way through the pores of material 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 surface subjected to dampness. The use of water repellent metallic soaps such as calcium and aluminium oleates and stearates is much effective in protecting the building against the ravage of heavy rain. Bituminous solution, cement coating, transparent coatings, paints and varnishes fall under this category. In addition to other surface treatment given to walls, the one commonly used in lime cement plaster. The walls plastered with cement, lime and sand mixed in proportions of 1:1:6 is found to serve the purpose of preventing dampness in wall due to rain effectively.

                                           Surface treatment

5.      Guniting

This consists in deposing an impervious layer of rich cement mortar over the surface to be water proofed. The operation is carried out by use of  a machine known as cementgun. The assembly broadly consists of a machine having arrangement for forcing the mixture under pressure through a 50 mm dia flexible hosepipe. The hosepipe 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 usually taken in proportion of 1:3 to 1:4 are then fed into the machine. This mixture finally shot on the prepared surface under a pressure of 2 to 3 kg/cm2 by holding the nozzle of the cement gun at a distance of 75 to 90 cm from the working face. The quantity of water in the mix can be controlled by means of regulating value 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 the treated surface becomes waterproof.

  

Guniting

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 outer to the inner wall.

                           Cavity wall construction