Liquid or sheet? Choosing a waterproofing membrane

Most membrane failures aren’t caused by a bad product. They’re caused by a good product used in the wrong place: a membrane that can’t handle sun left exposed on a deck, a rigid coating over a substrate that moves, or a sheet system tiled over when it was never designed for it. Choosing the right type for the job is half the work.

This guide compares the main types in general terms and covers the questions to ask before you choose. It’s not a substitute for the system’s data sheet or your local building rules.

Liquid-applied membranes

Liquid membranes are brushed, rolled, troweled or sprayed on, and cure into a seamless skin. Their big advantage is detail work: they wrap around penetrations, corners and odd shapes without seams. Their weakness is that the thickness depends entirely on the person applying them.

Polyurethane

• Very flexible and stretches well over small movement and cracks
• Good water resistance, and many are suited to ponding or wet conditions
• Often used on roofs, balconies and wet areas
• Many types break down in sunlight unless covered or given a UV-stable topcoat, so check whether yours is rated for exposure
• Some cure using moisture in the air, so cure time changes with weather, and some give off strong fumes indoors

Acrylic

• Water-based, low odour, and easy to apply and clean up
• Common under tiles in internal wet areas such as showers and bathrooms
• Cures by drying, so it can be slow in cold or damp conditions, and rain before it cures can wash it off
• Not usually suited to permanent immersion or water sitting on it for long periods unless the data sheet says so

Cementitious

• Cement-based, often modified with polymers to give some flexibility
• Bonds well to concrete and masonry, and can usually go on damp surfaces
• Some types are made for water tanks, pools, retaining walls and resisting water pressure from behind (negative side)
• More rigid than polyurethane or acrylic, so less suited to substrates that move or crack

Hybrid

Hybrids combine chemistries, such as polyurethane with acrylic, or cement with a high polymer content, to get a mix of properties. “Hybrid” isn’t a standard term, so read what each product is actually made for.

Sheet membranes

Sheet membranes come in rolls made in a factory, so the thickness is consistent. Their weak point is the seams and the detailing, where sheets are joined or have to turn corners and go around penetrations.

Bituminous torch-on

• Thick, tough, and long-lived on roofs, podiums and below-ground work
• Mineral-surfaced cap sheets stand up to sun
• Installed with a flame, so it’s hot work with a real fire risk, and it isn’t allowed or practical in some places

Self-adhered bituminous

• Peel-and-stick, with no flame needed
• Widely used below ground, on foundation walls and as underlays
• Usually needs a primer, a clean dry surface and warm enough temperatures to stick properly
• Generally not designed to be left exposed to sun long-term

PVC, TPO and EPDM

• PVC and TPO are single-ply thermoplastic sheets with seams welded by hot air. Mostly used on commercial roofs. Some PVC sheets can be damaged by contact with bitumen or certain insulation, so check compatibility.
• EPDM and butyl are synthetic rubber sheets. Very flexible, good in sun, and used on roofs, gutters, decks and ponds. Seams are glued or taped.
• All of these handle exposure well, but most are not designed to be tiled over directly.

Liquid vs sheet at a glance

	Liquid-applied	Sheet
Thickness	Depends on the applicator	Set in the factory
Seams	None	Lapped, welded, glued or taped
Complex shapes and penetrations	Easy to detail	Harder, more labour
Large flat areas	Slower, more coats	Fast
Weather during install	Needs dry conditions to cure	Some types less sensitive
Under tiles	Common, if the system allows it	Only if the system is designed for it

Matching the membrane to the job

Work through these questions before you choose:

What’s the substrate?

Concrete, plywood, fibre cement and masonry all behave differently. Some membranes need a particular primer, a minimum concrete age or a maximum moisture content. Plywood moves with moisture and temperature, so it needs a membrane flexible enough to cope. The data sheet will list approved substrates. If yours isn’t on it, choose another system.

How much will it move?

Timber framing, large slabs and junctions between different materials all move. A rigid membrane over a moving substrate will crack. Choose a membrane with enough flexibility for the movement expected, and detail joints, corners and changes of material with the system’s bandage or bond-breaker.

Will it see the sun?

An exposed roof or deck needs a membrane rated for UV exposure, or a protective topcoat, tiles or a covering. A membrane designed to sit under tiles can break down quickly if left exposed.

Will people walk on it?

Foot traffic, furniture and dropped tools damage membranes. A trafficable deck needs a membrane rated for traffic or a protective wearing surface. A roof that only gets maintenance visits has different needs.

Will water sit on it?

Even with falls, water ponds in low spots. Choose a membrane suited to ponding if there’s any chance of it. Better still, get the falls right before you start. In New Zealand, the external moisture acceptable solution (E2/AS1) sets minimum falls for membrane roofs and decks.

Is it below ground or under water pressure?

Basements, retaining walls, planters, tanks and pools need membranes designed for constant contact with water, soil or pressure, plus proper drainage and protection boards. Many wet-area products are not suited to this.

Compatibility with tiles and adhesives

If you’re tiling over the membrane:

• Use a membrane the manufacturer approves for direct tiling
• Use an adhesive that’s compatible with that membrane. Many systems list the approved adhesives, or sell their own.
• Let the membrane cure fully before tiling
• Use flexible sealant at junctions and changes of plane, not grout

Tiles directly over a membrane on an external deck are a more complex case. In New Zealand they fall outside the E2/AS1 acceptable solution, so expect the design to need its own supporting evidence.

Follow the system and the local rules

The safest rule is: one manufacturer, one complete system, installed exactly as the data sheet says. Primer, membrane, reinforcing, sealants and adhesives are tested together. Mixing brands can cause adhesion failures and void the warranty.

Local rules also limit what you can use and who can install it:

• New Zealand: internal wet areas are covered by Building Code clause E3, and external membranes by clause E2. E2/AS1 only covers butyl and EPDM membranes over plywood within set limits, so other membranes on roofs and decks need other evidence of compliance, such as a product appraisal. Waterproofing on consented residential work can also be restricted building work, which must be done or supervised by a licensed building practitioner.
• Australia: domestic wet areas are covered by AS 3740, and external above-ground membranes by AS 4654.2, both referenced in the National Construction Code. Some states, including Queensland, have a specific waterproofing licence.

Keep a record

Note the product, batch numbers and the data sheet version, and take photos of each stage: substrate, primer, detailing and each coat. Many warranties depend on proof the system was installed correctly. For a membrane and detailing reference you can keep on your phone, see WaterProof Pro.