Damp is a symptom, not a diagnosis
A damp patch tells you that moisture has reached a surface. It does not tell you where that moisture came from, and the four or five possible sources need completely different remedies. That is why the same stain can lead to a £2,000 chemical damp-proof course in one quotation and a £90 gutter repair in another. Before anything is specified, the mechanism has to be established: moisture rising from the ground, water crossing the fabric from outside, moisture condensing out of the indoor air, a leaking pipe or fitting, or moisture still drying out of the construction. Three pieces of information do most of the work in separating them: where the damp appears, how high it reaches, and when it is at its worst.
Rising damp: constant, low and salt-marked
Genuine rising damp comes from groundwater moving up through porous masonry by capillary action. It is normally confined to the lowest part of the wall, rarely climbing more than about a metre because gravity and evaporation eventually balance the capillary pull. It tends to be present all year rather than appearing after rain, it is usually worse on walls in contact with damp ground, and it very often leaves a tide mark with a band of salt deposits at the top edge, because the water carries nitrates and chlorides out of the ground and leaves them behind when it evaporates. Those salts are hygroscopic, which means the plaster can stay damp to the touch long after the original source has been dealt with. Rising damp is real, but in the properties we inspect it is a good deal less common than the number of treatments sold for it would suggest.
Penetrating damp: follows the weather
Penetrating damp is liquid water crossing the building envelope. The distinguishing feature is timing: it appears or worsens during and after wet or windy weather and improves in dry spells. It can occur at any height, which immediately separates it from rising damp, and it is frequently found around a specific feature: below a gutter joint, beside a downpipe, under a window sill, at a chimney breast, around a flashing, or on the elevation that faces the prevailing wind. In cavity-walled properties the water may cross a bridge in the cavity, a defective cavity tray or a mortar snot at the base, so the internal patch may be some way from the external defect. Following the evidence back to the entry point matters more than treating the mark on the plaster.
Condensation: follows temperature and occupancy
Condensation forms when humid indoor air meets a surface cold enough to bring it to dew point. It is worst in cold weather, worst in rooms that are heated intermittently, and concentrated on the coldest surfaces: window reveals and frames, external corners, lintels, floor-to-wall junctions and the walls behind wardrobes and beds. It has no tide mark and no salt band, and its usual companion is black mould growth rather than blown plaster. The moisture comes from ordinary life indoors: cooking, showering, washing and drying clothes. Where glazing, insulation and draught-proofing have been improved without any matching improvement in how moist air leaves the building, condensation becomes almost inevitable, and no damp-proofing product will make any difference to it.
Leaks and residual construction moisture
Plumbing and drainage faults are the most under-diagnosed cause of internal damp. A weeping compression joint, a failed shower tray seal, a cracked waste pipe behind a boxing or a leaking heating pipe under a floor produces persistent, localised damp that does not respond to weather or to heating patterns at all. It should be ruled in or out early, because it is cheap to fix and expensive to ignore. Residual moisture is the other candidate in recently built or recently repaired properties: concrete, screed, plaster and masonry release water for months after completion, and in an airtight house that moisture shows up on the coldest surfaces during the first winter or two. Recognising it prevents a drying-out process being mistaken for a defect.
Reading the pattern before anyone lifts a meter
A meter reading on plaster is a data point, not an answer: it responds to salts and to surface conditions as well as to water. The pattern is more reliable. Damp confined to the bottom 600mm of every external wall, present all year and salt-marked, is consistent with rising damp or with bridging. Damp at first-floor level that follows storms points to the roof, the rainwater goods or the elevation. Mould in the corner of a bedroom that vanishes in July points to a moisture-balance problem. Damp that is always in the same 300mm square, regardless of season, points to a pipe. When two or three of these mechanisms are present at once, which is common, the order in which they are addressed makes a considerable difference to the cost.
What to record, and when to get an inspection
Before commissioning anything, spend a fortnight collecting evidence. Photograph each affected area with something in frame for scale, note the height the damp reaches, write down the date and the weather, and check the obvious external items: gutters, downpipes, gullies, ground levels against the wall, air bricks, pointing and sill details. Note whether extract fans in the kitchen and bathroom actually clear air to outside and how long they run. That record turns a vague complaint into something a surveyor can work with, and it sometimes solves the problem on its own. If the damp is spreading, if timber is involved, if a sale depends on it, or if you have had contradictory advice, an inspection that measures and reports the evidence is the cheapest way to avoid paying for the wrong remedy.
Limitations of general guidance
This article is general guidance. It cannot confirm the cause of dampness in a specific property. Recommendations should follow an inspection of the building, its external environment and its internal conditions.
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