Key takeaways
- A balcony drains only if the surface falls to the outlet, so the falls must be built in and checked with a level before the membrane goes down.
- The outlet, the overflow and the height of the flange relative to the finish are the details that cause the most expensive balcony leaks.
- Upstands, corners and door thresholds need their own sequence, because water runs along the wall junction and under the sill if the membrane is not dressed correctly.
- Every penetration through the membrane, including balustrade posts and fixings, should be treated as a deliberate detail and recorded as the work goes.
- A flood test before tiling is the cheapest insurance on a balcony, because it shows a fault while it can still be fixed without lifting the finished surface.
An exposed balcony is the hardest waterproofing job on a residential build. It takes the weather from the side and from above, it carries foot traffic and furniture, and it has more junctions than almost any other part of the house. When it leaks, the water runs into the wall below, the ceiling of the room under it, and eventually the frame. The first sign is usually a stain on a ceiling that is months old, which is a poor position to be in when the homeowner wants to know why.
Most balcony failures are not caused by a bad membrane. They are caused by a membrane that was laid on a surface that did not fall to the outlet, around an outlet that was set at the wrong level, against an upstand that was too short, or through a balustrade fixing that was never sealed. Each of those faults is a detail, and each is easy to get right if the sequence is clear before the first roller goes down.
This article sets out the balcony detail in the order it should be built: the falls, the outlets, the upstands and penetrations, the membrane, the test, and the tiling that goes on top. It is written for a licensed builder who already knows how to lay a membrane and wants a clear picture of where balcony work fails. The Building Commission NSW defects library is a useful reference for the faults that show up on balconies in practice, and the detail below is written with those faults in mind.
Falls first, because water only goes where it is sent
A balcony drains only if the surface falls in the direction you want the water to go. The fall has to be built into the structure or the screed before the membrane goes on, and it has to be checked with a level before anything is fixed down. The minimum fall is set by the membrane system manufacturer and the relevant standards, and the builder should confirm the figure in the current product documentation rather than work from memory or from the last job.
The common fault is a surface that looks like it falls but holds water in a dish near the middle, or in a corner where the fall runs the wrong way. Ponding water sits on the membrane for long periods. Ultraviolet light and foot traffic wear the surface, and any small opening in the membrane lets that standing water through to the structure below. A balcony that holds water is a balcony where a leak is waiting to happen, even if the owner has not yet seen the damage.
Check the falls at the stage where they can still be corrected. Lay a straight edge across the surface in several directions, look at the falls from the door threshold, and check that no low spot has been created by a fixing or a patch. A balcony that is set out with the right falls from the start rarely needs remedial work later, and remedial work on a balcony usually means lifting tiles, which is expensive for everyone.
Outlets: the most important detail on the balcony
The outlet is where the water leaves the balcony, and it is where the most expensive mistakes are made. The outlet must sit at the lowest point of the falls, so that every part of the membrane drains to it. The flange of the outlet has to be bonded to the membrane in the way the manufacturer specifies, with the membrane lapped over the flange in the right direction, so that water running across the surface flows over the seal rather than under it.
The height of the outlet relative to the finished surface matters as much as its position. An outlet set too high leaves a pool that never drains, and an outlet set too low can trap debris and stop the water from leaving at all. Set the outlet to the level that the drainage design calls for, and check that the strainer sits flush with the tile finish so that leaves and grit do not build up around it. An outlet that stands proud of the finish is a trip hazard as well as a drainage fault.
Where a balcony has a second outlet for overflow, the same discipline applies. Overflow outlets are often forgotten in the first build and then found missing when a heavy storm fills the balcony to the wall. The overflow should be above the main outlet, clear of the tiling, and sealed like the main outlet. Check that the overflow discharges somewhere it will not run back under the wall.
Right detail against the common fault
| The detail that holds | The fault that leaks |
|---|---|
| Falls checked with a level before the membrane goes down | A dish near the middle that holds water for years |
| Outlet set at the lowest point and at the designed height | Outlet set high, leaving a pool that never drains |
| Membrane turned up the wall and capped at the top | A short upstand with no flashing, so water runs behind it |
| Threshold dressed with membrane under or up the sill | Threshold tiled higher than the membrane, giving water a direct path |
| Penetrations set before the membrane and sealed in the system way | Fixings screwed through finished tiles after the membrane is down |
Upstands, corners and the wall junction
Water on a balcony does not stay on the floor. It runs along the joint where the balcony meets the wall of the house, and it finds any gap at the top of the upstand. The membrane has to turn up the wall by the height the manufacturer specifies, and the top of the upstand has to be protected by a flashing or a capping that sheds water away from the wall rather than behind it. A short upstand, or an upstand with no top flashing, is one of the most common reasons a balcony leaks into the room below.
