Both rising damp and cavity flashing failure produce damp patches on internal walls and white staining on external brickwork. They are confused regularly, including by building inspectors and damp-proofing salespeople, and the consequence of getting the diagnosis wrong is expensive: the wrong repair will fail to solve the problem, and you’ll be back paying for the right one a year later. Here’s how to tell them apart before any work starts.
What is rising damp?
Rising damp is moisture from the ground travelling upward through brickwork by capillary action. The bricks and mortar act like a wick, drawing groundwater up through the wall until the rate of upward movement equals the rate of evaporation from the wall surface. The line where that happens is the visible damp height, usually somewhere below 1 metre from the slab.
The classic signature is a progressive tide mark: damp at the very base, fading upward, with efflorescence concentrated near the floor. It’s most common in pre-1945 buildings that were built without a damp-proof course, or where the original DPC has degraded. Sydney’s older Federation and Victorian stock is the typical candidate.
What is cavity flashing failure?
Cavity flashing failure is water getting into the wall cavity through the outer brick leaf, then failing to exit through weep holes the way it’s supposed to. Instead of draining out, it pools at the flashing level (or wherever the flashing has failed) and saturates the inner leaf at that height. The damp shows up at a defined horizontal band, the flashing level, not progressively from the base.
It’s most common in Sydney buildings built between 1960 and 1995, where the original lead, bitumen, or metallic flashings have aged out. Coastal exposure accelerates it. We covered the full picture of cavity flashing failure here.
Side-by-side comparison
| Rising damp | Cavity flashing failure | |
|---|---|---|
| Height of damp patch | Below 1m, from the base | At a specific height, the flashing level |
| Pattern | Gradual fade, tide mark | Defined horizontal band |
| External staining | Efflorescence at the base | Efflorescence at the flashing level |
| Weep holes | Not relevant | Often absent or blocked |
| Common building age | Pre-1945 | 1960–1995 |
| Triggered by rain? | Constant, not weather-driven | Worse after rain events |
| Repair | DPC injection or replacement | Cavity flashing replacement |
Why getting it wrong is expensive
The most common mis-fix is injecting a chemical damp-proof course into a wall that actually has failed cavity flashing. The injection treats the bottom of the wall, but the water is entering at the flashing line a metre up, it never gets near the injected band. Six months later the same damp patch returns. The owner has paid for an unnecessary DPC injection and the actual repair is still ahead of them.
The reverse is less common but does happen: replacing flashing on a building that actually has rising damp. The flashing repair won’t hurt anything, but the damp keeps rising and the symptoms keep returning.
The age of the building usually narrows it down quickly. Pre-war stock with no damp course almost always means rising damp. Post-1960 cavity construction with damp at a consistent height usually means flashing failure. The grey zone is buildings around 1945–1960, where both are possible.
When to get a professional assessment
Before committing to any damp repair, get a specialist remedial assessment. We do this for clients across Sydney Metro, often without a site visit, just from photos. The diagnosis is the easiest part to get right; getting it right saves the wrong repair from getting paid for.
Send photos of the affected area both internally and externally through our Project Brief form, or call 0485 672 664. We’ll come back within 24 hours with our read on which problem you have and what the right fix involves. If we can’t tell from photos, we’ll say so, and we’ll arrange a free site visit if needed.