Key takeaways
- Water enters every masonry cavity wall through the outer leaf, so the design must provide a continuous drainage path rather than rely on the wall staying dry.
- Flashing must be continuous, correctly graded to the outlet, and lapped so that water cannot track back into the inner leaf.
- Weep holes are the final outlet, and their spacing, height above the flashing, and freedom from mortar blockage determine whether the cavity actually drains.
- The cavity must be kept clear of mortar droppings and debris, because a blocked cavity turns a drained wall into a reservoir.
- Getting drainage detailing wrong rarely shows up at practical completion; it surfaces as damp, efflorescence, or timber decay after the owner has moved in, and the builder carries the cost of rectification.
Water gets into masonry. That is not a defect; it is a property of the material and the way walls are built. A brick or block outer leaf absorbs rain, and under wind pressure some of that moisture migrates through the leaf and into the cavity. The question that matters on site is not whether water enters, but where it goes once it is inside.
In a cavity wall, the answer is meant to be: down the inside face of the outer leaf, onto a flashing, along the flashing to a weep hole, and out. Every part of that path has to be built deliberately. If any part is missing, discontinuous, or blocked, the wall stops behaving as a drained assembly and starts behaving as a reservoir. The water then finds its own way out — usually into the inner leaf, the frame, the insulation, or the floor structure.
This article deals with the drainage end of the detail: the flashing that collects water, the weep holes that discharge it, and the cavity that has to stay clear for any of it to work. It does not cover the full range of damp-proofing or the selection of the outer leaf itself. The focus is on where water is meant to leave the wall assembly, because that is where most of the callbacks originate.
Why cavity masonry is designed as a drained assembly
The cavity exists because masonry is not waterproof. A single-leaf wall relies on the material and the render or paint coating to keep water out, and in driving rain that is a losing proposition. A cavity wall accepts that the outer leaf will get wet and provides a second line of defence: an air space, an inner leaf that is kept dry, and a drainage path at the base.
This is why the flashing and weep holes are not optional extras or belt-and-braces measures. They are the mechanism that makes the cavity work. Without them, water that enters the cavity has nowhere to go except down and inward. It accumulates at the base of the cavity, saturates the inner leaf, and eventually appears as damp patches on the internal face, lifting paint, staining plasterboard, or rotting timber floor framing.
The same principle applies above openings. A lintel over a window or door interrupts the downward path of water in the cavity. Water running down the inside face of the outer leaf above the opening has to be intercepted before it reaches the lintel and diverted out. That is why a flashing and weep holes are required at the head of openings, not just at the base of the wall.
Where flashing is required in a cavity wall
Flashing is required wherever water running down the cavity needs to be intercepted and redirected to an outlet. In practice, that means at least at the base of the wall, at the head of every opening in the outer leaf, and at any horizontal interruption such as a change in wall type, a shelf angle, or a roof-to-wall junction.
At the base of the wall, the flashing sits on top of the inner leaf or the slab edge and extends across the cavity and into the outer leaf. It must be continuous around corners and along the full length of the wall. A break in the flashing is a break in the drainage path, and water will find it.
At the head of openings, the flashing sits above the lintel or the opening frame and extends into the outer leaf. It has to be integrated with the lintel and with any damp-proof course so that water cannot track behind the flashing and into the opening.
The flashing material has to be suitable for the exposure and compatible with everything it touches. Where dissimilar metals are in contact, corrosion can destroy the flashing long before the wall reaches its design life. Check the manufacturer’s literature and the NCC for the material and the minimum fall.
The drainage path in a cavity masonry wall
- 1Water enters the outer leafDriving rain is absorbed by the masonry and migrates inward through the leaf and any gaps in the mortar.
- 2Water reaches the cavity faceIt runs down the inside face of the outer leaf rather than crossing the cavity as a sheet.
- 3Flashing intercepts the waterA continuous flashing at the base of the cavity, or above an opening, collects what has run down.
- 4Flashing grades to the outletThe flashing falls towards the outer leaf so water moves to the weep holes rather than pooling on the membrane.
- 5Weep holes dischargeWater exits through the open perpend joints at the base of the outer leaf and drains clear of the wall.
Flashing grades, laps and continuity
The single most common flashing defect on site is a fall in the wrong direction, or no fall at all. Water does not run uphill, and a flashing laid flat or with a slight backward slope will hold water on the membrane. That water eventually finds a lap, a puncture, or a fixing and gets into the inner leaf.
The flashing must be laid to the fall the manufacturer and the NCC require, and it must be supported so it cannot sag between supports. A membrane that droops between the brick ties will pond, even if it was laid to the correct fall.
Laps must run in the direction of water flow, with the upper sheet overlapping the lower one. The lap width and the method of sealing depend on the material; some flashings are sealed with a compatible adhesive, some are folded, and some rely on a mechanical clamp. Whatever the method, it has to be continuous. A lap that is sealed at the ends but open in the middle is still a leak.
