Why does my brick wall only leak in storms from one direction?

Wind pressure drives rain past the outer leaf of brickwork only on the exposed orientation, and a failed cavity flashing then delivers that water inside. Leaks that appear only after wind-driven rain from a particular direction are a classic signature of a cavity flashing defect rather than a plumbing problem.

Can't we just seal the outside of the brickwork?

Sealing the face treats the symptom and can make things worse by trapping moisture. Cavity walls are designed on the assumption that some water gets past the outer leaf and is caught and drained back out by flashings and weep holes. The lasting fix is restoring that drainage system, not trying to make the brickwork waterproof.

Why is the damp patch inside nowhere near the actual defect?

Water that gets past a failed flashing tracks along the flashing line, the top of framing members or the slab before it shows itself. The internal symptom is downstream of the entry point, which is why cavity flashing leaks are so often misdiagnosed and why staged water testing or inspection openings are used to pin down the true entry path.

Is retrofitting a cavity flashing a big job?

It involves removing courses of the outer-leaf brickwork in short alternating stages, installing the new flashing with the correct upstand, laps, stop ends and weep holes, and reinstating the bricks. It is not trivial — over openings it needs propping and engineering input — but it is routine remedial work, and it is the only repair that actually lasts. Surface sealants over the brickwork do not fix a missing flashing.

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Leaks Caused by Cavity Flashings: The Hidden Component Behind Persistent Brick Wall Leaks

The Wall Is Supposed to Let Water In

Brick veneer and cavity masonry walls are not designed to be waterproof at the outer face. Wind-driven rain will get past the outer leaf — through hairline cracks, porous mortar joints and the bricks themselves. The wall works because of what is inside it: the cavity is a drainage plane, and cavity flashings are the trays that catch the water that gets in and direct it back out through weep holes before it can reach the inner leaf, the frame, or the interior.

When a flashing is missing, damaged or badly detailed, the wall's drainage system fails at that point — and the water the wall was always expected to catch ends up inside the building instead. Cavity flashing failures are among the most common causes of persistent, hard-to-trace leaks in brick buildings, precisely because the failed component is buried inside the wall where nobody can see it.

Where Flashings Live — and Fail

Flashings are required wherever water collected in the cavity must be discharged, or wherever the cavity is interrupted:

Slab edges and the base of walls — the damp-proof course and flashing that step water out at the bottom of the cavity.
Window and door heads — over the lintel, catching water above the opening.
Window sills — under the sill, catching water that gets past the sill joint.
Shelf angles and slab supports in multi-storey cavity construction.
Roof/wall junctions and parapets on more complex buildings.

The Common Failure Modes

Missing entirely. The most common finding on opening up — no flashing was ever installed at the sill, head or slab edge. Prevalent in volume-built housing and older stock.
No upstand. Flashing laid flat instead of turned up against the inner leaf or frame — water simply runs over the back edge and into the building.
No stop ends. Flashing not turned up at its ends, so collected water pours off the end of the tray into the cavity. Classic at window heads — the leak appears beside the opening rather than under it.
Not lapped or sealed at joints and corners — water exits at every discontinuity.
Terminated inside the wall rather than carried through the outer leaf to the face, so the collected water is released straight back into the brickwork.
Blocked or missing weep holes — mortar-filled, rendered or painted over, or never formed. The flashing tray fills up and overflows inward.
Bridged cavity. Mortar droppings and debris sitting on the flashing form a bridge that carries water across to the inner leaf and blocks the weeps.
Construction damage — flashings punctured, torn or displaced during bricklaying.
Material degradation — older bitumen-coated flashings embrittling and splitting with age.

How to Recognise a Cavity Flashing Leak

The tell-tale pattern is rain-dependent, wind-direction-dependent internal moisture at predictable locations:

Damp patches or staining at window and door heads after wind-driven rain — often only in storms from one direction.
Damp at sill level below windows, blistering paint on the reveals.
A horizontal damp or efflorescence line at slab edge or damp-proof course level.
Damp skirtings and carpet at the base of ground-floor walls.
Efflorescence or staining bands on the external brickwork at flashing levels.
Leaks that defy plumbing investigation — no fixture anywhere nearby, and it only appears after rain.

Because the symptoms show up downstream of the failure, the wet patch inside rarely sits directly opposite the defect in the wall — which is why these leaks get misdiagnosed for years.

How the Leak Is Traced

Targeted water testing — staged hose testing that isolates one element at a time (sill, then head, then wall face), working bottom-up, to reproduce the leak and pin the entry path.
Moisture meter and thermal imaging mapping from inside to trace the wet path back towards its source.
Borescope inspection through a weep hole or a small drilled port, to view the cavity directly: is there a flashing, is it bridged with mortar, where does it end?
Inspection openings — removing selected bricks (or a section of internal lining) at the suspect flashing line. Direct verification, and usually necessary before a repair can be specified honestly.

The Remedial Fix

The uncomfortable truth first: flashings buried in a masonry wall cannot be repaired from the surface. Sealant smeared on the outside of the brickwork does not fix a missing tray inside the cavity — at best it redirects the water for a season. The repairs that last:

Retrofit flashing by staged brick removal. The standard repair. Courses of the outer leaf are removed in short alternating sections — so the wall above stays supported — and the new flashing is installed with a proper upstand against the inner leaf, lapped and sealed joints, stop ends, and carried through to the face of the wall. Weep holes are formed and the brickwork reinstated. Over openings this is combined with lintel work and needs temporary propping designed for the opening — engineering input, not improvisation.
Localised head or sill flashing replacement where the defect is confined to a single opening — the same principles at smaller scale.
Clear the drainage where the flashing exists but has been defeated: rake out and re-form blocked weep holes, remove mortar bridging through openings, clean the tray.
Interim measures. Water repellents or face sealing can reduce inflow while a staged repair program is planned — common on strata buildings — but they are management, not repair, and they must never seal the weep holes.

Every retrofit must preserve the wall's logic: water gets in, is caught, and drains out. Details that trap water in the cavity — sealed weeps, cavity fill below flashing level — make the wall worse than before.

When to Involve an Engineer

Openings over structural lintels — propping design and lintel condition. Corroded steel lintels and missing head flashings very frequently co-exist, and the brick removal is the opportunity to deal with both.
Multi-storey cavity walls and shelf angles, where the flashing interacts with the structure supporting the brickwork.
Recurring leaks where previous sealant "fixes" have failed and an evidence-based diagnosis and scope is needed before more money is spent.
Strata buildings where the owners corporation needs an independent diagnosis, a specification, and a staged repair program contractors can price against.

Key Takeaways

Cavity walls are designed to let some water past the outer leaf; flashings and weep holes are the drainage system that gets it back out.
A missing, flat, unstopped or bridged flashing turns that design assumption into a leak.
Rain-dependent, wind-direction-dependent damp at heads, sills and slab edges is the classic signature — and the wet patch is rarely opposite the defect.
Diagnosis is staged water testing, borescope inspection and inspection openings — not guesswork.
The lasting fix is retrofitting the flashing by staged brick removal with correct upstands, laps, stop ends and weep holes. Surface sealants are interim management at best.
Never seal weep holes. They are the outlet, not the problem.

Chasing a leak that only appears after wind-driven rain? Contact our facade and waterproofing team for staged water testing, a cavity inspection and a repair scope that ends the cycle.