What is rising damp?
Brick and stone are porous. Moisture in the ground beneath a wall is drawn up through the masonry by capillary action, the same way a paper towel soaks up water. To stop this, masonry walls are built with a damp-proof course (DPC): a barrier layer just above ground level.
In many older Sydney homes, particularly terraces, cottages and walk-up apartment blocks, the original DPC has deteriorated, was never installed, or has been bridged by raised garden beds, paving, render or fill that let moisture bypass it. The result is damp that climbs the wall carrying salts from the ground, which crystallise as the wall dries and destroy plaster and paint.
Signs of rising damp
- Tide mark: a horizontal stain line, usually under about a metre high
- Bubbling, blistering or flaking paint low on walls
- Crumbling, powdery plaster that sounds hollow
- White salt deposits (efflorescence) on walls
- Rotting skirting boards and floor timbers near walls
- Musty smell and mould near floor level
- Spalling bricks and failing mortar outside at the base of walls
Is it really rising damp?
Not all damp low on a wall is rising damp. Misdiagnosis is common, and treating the wrong problem wastes money. Before recommending a DPC we check for:
| Possible cause | Clues | Fix |
|---|---|---|
| Bridged damp course | Garden beds, paving or render above DPC level | Remove bridging, sometimes no injection needed |
| Leaking pipes | Localised damp near plumbing | Plumbing repair |
| Failed waterproofing | Damp beside showers or below balconies | Waterproofing |
| Gutters and downpipes | Damp below overflowing gutters | Gutter repairs |
| Poor subfloor ventilation | Damp floors, musty smell, mould | Subfloor ventilation |
| Condensation | Damp on cold walls, worse in winter | Ventilation and heating |
How we treat rising damp
1. Diagnose
Moisture readings, salt checks and an external inspection to confirm the source and extent.
2. Remove bridging
Lower garden beds and paving, cut back render and clear weep holes where they bypass the DPC.
3. Install a new damp-proof course
A chemical DPC is injected into the mortar course at the correct level along affected walls.
4. Remove contaminated plaster
Salt-damaged plaster and render are stripped back to masonry, typically above the visible damage line.
5. Salt-resistant render
Walls are re-rendered with a salt-resistant system so finishes don't blow as the wall dries. See rendering.
6. Repair and finish
Skirting, floor timbers and joinery replaced, then walls painted.
Painted over it before and it came back?
That's what happens when the cause isn't fixed and salt-resistant render isn't used. We do both.
Why use a multi-trade builder for damp?
Rising damp is rarely just a damp problem. By the time it's noticed there's usually blown plaster, rotten skirting, damaged floorboards, sometimes decayed bearers, and walls that need repainting. A damp-only contractor treats the wall and leaves the rest to you.
We handle the full job: diagnosis, DPC, render, carpentry, ventilation and painting, sequenced properly under one quote. If damp has reached power points or wiring, our electricians check and repair those too.
Rising damp in strata buildings
Many older Sydney walk-up blocks have rising damp in ground floor units, hallways and stairwells. Treatment often involves common property walls, so it's usually managed through the owners corporation. We provide reports and quotes for strata managers and carry out the work with minimal disruption to residents. See strata maintenance.