Semi-rigid damp-proof courses - Mastic asphalt.

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Mastic asphalt, spread hot in one coat to a thickness of 13mm, is a semi-rigid dpc, impervious to moisture and water. Moderate settlement in a wall may well cause a crack in the asphalt through which moisture or water may penetrate. It is an expensive form of dpc, which shows on the face of walls as a thick joint, and it is rarely used as a dpc.

Polymer-based sheets.

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Polymer-based sheets are thinner than bitumen sheets and are used where the thicker bitumen dpc mortar joint would be unsightly. This dpc material, which has its laps sealed with adhesive, may be punctured by sharp particles and edges.

Polythene sheet.

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Polythene sheet for use as a dpc should be black, low density polythene sheet of single thickness not less than 0.46 mm, weighing approximately 0.48 kg/m^2. Polythene sheet is flexible, can withstand distortion due to moderate settlement in a wall without damage and is an effective barrier against moisture. It is laid on an even bed of mortar and lapped at least the width of the dpc at running joints and intersections. Being a thin sheet material, polythene makes a thinner mortar joint than bitumen dpc, and is sometimes preferred for that reason.

Its disadvantage as a dpc is that it is fairly readily damaged by sharp particles in mortar or the coarse edges of brick.

Bitumen damp-proof course.

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(1) bitumen dpc with hessian base
(2) bituman dpc with fibre base

(3) bituman dpc with hessian base and lead
(4) bitumen dpc with fibre base and lead.

Bitumen dpcs are reasonably flexible and can withstand distortion due to moderate settlement in walls without damage. They may extrude under heavy loads without affecting their efficiency as a barrier to moisture. Bitumen dpcs, which are made in rolls to suit the thickness of walls, are bedded on a level bed of mortar and lapped at least 100 mm or the width of the dpc at running joints and intersections.

Bitumen is much used for dpcs because it is at once economical, flexible, reasonably durable and convenient to lay. There is little to choose between hessian or fibre as a base for a bitumen dpc above ground. The fibre base is cheaper but less tough than hessian.

The lead cored dpc, with a lead strip weighing not less than 1.20 kg/m^2, joined with soldered joints, is more expensive and more effective than the bitumen alone types. It is generally used as the horizontal dpc for houses.

The combination of a mortar bed, bitumen dpc and the mortar bed over the dpc for brickwork makes a comparatively deep mortar joint that may look unsightly.

Copper.

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Copper as a dpc should be annealed, at least 0.25 mm thick and have a nominal weight of 2.28 kg/rn2. Copper is an effective barrier to moisture and water, it is flexible, has high tensile strength and can suffer distortion due to moderate settlement in a wall without damage. It is an expensive material and is little used today as a dpc above ground. When used as a dpc, it may cause staining of wall surfaces due to the oxide that forms. It is spread on an even bed of mortar and lapped at least 100 mm or the width of the dpc at running joints and intersections.

There are four types of bitumen dpc, as follows:

Flexible dpcs Lead.

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Lead for use as a dpc should weigh not less than 19.5 kg/rn2 (Code No 4, 1.8 mm thick). Lead is an effective barrier to moisture and water. It is liable to corrosion in contact with freshly laid lime or cement mortar and should be protected by a coating of bitumen or bitumen paint applied to the mortar surface and both surfaces of the lead. Lead is durable and flexible and can suffer distortion due to moderate settlement in walls without damage. Lead is an expensive material and is little used today other than for ashlar stonework or as a shaped dpc in chimneys. Lead should be laid in rolls the full thickness of the wall or leaf of cavity walls and be lapped at joints along the length of the wall and at intersections at least 100mm or the width of the dpc.

Material for damp-proof courses above ground.

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It is convenient to group the materials used for dpcs as flexible, semirigid and rigid. Flexible materials such as metal, bitumen and polythene sheet can accommodate moderate settlement movement in a building which may fracture the semi-rigid material mastic asphalt and will probably fracture the rigid materials brick and slate.