Membrane types explained - polyurethane, acrylic, cementitious, sheet, bitumen

One of the most consequential decisions in any waterproofing job is which membrane type to use. The wrong choice can fail in years even with perfect application; the right choice will last decades. Here's a tradie's-eye view of the five major categories.

Polyurethane (liquid-applied)

What it is: A solvent-based or water-based polyurethane resin painted on in 2-3 coats, building up to a continuous elastic membrane around 1.2-1.5mm thick when cured.

Strengths: Exceptional adhesion to almost any substrate. Excellent stretch (10-200% elongation depending on product) so it follows substrate movement without cracking. Bonds to itself across coats, so detailing is clean.

Where to use: The default choice for residential balconies, bathroom and shower wet areas, planter boxes, and most situations under tile. The major Australian manufacturer brands (Crommelin, Sika, Wattyl, Ardex, Mapei, Bostik) all have well-tested PU systems.

Limitations: UV-sensitive — needs to be covered (by tiles, pavers, or a topcoat) within manufacturer-specified time, usually 14-90 days. Solvent-based versions have odour and require ventilation during cure.

Acrylic (liquid-applied)

What it is: Water-based acrylic emulsion, applied similarly to polyurethane but cures by water evaporation rather than chemical reaction.

Strengths: Cheaper than PU. Easier and faster to apply. Water-based, so almost no odour. Good for "lighter duty" applications.

Where to use: Internal residential wet areas only — bathrooms, laundries, internal walls.

Where NOT to use: External applications. Acrylic re-wets when standing water is present, slowly compromising the membrane. Don't use on balconies. Don't use anywhere with serious substrate movement — acrylic has lower stretch than PU.

Common misuse: Cheaper acrylic membrane used on balconies because "it's also liquid applied". This is one of the most common reasons we see for balcony failures — the membrane was a poor specification choice from day one.

Cementitious crystalline

What it is: Cement-based slurry containing reactive chemicals that penetrate into the concrete substrate and grow into the capillary pores, sealing them from the inside.

Strengths: Becomes part of the concrete — no surface film to fail. Self-healing of micro-cracks for years afterwards. Tolerates negative-side water pressure (i.e. water pushing on the membrane from the side it's not applied to).

Where to use: Concrete water tanks, retaining walls, basements, swimming pool structures, lift pits. Anywhere with hydrostatic pressure.

Limitations: Works only on cementitious substrates (concrete, masonry). Useless on plywood, fibre-cement or metal. Cosmetic finish — not designed to be a top surface.

Sheet membrane (PVC, TPO, EPDM)

What it is: Pre-manufactured plastic or rubber sheets in rolls (typically 1-2m wide), seamed by heat welding, solvent welding, or adhesive depending on the type.

Strengths: Consistent thickness (no chance of "thin spots" from poor application). Fast to install on large areas. Long warranty periods (20-30 years from some manufacturers). Some types are UV-stable so don't need covering.

Where to use: Large flat or low-slope roofs, podium slabs over basements, commercial roof gardens, civil works.

Limitations: Detailing at penetrations requires more skill than liquid systems — every pipe, vent and corner needs a hand-cut piece. Not practical for small residential jobs.

Bitumen (torch-on, self-adhesive, or peel-and-stick)

What it is: Bitumen-modified asphalt in sheets, either heat-welded (torch-on) or self-adhesive. Around 4mm thick.

Strengths: Very robust — physically tough, handles foot traffic during installation. Long service life when protected from UV (40+ years documented on European basement installations). Excellent for buried applications.

Where to use: Basements (above the membrane), buried slabs, planter boxes, parking structures. Anywhere durability matters more than fine detailing.

Limitations: Heavy. Hot work permit needed for torch-on. UV-sensitive (must be covered). Detailing at penetrations is laborious.

How to know your applicator is choosing wrongly

Three quick checks:

• Acrylic on a balcony — wrong choice. Always.
• PU under planter soil with no protection board — wrong. Soil will eventually damage the membrane.
• "One membrane for the whole house" — bathrooms, balconies and basements need different products. A quote that uses one system for all three is a quote that's specified by convenience, not by job.

Ask which membrane is specified, by brand and product name. Look up the manufacturer's TDS (technical data sheet) and check it's intended for your situation. If your applicator can't answer the question, they're picking by what's on the van, not what's right for the job.