Rising damp happens when groundwater is drawn upward through the tiny capillary pores in brick, block, or mortar — the same physical effect that lets a paper towel soak up a spill. In a properly built structure, a damp-proof course (DPC) — a continuous horizontal barrier of plastic, bitumen, or chemical cream built into the wall just above ground level — interrupts that capillary path before moisture can climb any higher.

Problems start when the DPC is missing, damaged, or bridged. In older buildings, the physical barrier can simply degrade over decades. In newer buildings, a far more common cause is 'bridging' — render, paving, soil, or a flowerbed built up against the wall so it sits higher than the DPC, giving moisture a path around the barrier rather than through it. Fixing the bridge, by lowering the external ground level or render line, is often the entire cure and costs a fraction of a full chemical re-treatment.

Before treating anything, get the diagnosis right. Rising damp is genuinely rare compared with its two look-alikes: penetrating damp, where rain gets in through a crack or failed render, and condensation, caused by humid air meeting a cold surface. All three produce damp patches, but rising damp is the only one that stays roughly constant year-round, typically shows a tide-mark no higher than about a metre above ground level, and is often accompanied by white salt deposits (efflorescence) as dissolved ground salts are left behind when the moisture evaporates.

Where an existing DPC has genuinely failed, a chemical DPC injection is the standard retrofit: holes are drilled along the base of the wall and a water-repellent cream or fluid is injected to form a new horizontal barrier without demolishing the wall. It's frequently paired with a damp-proof membrane applied before replastering, since the old plaster is usually contaminated with hygroscopic salts that will keep drawing in moisture even after the source is cut off.