Waterproofing Membranes Compared: Liquid, Sheet and Cementitious

There are three families of waterproof membrane used in Australian domestic wet areas. They all work. They fail for different reasons, and the right one depends on the substrate, the shape of the room and how much movement the structure will see.

Liquid-applied membranes

The most common choice in domestic bathrooms. A liquid membrane is rolled or brushed on in multiple coats and cures into a continuous flexible film.

Most are polyurethane or acrylic based. Polyurethane is generally more elastic and more tolerant of movement, which matters on timber floors. Acrylic systems are easier to work with and lower odour, and are often used on concrete substrates where movement is minimal.

What they are good at. Seamlessness. A liquid membrane goes around an internal corner, up over a hob, into a niche and around a waste without a single join. In a bathroom full of awkward shapes, that is a genuine advantage.

Where they fail. Application. The film thickness is created by the person holding the roller, so it depends entirely on coats applied and cure time allowed. Too few coats, or coats rushed on top of each other, and the membrane never reaches its specified thickness or never fully cures. This is the single most common failure we strip out.

Sheet membranes

Supplied in rolls and bonded to the substrate with an adhesive.

What they are good at. Guaranteed thickness. The sheet was manufactured to a consistent thickness in a factory, so there is no possibility of a thin patch behind the tiles. There is also no cure time in the same sense, so tiling can often follow sooner.

Where they fail. The joins. Every lap has to be bonded and sealed correctly, and every internal corner needs to be dressed or a preformed corner used. Complex shapes are far more work than they are with a liquid. A sheet system installed carelessly at the corners is no safer than a thin liquid.

Cementitious systems

Cement-based coatings, usually with a polymer additive for flexibility.

What they are good at. Bonding to masonry and concrete, and taking a tile bed directly. They are common on balconies and in commercial work.

Where they fail. Movement. Cementitious systems are comparatively rigid. On a timber floor that flexes underfoot, a rigid membrane can crack at exactly the junction where you least want it to. For that reason they are less commonly used on suspended timber floors in domestic bathrooms unless modified for flexibility.

What actually decides the outcome

Nearly every membrane sold through a trade supplier in Australia will perform if it is installed the way the manufacturer specifies. Almost every failure is an installation failure.

The things that decide whether a wet area lasts:

• Primer. The right one, applied first. Without it the membrane can lose adhesion and water tracks underneath it, which is worse than no membrane at all because it spreads.
• Reinforcement at junctions. Internal corners and wall-to-floor junctions need bond breakers or reinforcing fabric so the membrane can stretch as the building moves rather than tearing.
• Coats and cure. The specified number of coats, with the specified time between them, and a full cure before tiling. How long that takes is covered here.
• Penetrations. Wastes, tap penetrations and fixings all need dressing. Every hole is a leak unless it is treated as one.
• Compatibility. Membrane, primer and adhesive from a compatible system.

Which to ask for

For a standard domestic bathroom on a timber or concrete floor, a quality liquid polyurethane system applied to the manufacturer's specification is the sensible default, and it is what most of our wet areas get.

What matters more than the brand is whether whoever applies it will allow the time it needs. If a quote has the tiler waterproofing in the morning and tiling in the afternoon, that is the answer to a different question.

The requirements that govern all of this are set out in AS 3740, and if you want it applied properly, that is our waterproofing service.