Liquid-applied membrane

A liquid-applied membrane (LAM) is a waterproofing system applied wet to the substrate by brush, roller, trowel, or spray, then cured to a seamless flexible film. It is the alternative to sheet membranes (rolled-out factory-formed sheets), and the residential default for complex wet-area junctions, balconies, and below-grade tanking where sheet systems are awkward.

Common chemistry types:

Polyurethane (PU) based: high elongation, good UV resistance, used for balconies, podiums, planter boxes. Two-pack (resin + catalyst) or single-pack moisture-cure.
Acrylic based: water-based, low odour, easy to apply, used for internal wet areas (bathrooms, laundries). Lower elongation than PU.
Modified bitumen based: hot- or cold-applied, used for below-grade and roof terrace waterproofing.
Hybrid PU-cement: cementitious base with PU resin, used over green or damp concrete substrates.

Each chemistry has its own application thickness, recoat window, and substrate compatibility. Cross-substituting (e.g. acrylic where PU was speced) is a defect.

Why LAM beats sheet membrane in some applications:

Complex junctions: shower hobs, floor wastes, around plumbing penetrations, internal corners. Sheet membrane needs cuts and patches at every internal corner; LAM goes around as one continuous film.
Irregular substrates: render with bumps, screed with steps, masonry with profiled blocks. Sheet needs perfect prep; LAM follows the substrate.
Site repair: small puncture in a finished LAM is repaired with a brush-on patch; sheet membrane patches require lapping and tape.
No seams: LAM is monolithic. Sheet has seams; seams are the failure points.

Where sheet membrane beats LAM:

Large flat areas (large balconies, podiums): sheet rolls out faster than brush-application.
Cold weather application: LAM cure depends on temperature and humidity; sheet doesn’t.
Reliable thickness: sheet is factory-controlled; LAM thickness depends on the applicator’s skill.

Application rules under AS 3740 / AS 4858:

Substrate prep: clean, dry, free of contamination. Many LAM systems require a primer coat before the main membrane.
Wet-film thickness (WFT): typically 0.6 to 1.5 mm per coat depending on chemistry. Most systems require two coats for a total dry-film thickness of 1.5 to 2.5 mm.
Coverage rate: stated on the product datasheet in m² per litre; track usage to confirm coverage.
Reinforcement: at internal corners, hobs, and plumbing penetrations, embed a reinforcing fabric in the first wet coat to spread movement stress.
Cure time: typically 24 to 72 hours between coats and before flood-testing. Working in high humidity or cold extends cure time substantially.
Flood test: 24 to 48 hours of standing water at design depth before tiling. Mandatory under AS 3740 for wet areas.

Common defects:

Insufficient WFT: applied too thin; fails the dry-film thickness gauge measurement.
Pinholes from over-thinning or applying onto bubbling primer.
Missing reinforcement at corners: cracks at junctions within years.
No flood test before tiling: hidden pinholes show up as leaks years later.

For builders. Hire a licensed waterproofer for the LAM application. The build pack should include the waterproofer’s certificate of compliance with the membrane brand, application date, and flood-test result. Most state regulators treat the certificate as essential evidence at OC.

Also known as: LAM, liquid membrane, paint-on waterproofing.

Category: Materials / waterproofing / membranes.

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