Continuity at corners and penetrations is where most failures start. A flashing that stops short of a corner and relies on the next length to cover the gap will leak at the joint. The same applies where pipes, conduits, or structural fixings pass through the flashing line. These penetrations need to be detailed and sealed, not simply pushed through the membrane.
Weep holes: the outlet that makes the system work
Weep holes are the final element of the drainage path. They are open perpend joints, or formed vents, left in the outer leaf at the level of the flashing. Their job is to let water out and to let air move through the cavity so it can dry.
The spacing and size of weep holes are set by the design and the NCC. They need to be frequent enough that water does not have to travel far along the flashing to reach one, and they need to be large enough to discharge the volume of water that can arrive during a storm. They also need to be positioned immediately above the flashing, so that water sitting on the membrane can flow directly out rather than having to dam up to reach the opening.
Weep holes get blocked. Mortar droppings fall into the cavity and set at the base, covering the flashing and sealing the outlets. Render or paint applied over the outer leaf can bridge the openings. Insects and debris can block them over time. Any of these turns a drained wall into a wall that holds water.
On site, the practical defence is to form the weep holes properly and then protect them. Some builders use proprietary weep hole formers or mesh inserts that keep mortar out while allowing water through. Others rely on careful bricklaying and a clean-out at the base of the cavity before the wall gets too high. Either way, the check is the same: after the outer leaf is complete, every weep hole should be open and should drain.
Site checks before the outer leaf goes up
- Confirm the flashing material type and that it is suitable for the exposure and the metals it touches.
- Check that the flashing is laid to the fall the manufacturer and the NCC require, with no ponding areas.
- Verify that laps run in the direction of water flow and are sealed or folded as the detail specifies.
- Make sure the cavity is wide enough to remain clear after the tie and mortar installation.
- Check that weep holes are formed at the spacing the design shows and sit immediately above the flashing.
- Confirm that the cavity is cleaned out before the wall reaches a height where it cannot be reached.
- Inspect the base of the wall after the outer leaf is complete to confirm every weep hole is open and draining.
Keeping the cavity clear and the path open
A cavity that is full of mortar droppings and offcuts is not a cavity. It is a solid mass with a damp interface, and water sitting in it will migrate to the inner leaf. The drainage detail assumes an open air space, and that space has to survive the build.
Mortar droppings are the usual culprit. They fall from the trowel as the bricklayer works and collect on the flashing and the ties. If they are not cleaned out, they set hard and block the base of the cavity. On a long wall, this can happen in a matter of hours.
The standard site controls are straightforward: keep the cavity width as designed, use a cavity board or a batten to catch droppings as the work proceeds, and clean out the base of the cavity before it becomes unreachable. On a multi-storey wall, that means cleaning at each lift, not just at the top. Once the wall is up, there is no practical way to remove a set mortar bed from the base of the cavity.
Cavity ties also matter. They must be installed to the correct fall so that water running down them drips off rather than tracking across to the inner leaf. A tie that slopes towards the inner leaf is a bridge for water.
Flashings at openings, roof junctions and changes in plane
The base of the wall gets the most attention, but the same detailing is needed wherever the wall changes direction or meets another element. At the head of an opening, the flashing has to be integrated with the lintel and the damp-proof course. At a roof-to-wall junction, the flashing has to connect the roof covering to the wall so that water running down the wall cannot get behind the roof. At a change from masonry to another cladding, the flashing has to transition between the two drainage systems.
These junctions are where trades meet, and that is where responsibility gets blurred. The bricklayer installs the flashing at the base of the wall, but the roof plumber installs the roof flashing, and the window installer sets the window. If the interfaces are not coordinated, water can track from one system into another and appear in a place that nobody expects.
The defence is a single, continuous detail that runs from the roof to the base of the wall, with each trade knowing where their element starts and finishes. On site, that means marking up the drawings, holding a pre-start with the relevant subbies, and inspecting the flashing before it is covered by the next layer. Once the outer leaf is up, the opportunity to correct the detail is gone.
Worked example: a two-storey brick veneer in western Sydney
A builder in western Sydney is completing a two-storey brick veneer house on a sloping block. The lower level is on a slab, the upper level is on timber floor framing, and the external walls are cavity brickwork with a rendered finish. The site is exposed to driving rain from the west.
At the base of the lower level, the flashing is installed correctly: continuous, graded to the outer leaf, with weep holes at the specified spacing. The bricklayers use a cavity board and clean out the base at the end of each lift. That part of the wall is fine.
The problem is at the upper floor level. The floor framing bears on a steel beam that runs along the western wall, and the cavity is interrupted at that point. The flashing over the beam is installed in short lengths, with the laps running in the wrong direction because the bricklayer was working from both ends towards the middle. No weep holes are formed above the beam, on the assumption that the wall above is protected by the roof and the upper flashing.
