Waterproofing is the hidden layer that stops water getting into the structure of a building. You rarely see it. It sits under tiles, beneath roof coverings, behind planter boxes and against basement walls. When it's working, nobody thinks about it. When it fails, water finds its way into concrete, steel, timber and the apartment below, and the repair bill grows every month it's left.
What waterproofing actually does
Concrete, brick, render and grout are all porous to some degree. On their own, they let water through. A waterproofing membrane is a continuous barrier laid over or against these materials so water is directed to a drain or away from the building instead of soaking in.
Continuous.
Most failures aren't a membrane wearing out across its whole area. They happen at the details. A good membrane badly detailed will still leak.
The main types of waterproofing membrane
Each system suits different jobs. What matters is where it goes, how it's protected and how water drains off it.
Liquid-applied
Balconies · bathrooms · planters · roofsPolyurethane, acrylic or hybrid, brushed or rolled on in coats, so it handles complex shapes and penetrations well. Thickness and cure time must be right or it fails early.
Torch-on (bitumen sheet)
Flat roofs · podiums · large decksRolls of modified bitumen heat-bonded to the surface. Very durable when the laps and edges are sealed properly.
Synthetic sheet
Roofs · podiums · exposed decksPVC, TPO or EPDM sheets, welded or glued at the joins. Consistent thickness, but the joins are the weak point.
Cementitious
Wet areas · water tanks · some below-ground workA cement-based coating. Rigid, so it can crack if the substrate moves.
Crystalline & tanking systems
Basements · lift pits · retaining wallsDesigned to resist water pushing in under pressure from the ground side.
The right choice depends on whether it will be exposed or under tiles, how much the structure moves, whether there's foot traffic, and how water will drain off it.
The Australian Standards that apply
Covers waterproofing of internal wet areas such as bathrooms and laundries.
Cover membranes for external above-ground use, including balconies, decks, podiums and planter boxes: the materials, and how they must be designed and installed.
These standards set things like minimum falls to drains, how far membranes turn up walls, and how thresholds and penetrations are detailed. Many older Sydney buildings were waterproofed before current standards, which is one reason leaking balconies are so common in apartment blocks from the 1960s to the 1990s.
In NSW, waterproofing is classed as specialist work under the Home Building Act, so check that whoever does yours holds the right licence.
How long does waterproofing last?
It depends far more on installation than on the product. A well-detailed membrane, protected by tiles or a screed and with water draining off it properly, can last decades.
The same membrane laid with poor falls, thin coats or unsealed edges can start leaking within a few years.
What shortens a membrane's life
- Water ponding instead of draining away
- UV exposure on membranes not designed to be left exposed
- Building movement cracking rigid membranes
- Tiles, grout and drains replaced without redoing the membrane underneath
- Planter boxes, where soil and roots keep the membrane wet all the time
Signs your waterproofing has failed
- Stains, bubbling paint or mould on the ceiling below a balcony, bathroom or roof
- White powdery deposits (efflorescence) on tiles, grout or the underside of slabs
- Drummy or loose tiles that sound hollow when tapped
- Rust stains or cracking on concrete edges, which can mean water is reaching the steel
- Damp, musty smells or peeling paint low on basement walls
- Water still pooling on a balcony hours after rain
Don't ignore early signs. Water that gets past a membrane often reaches the steel reinforcement in concrete. That's how a leak turns into concrete cancer, a much bigger and more expensive repair.
Why patching rarely works
Regrouting, applying a clear sealer over tiles or caulking a crack can slow a leak for a while. But if the membrane underneath has failed, water keeps getting in. Proper remedial waterproofing usually means:
- Take up the tiles
- Repair the substrate
- Fix the falls
- New membrane to standard
- Re-tile
The cost is in the removal and rebuild, not the membrane itself. So it's worth doing once, properly, with the details right.
Waterproofing on the Northern Beaches
Coastal buildings work their waterproofing harder. Salt air, wind-driven rain and large temperature swings on exposed balconies and roofs all speed up wear. Many Northern Beaches apartment blocks also date from an era of minimal waterproofing detailing.
Regular inspections, especially of balconies and planter boxes, catch problems while they're still small.
Remedial builder Northern BeachesWaterproofing FAQ
Can you waterproof over existing tiles?
Some systems are designed to go over tiles, but they only work if the tiles are well bonded and the falls are right. If tiles are drummy or water is already getting under them, they need to come up.
How do I know where a leak is coming from?
Leaks often show up a long way from where water gets in. A remedial builder will check drains, falls, thresholds, penetrations and the membrane edges, sometimes with moisture meters or flood testing.
Is waterproofing covered by strata insurance?
Usually not. Gradual leaks and failed membranes are normally treated as maintenance or defects rather than insured events. Check your policy.
Who's responsible for a leaking balcony in a strata building?
It depends on your by-laws and what's common property. Membranes are often common property, which makes them the owners corporation's responsibility.
Leaks, stains or drummy tiles?
Stamm Remedial diagnoses the cause and repairs waterproofing on balconies, roofs, planter boxes and basements across Sydney and the Northern Beaches.
