The mistake we see most is treating the symptom. Stain block, a dehumidifier and a fresh coat of paint will hide a patch for a few weeks and then it comes back darker, because the water is still arriving. Trace it first, then repair.
Read the pattern before you touch anything
A patch that is worst after rain, has a hard tide line and sits high on a wall is penetrating water, usually through a failed detail outside. A patch low down that rises no more than about a metre and shows a salt band is a ground water or bridged damp course issue. A patch that is worst in cold weather, spreads evenly over a cold surface and grows black spot mould is condensation, not a leak.
A patch under a bathroom, kitchen or new radiator run that stays the same size whatever the weather is a plumbing leak until proven otherwise. This is the single most common cause after a refit.
- Worse after rain and high on the wall: penetrating water
- Low, with salting and a tide line: ground water or bridged damp proof course
- Cold weather, black mould, no tide line: condensation
- Constant, near new pipework: a plumbing leak
The defects we find after building work
Bridged damp proof courses come top. A new path, patio or render coat taken above the damp course lets water walk straight past it. So does rubble or insulation left in a cavity during the build, which forms a bridge for water to cross.
Then comes flashing and abutment work: a new lead flashing chased too shallow, a cavity tray missing where a new roof meets an existing wall, or a soil pipe that was reconnected without a proper seal. On new render, hairline cracking around a window reveal will pass water even though it looks dry from the ground.
How the cause gets proved
A moisture meter reading on its own proves very little, because salts in plaster read high whether or not the wall is wet. The useful tests are a hose test on the suspect elevation, lifting a small area of finish to see whether the wall behind is wet or just the plaster, and isolating pipework to see if pressure holds.
If the patch dries out over a warm week and returns after rain, that is penetrating water confirmed. If it never changes, look at plumbing. If it appears on every cold morning and fades by lunchtime, you have a ventilation and heating problem, not a leak.
Getting it repaired once
Repairs are usually small once the cause is known: lowering a path below the damp course, fitting the cavity tray that was left out, cutting out and re-forming a lead detail properly, or remaking a joint in a waste pipe. The expensive part is opening up in the wrong place, which is why we spend the time on diagnosis first.
Only once the wall has genuinely dried does replastering make sense. Skimming over a wet background traps the moisture and the patch comes straight back through the new finish.
Common questions
Is this rising damp?
Rarely. Genuine rising damp is far less common than the term suggests. A new patch after building work is much more likely to be a bridged damp course, a missing cavity tray, a flashing fault or a plumbing leak.
How long should I wait before replastering?
Until the wall itself is dry, not just the surface. Depending on wall thickness and the season that is usually several weeks after the cause has been fixed, and longer on solid masonry.
Can a damp patch appear from the building work drying out?
Yes. New screed, plaster and blockwork all release a lot of water. That shows as general dampness and heavy condensation rather than a defined patch in one place, and it settles over a few months.
Will a dehumidifier solve it?
It helps with construction moisture and condensation. It does nothing about water entering from outside or a leaking pipe, and running one can mask the pattern you need in order to diagnose the cause.
Do you charge to look at a damp patch?
No. The site visit and the written quote are free, and if the honest answer is that it is condensation and needs ventilation rather than building work, we will tell you that.