Corners need the same care. Internal corners should be formed with the reinforcement the membrane system calls for, and external corners need the membrane to wrap round the edge without a gap. Where the balcony meets a door, the threshold is the critical junction. The membrane should run under the door sill or be dressed up the face of the sill, and the sill itself should drain outwards. A door threshold that sits lower than the membrane is a direct channel for water into the house.
Plan the junctions on the drawings, not on the day. The corner, the door, the wall and the outlet are four separate detail conditions, and each one needs its own sequence. Walk through the sequence with the tilers and the carpenter before anyone starts, so that the membrane is not lapped in the wrong direction because the tiler was in a hurry to get to the floor.
Penetrations: balustrades, posts and services
Every penetration through the membrane is a potential leak. Balustrade posts, handrail fixings, light fittings, pipes, and the fixings for a pergola or a privacy screen all pierce the surface, and each one needs to be treated as a deliberate detail. The fixing should be set in place before the membrane goes down where possible, with the membrane dressed around it and sealed in the way the system requires. A fixing that is screwed through the finished tile after the membrane is down is almost always a leak waiting to happen.
Balustrade posts are the most common offender. Many posts are fixed by bolting through the balcony slab, and the bolt holes become the path for water if they are not sealed. Where the post is fixed to the edge of the slab, the membrane should be taken over the edge and the post base set onto a sealed pad or sleeve. Check the details in the product literature for the system you are using, because the approach differs between products.
Keep a record of every penetration as you go. A simple sketch with the location of each fixing, and a note of the sealing method, is far more useful than a memory of what was done in the wet. When a balcony leaks five years later, the first question is always where the water came in, and a good record answers it in minutes.
The membrane: sequence, coverage and drying
The membrane is the part of the job that people remember, and it is only as good as the surface beneath it. Prepare the substrate so that it is clean, sound and free of sharp edges, and check that the primer or priming coat has been applied where the system requires it. Apply the membrane in the coats and thicknesses the manufacturer specifies, and do not try to save a coat on a balcony that will see sun and traffic every day. Thin membranes are a common cause of balcony failures that appear well after handover.
Allow the membrane to cure for the period the manufacturer sets out before tiling. Rain during the curing period is a separate hazard, and a membrane that is wet when it should be dry can lose adhesion at the edges. Keep the balcony covered or protected while it cures, and mark the area so that tradespeople do not walk across it before the surface has set. The sequence matters more than the speed, and a rushed balcony usually costs more to repair than the day it saved.
Before anything goes over the membrane, carry out a flood test. Seal the outlets, block the overflow, and fill the balcony with water to the depth the system specifies, then leave it for the test period set in the product documentation. Mark the water level at the start, and check the surface, the upstands and the junctions at the end. A flood test is the only way to know that the membrane is watertight before the tiles hide it, and it is the cheapest insurance on the job.
Tiling on top: falls, movement and joints
The tiles do not waterproof the balcony. They protect the membrane from wear, and they add movement that the membrane must accommodate. The fixing method depends on the tile and the substrate, and the tile adhesive should be the type the membrane system approves. Lay the tiles to the falls that were set out at the start, and check the falls again once the tiles are down, because a tile that is set slightly high at the outlet will hold water even if the surface looked right before.
Movement joints need careful planning on a balcony. The slab moves with temperature and load, and the tiles above it move at a different rate. Joints in the tiling should line up with the movement joints in the structure and be sealed with the flexible sealant the system calls for. The detail is covered in more depth in the article on tile lipage and movement joints, and the same logic applies to a balcony as to a wet area, with more exposure to sun and rain.
Finish the tiling with a clean perimeter. Where the tiles meet the wall or the door, use the sealant and trim that suits the detail, and do not fill a gap with a rigid grout that cannot move. The perimeter seal is the last defence against water running behind the tiles, and it is the part of the job that the owner will see first when the balcony is handed over.
Lead in. The mistake that costs the most is tiling over a membrane that has not been flood tested. A balcony leak that is found after the tiles are down can mean lifting the whole surface, and a waterproofing failure of that kind could be classified as a major defect, which brings the homeowner, the builder and the insurer into a dispute that could have been avoided with a bucket of water and a day.
