There are four types of bitumen dpc, as follows:
(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/ m2, 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.