HORIZONTAL DAMP PROOF COURSE (DPC)
Capillary action can cause moisture to rise vertically through a structure to a height of around 1000mm. Cavity walls must have a horizontal barrier inserted at the base of the wall, to prevent moisture from the ground rising to a point where the internal living space becomes damp. A horizontal damp proof course (DPC) is positioned in both internal and external leaves of a cavity wall at a level that is no lower than 150mm above finished ground level, as required by the Building Regulations. The Building Regulations further require that the DPC be positioned flush with the face of the wall, within a tolerance of + or – 2mm. The damp proof course should also be free from tears and punctures and be uniformly bedded.
The most common material used to form a horizontal damp proof course is a flexible DPC felt, which is actually not a felt-like material at all but a thin, ‘polymeric’ sheet plastic supplied in rolls of varying widths from 100mm up to 600mm. Both sides of the felt have a fine, raised diamond pattern in order to provide a key, as the material is designed to be sandwiched within a 10mm bed joint of mortar. This is achieved by spreading a thin bed joint of mortar, rolling out the felt on top and then smoothing it out and flattening it using the bedding face of a brick. Care must be taken not to damage the felt during this part of the process. Another thin bed joint of mortar is spread on top of the felt for bedding the next course of bricks. Under no circumstances should the felt be laid directly on top of a course of brickwork with no mortar beneath it.
Fig. 168 DPC felt in different roll widths.
At corners and joints in straight runs, DPC felt must be overlapped by a minimum of either 100mm or the width of the felt, whichever is the greater. This minimum overlap is derived from the notion that water can travel up to 100mm horizontally by capillary attraction. It is good practice to exceed the minimum overlapping requirement wherever possible.