A damp patch on a wall is a symptom, not a diagnosis. Three very different things produce it: water climbing up from the ground, water pushing through from the other side, and moisture condensing out of the air. Each has its own fix, and none of the fixes work on the other two. Repainting over the wrong one is the most common way home owners spend money twice.
Rising damp: water climbing from below
Masonry is porous. Where a wall sits in damp ground with no working damp-proof course, water wicks upward through the bricks or blocks. As it evaporates from the wall face it leaves behind the salts it carried, which is why rising damp so often comes with a white, fluffy or crusty deposit and paint that bubbles and flakes.
- A fairly level tide mark, typically up to about a metre above the floor, on ground-floor walls only.
- Worst at the base of the wall and fading upward, rather than patchy.
- Salt deposits, blistering paint and crumbling plaster or render.
- Skirting boards that are damp, stained or rotting at the bottom.
It is common across the Gold Coast and coastal Brisbane suburbs, where older homes on sandy, high-water-table ground were built with damp-proof courses that have since failed or were bridged by later paving and garden beds.
Lateral damp: water pushing through from the side
Where a wall retains soil, or the ground outside sits higher than the floor inside, water in that soil presses sideways through the wall. Basements, garages cut into a slope, and the downhill side of split-level homes are the classic cases. Unlike rising damp there is no tide mark; the damp follows the soil level outside and the wettest spots line up with where water collects in the ground.
- Damp that matches the outside ground level, not the floor level.
- Worse after rain, and worst where garden beds, downpipes or paving direct water at the wall.
- Often on one wall or one corner, not all around the room.
- Can be accompanied by efflorescence, but the pattern is a patch or streak, not a band.
Condensation: moisture from the air
Warm, humid air meeting a cold surface leaves its moisture there. In Queensland this is a genuine problem in air-conditioned rooms, poorly ventilated bathrooms, built-in wardrobes against external walls, and any room that is closed up for weeks. The wall is not leaking; it is simply the coldest thing in a humid room.
- Black spotty mould rather than salts or blistering.
- Worst in corners, behind furniture and at the top of walls or on ceilings, not at the base.
- Comes and goes with the weather and how the room is used.
- Often several rooms at once, on internal walls as well as external ones.
Why the distinction changes the fix
Rising damp needs the path from the ground interrupted, typically by installing a new damp-proof course, plus attention to the ground and paving levels that bridged the old one. Lateral damp needs the water stopped on the outside or the wall waterproofed from the inside as a designed system, along with fixing whatever is delivering water to that wall. Condensation needs ventilation, insulation or a change in how the room is heated and cooled. Applying a damp-proof course to a condensation problem does nothing; tanking a wall with rising damp moves the moisture somewhere else.
A proper investigation maps where the moisture is, at what heights, in what pattern, and how it changes with weather, then reads that pattern against the building. That is the job HydroIQ does. The repair, once the cause is known, is carried out by a licensed contractor such as our sister company S&S Remedial.
Common questions
General information for South East Queensland home owners, not advice about your building or a legal matter. Standards and time limits change; check the current edition or rule before relying on a figure here.