Falls on the balcony during construction
The balcony is also a place where people fall during construction. The Safe Work Australia code of practice on managing the risk of falls in housing construction sets out how to protect workers at the edges of elevated work, and a balcony slab with no guardrail or edge protection is exactly the kind of place where a fall happens. Put the edge protection in before the membrane work starts, and keep it in place until the balustrade is fitted and inspected.
Plan the access as well. Materials stacked on a balcony during the membrane stage can damage the surface and create a trip hazard, and a ladder placed on a freshly laid membrane can puncture it. Keep the crew route clear, lay down protection where people must walk, and make sure that the people working on the membrane are not also carrying tiles and tools across the same surface.
The balcony sequence, in order
- 1Set the fallsForm the surface to fall to the outlet, and check it with a straight edge and a level before anything is fixed.
- 2Fit the outletsSet the outlet at the lowest point and at the height the drainage design calls for, and bond the flange as the system requires.
- 3Dress the junctionsTurn the membrane up the walls and over the thresholds, and cap the upstands so that water sheds away from the wall.
- 4Lay and cure the membraneApply the coats the manufacturer specifies and allow the curing period before anyone walks on it.
- 5Flood test, then tileSeal the outlets, fill the balcony to the set depth, check the surface, and only then fix the tiles to the original falls.
Worked example: a balcony that leaked after handover
Consider a builder in inner Sydney who completed a first-floor balcony over a living room. The membrane was laid over a surface that looked level, the outlet was set in the right place, and the tiles went down a week later. Six months after handover the owner noticed a stain on the ceiling below the balcony door. The builder inspected the area and found that the falls near the door ran back towards the house, and that the threshold had been tiled a little higher than the membrane.
The remedy was expensive because the fault was built into the sequence. The tiles had to come up around the door, the threshold had to be rebuilt, the membrane had to be dressed up the sill, and the falls had to be corrected with a new screed layer before the membrane was relaid. The builder kept the owner informed in writing at each stage, and the cost was shared in a way that reflected the work that had been done incorrectly. The lesson was that the threshold and the falls are a single detail, and both should have been checked before the tiling started.
A flood test at the end of the membrane stage would have shown the problem in an afternoon. The builder now runs one on every balcony, and records the result in the job file, so that the next owner can see the test was done.
Where the detail sits in the wider waterproofing picture
Balcony waterproofing is one part of a wider set of wet area and exposed area details, and the same discipline applies to all of them. The guide to wet area waterproofing membranes covers the membrane side of the job, and the principles of falls, outlets, upstands and sealing carry across. The balcony is harder than most wet areas because it is exposed to the weather, so the margin for error is smaller.
The balcony waterproofing defects guide is a useful companion when you are checking an existing job, and it is worth reading alongside the sequence above.
If you need to keep your licence current, the PointCPD guide to NSW builder CPD explains the renewal requirement in plain terms. Waterproofing detail changes as products and practice change, so keeping the learning current is part of doing the work properly.
Questions builders ask
How do I check the falls on a balcony before the membrane goes down?
Use a straight edge and a level across the surface in several directions, and look at the falls from the door threshold. Check that no low spot has been created by a fixing or a patch, and correct any dish or reverse fall before the membrane is laid.
Where should the balcony outlet sit?
The outlet should sit at the lowest point of the falls so that every part of the membrane drains to it. Its height should match the drainage design, and the strainer should sit flush with the tile finish so that it does not trap debris or stand proud as a trip hazard.
How high should the membrane turn up the wall at a balcony?
The membrane should turn up the wall to the height the manufacturer of the system specifies, and the top of the upstand should be protected by a flashing or capping that sheds water away from the wall. Check the product documentation for the exact requirement, because it differs between systems.
What is a flood test and when should it be done?
A flood test seals the outlets, blocks the overflow and fills the balcony with water to the depth the system specifies, then leaves it for the set test period. It should be carried out after the membrane is cured and before any tiling goes down, and the result should be recorded in the job file.
Why do balcony waterproofing faults cost so much to fix?
Most balcony faults are built into the sequence, such as the falls, the threshold or the outlet, and fixing them usually means lifting the tiles and sometimes the screed. A leak that is found after handover can also lead to a dispute about who pays, which adds time and cost on top of the repair.
Do I need edge protection on a balcony during construction?
Yes, where the balcony edge is open and the work is at height, edge protection must be in place to protect workers. The Safe Work Australia code of practice on managing the risk of falls in housing construction sets out the approach, and the protection should stay until the balustrade is fitted and inspected.
Sources
- Building Defects Library · Building Commission NSW · 2025-06-25
- Continuing professional development for builders and swimming pool builders · NSW Government
- General building work licensing · NSW Government
Published 12 October 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.
