A damp patch on the inside of an external wall.

This is one of the most misdiagnosed problems in Sydney brickwork. Most repairs treat the symptom, and the damp comes back. Here is how a brick wall is supposed to keep water out, what goes wrong, and why the position of the patch matters more than the patch itself.

The signs, in plain terms.

A damp or discoloured patch on the internal face of an external wall.

Paint blistering, bubbling or peeling, or wallpaper lifting away from the wall.

A musty smell in the room that does not clear, no matter how much you air it.

Salt or a fine white powder appearing on the internal plaster.

Mould in a corner of the room or at skirting level.

The patch is often worse after sustained rain, improves in dry weather, and then returns.

That last point is important. A patch that reacts to rainfall is telling you the water is coming through the wall from outside, not rising from the ground or condensing out of the air.

The cavity is not a mistake.

Most Sydney brick homes from the 1960s onward are cavity walls: two skins of brickwork with a deliberate gap between them. The outer skin faces the weather, the inner skin holds up the house and carries your plaster and paint.

The gap exists because water will always get through the outer brick skin. Brick and mortar are porous, and in driving rain, moisture tracks through. The cavity gives that water somewhere harmless to go: it runs down the inside face of the outer skin without ever touching the inner wall.

At the base of the cavity, and above every window and door opening, a waterproof membrane called a cavity flashing collects that water and directs it back outside through the weep holes, the small gaps you can see in the mortar joints near the base of the wall. The system is simple and it works well, right up until the flashing fails.

When the flashing fails, the water it was collecting has nowhere to go but across to the internal leaf. That is when a patch appears on the inside of the wall.

What goes wrong inside the cavity.

Original flashings in Sydney homes from roughly 1960 to 1995 were commonly lead, bitumen impregnated felt or similar materials. All of them deteriorate: they crack, corrode or tear over time, and once they do the water finds the gap.

Flashings are frequently damaged during later works. A plumber cutting into the wall, a new window, an air conditioning penetration. None of these trades reinstate cavity flashing, because it is not their job to.

Mortar droppings left in the cavity during the original construction can bridge the gap between the two skins, giving water a path straight across to the inner leaf even where the flashing itself is sound.

Blocked or painted over weep holes stop the system draining. The water collects on the flashing with nowhere to exit, and eventually finds its way inward.

What it usually gets blamed on instead.

Failed cavity flashing sits behind the wall where nobody can see it, so the visible symptom gets attributed to something more familiar. These are the four most common misdiagnoses, and how to tell them apart.

Blamed on rising damp

Rising damp works upward from ground level and is usually confined to the bottom of the wall, often with a tide mark. A patch appearing higher up the wall, particularly near or above a window head, is not rising damp.

Blamed on the roof or gutters

Worth ruling out, but a roof leak usually tracks to a ceiling or the very top of a wall, and it moves with the leak. A failed flashing produces a patch at a consistent point on one wall, at or below the flashing line.

Blamed on condensation

Condensation is generally spread across a cold surface rather than concentrated in one patch. It is worse in winter, and it favours bathrooms, kitchens and the wall behind furniture. A single persistent patch that reacts to rainfall is not condensation.

Treated by painting or sealing the inside

This is the most common response, and it fails. The water source is untouched, so the moisture simply appears elsewhere on the wall, or builds up behind the new coating until it blisters off. It is the clearest example of treating the symptom instead of the cause.

The position of the patch relative to the windows, the floor level and the weep holes is usually what identifies the cause. That is why an inspection matters more than a photograph for this particular problem, although photos are still the right place to start.

What replacing a cavity flashing involves.

Identify the affected run of wall. We confirm the point of water entry and how far along the wall the flashing has failed, so the repair covers the actual problem and no more.

Remove the brick courses at the flashing line. The bricks are taken out carefully and set aside for reuse wherever they are sound.

Clean out the cavity. Mortar droppings and debris bridging the gap are removed, because a bridged cavity leaks even with new flashing in place.

Install the new flashing system. The new membrane is correctly lapped along its length, with pressed sill flashings where the detail requires them, so water is caught and directed outward.

Reinstate the weep holes. Weep holes are formed at the correct spacing so the cavity can drain, which is the whole point of the system.

Rebuild and repoint. The brickwork is rebuilt and repointed in mortar matched to the existing wall.

Internal make-good. The affected internal plaster and paint will need making good once the wall has dried out. That is generally a separate trade, and we will tell you up front so it is in your budget, not a surprise.

You can read more about how we carry out this work on our cavity flashing replacement service page. And because the flashing above an opening protects the steel lintel underneath it, a failed flashing and a cracked lintel line above a window often turn up together.

What affects the price.

Every job is priced individually, because these variables genuinely change the scope of work:

The length of wall affected. A few metres of failed flashing is a different job to a full elevation.

Working height and access. Base of wall work at ground level is simpler than flashing over a first floor window.

Whether the failed flashing is at base level or over openings, which changes the detailing required.

Whether pressed sill flashings are required at windows.

How much brickwork has to be removed and rebuilt to get at the flashing line.

We quote a fixed price after inspecting the site, and the quote sets out exactly what is and is not included.