Materials Used For Damp Proofing (DPC) | What Are The Materials Used For DPC? - HOMESLIBRO

MATERIALS USED FOR DAMP PROOFING (DPC)

There are various materials, which are used as damp-proof courses depending upon the location, economy and degree of damp prooﬁng desired. However, while selecting a particular damp-prooﬁng material, the following requirements of an ideal damp-prooﬁng material should be remembered.

1. The material for DPC should be impervious and durable, i.e., the material should be eﬀective during the useful life of the building.

2. The material should be capable of resisting both dead loads and superimposed loads without being disintegrated.

3. The material should remain steady in its position, without any movements, so that the walls overlying the DPC do not develop any cracks.

4. For DPC above the ground level, with wall thickness up to 40 cm, any material listed below for DPC can be used.

5. For DPC to be laid over larger areas, such as ﬂoors and roofs, and thicker walls, a DPC material that provides a lesser number of joints should be used, such as mastic asphalt, bitumen sheeting and plastic sheeting.

6. The material for parapet walls and in other situations where diﬀerential thermal movements are expected due to exposure should be of ﬂexible material, like mastic asphalt, bituminous felt and metal sheets.

7. In water-retaining structures or situations, a jointless DPC should be provided to take care of the risk of leakage.

8. In the cavity or hollow walls, the cavity over the door or window openings should be bridged by ﬂexible materials, like bitumen sheets, strips of lead and copper.

The materials generally used for DPC are listed below.

1. Flexible materials: Hot bitumen, bituminous felts, bituminous sheet, plastic sheet (polythene sheets), metal sheets of lead, copper, etc.

2. Semi-rigid materials: Mastic asphalt or a combination of materials or layers.

3. Rigid materials: Use of I-class bricks, stones, slates, etc.

Hot bitumen or hot asphalt

This is a ﬂexible material, which is ﬁrst heated and then spread over the bedding of concrete or mortar (i.e., over walls). This should not be applied in thickness less than 3 mm. Bitumen or asphalt forms an excellent damp-proof course, as it oﬀers an impervious, indestructible and tough surface.

Bituminous felts (6 mm thick sheet or asphaltic felt)

This is also a ﬂexible material, which is available in rolls of normal wall widths. For placing this in position, ﬁrst a layer of cement mortar is laid on the brickwork and then DPC is bedded on it. An overlap of 10 cm in case of joints and full-width overlap in case of angles and crossings should be provided. Bitumen felt is capable of accommodating slight movements but cannot withstand heavy loads.

Sheets of lead, copper and aluminium (metal sheets)

These are used as membranes for damp prooﬁng and are of ﬂexible type.

Sheets of lead

The thickness of the sheet should be such that the weight of the sheet is not less than 20 kg/m2. These are spread on the walls and overlapped at the joints. The sheets of lead should be embedded in lime mortar and not in cement mortar (because cement chemically reacts with lead and destroys it). The surfaces of lead should be protected by a coating of bitumen against corrosion. DPC formed by lead sheet provides an impervious and highly resistant surface against lateral movements.

Sheets of copper (minimum 3 mm thick)

Like lead sheets, these are spread, lapped and jointed. They are embedded in lime or cement mortar. This is another excellent DPC material that possesses characteristics such as high durability, good resistance to dampness and high resistance against sliding action.

Sheets of aluminium

These can also be used for DPC but not as good as to lead or copper sheets. These sheets should be protected with a layer of bitumen.

Combination of sheets and bituminous felts

A lead foil is sandwiched between the sheets of asphalt or bituminous felt. This combined sheet is known as ‘lead core’ and DPC of this core possesses the characteristics of easy laying, durability, eﬃciency and economy.

Mastic asphalt

This is obtained by heating the asphalt with sand and mineral ﬁllers. This is a semi-rigid material and forms an excellent impervious layer for damp prooﬁng, i.e., DPC. However, it requires special care in its laying. Good mastic asphalt has many characteristics, such as high durability, excellent water-prooﬁng quality and reasonable elasticity. This can withstand only very slight distortion and is liable to lateral movements under heavy pressures or very hot climates.

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Bricks

Good dense bricks, which absorb water less than 4 per cent of their weight, are suitable as a DPC material at places where the damp is not excessive. The joints are left open. They are widely used for providing or inserting a DPC membrane in an existing wall.

Stones

Generally, dense and sound stones, such as granite, trap and slates, are laid in cement mortar in two courses to form an eﬀective DPC. The stones are used for the full width of the wall. While laying the stones, care should be taken in breaking the continuity of vertical joints.

Cement concrete layers

A cement concrete layer, having mixed proportions 1:2:4 (1 cement:2 sand:4 aggregates) with waterprooﬁng agents, is used as DPC at the plinth level. It is eﬀective in stopping the water rising due to capillary action but allows the water to pass through the cracks, etc., and hence is suitable as DPC material where dampness is not in excess. The concrete layer is used as a horizontal main DPC in thickness varying from 4 to 15 cm followed by two coats of hot bitumen paint.

Mortar

It is used in two ways, either (i) as a bedding layer for taking up other types of DPC or (ii) as a waterprooﬁng plaster. For the bedding layer, the mortar is prepared by mixing cement and sand in proportions 1:3 and adding slight lime to increase the workability. For waterprooﬁng plasterwork, the mortar is prepared by mixing either of the following:

a. 1 cement:2 sand pulverized alum and soap water,

b. Cement: sand in proportions 1:3, with patented waterprooﬁng material like pudio, dampro and sika. After applying this plaster in 2–4 cm thickness, it is painted with two coats of hot bitumen.