Damp-proof course (DPC)
An impermeable layer in masonry walls that prevents moisture rising from the ground. Pre-1940s homes often lack a working DPC.
A working DPC sits a few courses up from ground level, breaking the capillary path that allows water to wick upward through masonry. Modern DPCs are bitumen, polythene, or chemical injection. Older homes used slate, bituminised felt, or lead - all of which can fail over time.
Failed DPCs cause rising damp and salt damp. Symptoms appear as tide-mark stains on lower walls, blown plaster internally, efflorescence externally.
Retrofit options: chemical injection DPC (common, moderately effective), physical insertion (highly effective but very expensive), or in some cases sufficient improvement of sub-floor ventilation and drainage to make the DPC issue moot.
Salt damp
Salt-laden moisture wicking up from the ground into masonry walls. Visible as white crystalline efflorescence on lower courses, blown render, or paint flaking on internal walls.
Rising damp
Moisture rising vertically through masonry walls from the ground, in the absence of a working damp-proof course.