Damp patches on the internal wall of a Sydney home or strata building are one of the most common complaints received by strata managers and building owners. The first call is usually to a roof plumber. The second, when the roof checks out fine, is often to a damp specialist who recommends an injected damp-proof course. Sometimes both are wrong. In a lot of Sydney buildings put up between 1960 and 1995, the actual cause is failed cavity flashing, and the symptoms have been misdiagnosed for years.

What is cavity flashing and what does it do?

Most Sydney brick buildings of that era were built as cavity walls: two parallel skins of brickwork separated by a 40–75mm air gap. The outer skin handles the weather; the inner skin handles the structure and the lining behind it. The cavity in between is meant to stay dry. Wind-driven rain that gets through the outer skin should fall down inside the cavity, hit a waterproof flashing, and run back out through small openings in the brickwork called weep holes.

Cavity flashing is the membrane that catches that water. It sits at the base of every cavity, above every window and door head, around penetrations, and at any point where the cavity transitions vertically. When it’s working, the inner leaf of the wall stays dry no matter what the weather is doing outside.

Why does cavity flashing fail in Sydney?

Three main causes:

1. Age. The original flashings in mid-century Sydney brickwork were lead, bitumen-coated felt, or galvanised metal. All three have a finite life. Lead becomes brittle and cracks; bitumen felt dries out and tears; galvanised metal corrodes through. After 30–60 years, much of the flashing built into Sydney’s 1960–1990 stock is well past its design life.

2. Salt attack. Sydney’s coastal climate is unkind to original flashing materials. Wind-driven salt aerosol penetrates the cavity and accelerates corrosion of any metal flashing from inside. Coastal homes and unit blocks, particularly anywhere within 5km of the ocean, experience flashing failure at significantly faster rates than equivalent buildings inland.

3. Damage during other works. Plumbing penetrations, HVAC installations, internal wet-area renovations, and external rendering jobs all routinely puncture, displace, or bridge cavity flashings without anyone noticing. Most of these are done without ever opening the wall to confirm what’s behind it. Years later, the resulting failure is misdiagnosed as something else.

How to recognise cavity flashing failure

The visual clues are specific:

  • Damp patches at a consistent height. Cavity flashing failure shows up at the flashing line, usually a defined horizontal band, often near floor level or just above the slab. The damp doesn’t migrate progressively; it appears at one height and stays there.
  • Efflorescence on the external brickwork. The white crystalline staining that returns after cleaning, concentrated at the same horizontal level as the internal damp patch.
  • Damp that follows rain events. The wall gets wet after rain, dries out partially in dry weather, then gets wet again. The roof is fine but a particular wall keeps coming up damp.
  • Musty smell in adjacent rooms. Moisture is reaching the inner leaf and the room beyond.

The pattern is different to rising damp (which migrates upward from the base of the wall and shows a tide mark) and different to roof leaks (which generally show up high on the wall or at the ceiling). We’ve written a side-by-side comparison of rising damp vs cavity flashing failure for cases where it’s ambiguous.

What the repair involves

Cavity flashing replacement is invasive but straightforward. We remove the affected course or courses of bricks above the flashing line, take out the failed flashing, install new modern flashing membrane (alkathene, EPDM, or stainless step flashing depending on location), reinstate the bricks using matched mortar, and form weep holes at correct centres so the cavity drains properly going forward.

A typical residential job covering one wall takes one to three days. The work is scoped from the inside and outside, but the repair is mostly external. From the street, once the new mortar has weathered for a season, the work is usually invisible.

When to act

The longer failed cavity flashing is left, the more damage compounds. Internal plaster has to be replaced; mould remediation may be needed; persistent moisture in the cavity can rust embedded steel lintels and accelerate wall tie corrosion. The flashing repair itself is the cheapest part of the chain. Catching it early means the surrounding work stays minimal.

If you’re seeing the symptoms described above in a Sydney building from the 1960s through to the early 1990s, cavity flashing should be on the diagnosis shortlist. We can usually confirm or rule it out from clear photos within 24 hours. Send them through our Project Brief form or call us on 0485 672 664.