damp course

A damp course is a continuous impermeable layer built into the masonry of a wall or foundation to block the upward movement of ground moisture. This is distinct from weatherproofing: a damp course specifically stops capillary rise, the process by which water wicks up through porous masonry from the soil below, often reaching heights of one meter or more if left unchecked. Without a damp course, moisture will migrate into interior walls, rotting timber, corroding steel reinforcement, and causing efflorescence, damp patches, and mold.

The traditional damp course material is slate or tile, laid as a single layer in a bed of mortar about 150 mm above external ground level. Modern damp courses are more commonly polyethylene film, bituminous membranes, or dense engineering brick. The material must be continuous, lapped at joints by at least 150 mm, and slightly wider than the wall thickness so that moisture reaching the surface can run outward rather than being trapped internally. Flexibility is important: rigid sheets crack under settlement, allowing water to bypass the barrier.

Older buildings often have no damp course at all, or a failed one that has cracked, been bridged by mortar droppings, or penetrated by pipes. Retrofitting a damp course is difficult and expensive; it requires either cutting a horizontal channel into the existing wall and grouting in a resin-based or cream membrane, or injecting a chemical damp-proof course along the mortar bed. These methods are less reliable than original construction, and cavity bridging by debris, mortar, or debris remains a persistent problem.

Where it sits in the building envelope

A damp course is part of the base of the building structure. It works in concert with surface-level drainage, foundation ventilation, and the cavity in a cavity wall. If the cavity is bridged by mortar or thermal blocks, water crossing the outer leaf can reach the damp course and spread laterally. Conversely, if the damp course is missing or defective, dampness rises through the full thickness of the wall. The damp course is therefore the critical last line of defense against rising damp, distinct from the waterproofing of the foundation slab itself, which resists hydrostatic pressure from groundwater.

Building codes specify damp course installation as mandatory in new construction. British Standard BS 3921 covers brickwork; equivalent standards exist in most jurisdictions. The presence and condition of a damp course should be confirmed during structural survey, particularly in properties built before 1900, when the practice was not yet universal, and in post-war concrete block construction, where cement-rich mortar and high suction brickwork accelerated damp problems if no course was installed.