Two years after handover, the owner reports damp patches on the ceiling below the upper floor and musty smells in the bedroom. The cause is water entering the upper wall, running down the cavity, and being intercepted by the beam flashing. Because the laps run the wrong way and there are no weep holes, the water sits on the flashing and eventually tracks into the floor framing. The repair involves removing sections of the outer leaf, replacing the flashing, forming weep holes, and treating the affected framing.
The lesson is not that the base detail was wrong. It is that the drainage path has to be continuous through every interruption, and the builder has to inspect the flashing at each level before it is covered. A five-minute check before the next lift would have caught the lap direction and the missing weep holes.
Inspection, handover and the cost of getting it wrong
Flashing and weep hole defects rarely show up at practical completion. The wall looks fine, the render is painted, and the building is dry. The water that enters the cavity during the first major storm has to fill the available storage before it overflows into the inner leaf, and that can take months or years.
When it does show, the symptoms are familiar: damp patches on internal walls, efflorescence on the outer leaf, lifting paint, musty smells, and in severe cases, decay in timber framing or floor boards. By then, the owner is in occupation, and the builder is dealing with a rectification claim.
The cost of rectification is high because the wall has to be opened up. Removing and rebuilding sections of the outer leaf, replacing the flashing, and treating any water damage to the inner leaf or framing is slow, messy, and disruptive. It is also the kind of defect that can affect multiple units in a townhouse development, multiplying the cost.
The commercial reality is that the cheapest place to solve a drainage problem is on the drawing board, and the second cheapest is on site before the wall is built. After handover, the builder carries the cost of access, demolition, reconstruction, and make-good, along with the reputational damage.
What to check on site, and where to find the requirements
The specific requirements for flashing material, minimum fall, lap width, weep hole spacing, and cavity width are set out in the NCC and in the manufacturer’s installation instructions. Those documents are the authority, and they are updated regularly. Do not rely on a detail from an old project or a verbal instruction from a supplier.
Before the outer leaf goes up, walk the wall and check the flashing for continuity, fall, and lap direction. Check that the cavity is clear and that the ties are installed with the correct fall. Confirm that weep holes are formed at the required spacing and that they sit immediately above the flashing.
After the outer leaf is complete, check every weep hole from the outside. They should be open, free of mortar, and positioned so that water can run out. If the wall is rendered, make sure the render does not bridge the openings.
Keeping up with the current requirements is part of the job, and in NSW, that means completing the continuing professional development the Act requires. PointCPD provides a guide to how NSW builder CPD works, and the course catalogue covers the detailing and compliance topics that come up on site. For related reading on how water behaves at other junctions, see the articles on wall cladding fixing and clearances and metal roof flashing details that leak.
Questions builders ask
How far apart should weep holes be in a brick cavity wall?
The spacing is set by the NCC, the design drawings, and the manufacturer’s instructions for the flashing system. It is frequent enough that water does not have to travel far along the flashing to reach an outlet, and it is usually specified on the architectural drawings. If the drawings do not show it, ask the designer before the outer leaf goes up rather than guessing on site.
Can weep holes be formed with plastic inserts instead of open perpends?
Yes. Proprietary weep hole formers and mesh inserts are commonly used to keep mortar out of the opening while allowing water and air through. They must be installed so they do not block the flashing or reduce the effective opening. Check the manufacturer’s instructions for the correct installation and compatibility with the flashing material.
What happens if the flashing is laid with no fall or a fall towards the inner leaf?
Water will pond on the flashing instead of draining to the weep holes. Over time, it will find any lap or penetration and track into the inner leaf, causing damp patches, efflorescence, and potentially decay in timber framing. The defect is hidden until the wall is opened up, so the cost of correction is high.
Do I need flashing and weep holes above a window in a brick veneer wall?
Yes. Water running down the cavity above an opening has to be intercepted before it reaches the lintel and diverted out. The flashing at the head of the opening should be integrated with the lintel and the damp-proof course, with weep holes immediately above it. Omitting this detail is a common cause of damp around window heads.
How do I stop mortar droppings from blocking the base of the cavity?
Use a cavity board or batten to catch droppings as the bricklayers work, and clean out the base of the cavity before it becomes unreachable. On multi-storey walls, clean at each lift rather than waiting until the top. Once mortar has set at the base of the cavity, there is no practical way to remove it without demolishing the wall.
Where are the actual requirements for flashing and weep holes written down?
They are in the NCC and in the manufacturer’s installation instructions for the specific flashing system. Those documents set out the material, minimum fall, lap width, and weep hole spacing. Because they are updated over time, always check the current edition or the current manufacturer’s literature rather than relying on an old detail.
Sources
- Heat pump water heaters · energy.gov.au
- WoodSolutions: timber and moisture · Forest and Wood Products Australia
- Water Efficiency Labelling and Standards (WELS) scheme · Australian Government
- Cheaper Home Batteries Program · Department of Climate Change, Energy, the Environment and Water
Published 3 August 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.
