Waterproofing membrane systems: applications, limitations and selection

Membrane manufacturers each present their system as the preferred solution, and each is correct within the conditions that system was developed to address. The systems are not interchangeable, and the selection is governed by substrate condition, geometry, exposure and programme rather than by product performance data in isolation.

One qualification applies before any comparison of products. Most membrane failures are not product failures. They are failures of detailing, of substrate preparation, or of falls. A higher-specification membrane installed over an unresolved substrate reproduces the original failure at greater cost, because the mechanism that defeated the first installation is unchanged.

Liquid-applied polyurethane

System description: a fluid-applied membrane, rolled or sprayed, curing to a seamless elastomeric film. It is the most commonly specified system for balcony and terrace remediation.

Applications: complex geometry. Balconies incorporate upturns, penetrations, thresholds and internal corners, and a liquid system dresses into each of them without seams or welded joints. Elongation is sufficient to accommodate minor substrate movement. Detailing around balustrade posts and drainage outlets is straightforward relative to sheet systems.

Limitations: the system is sensitive to substrate preparation and to application conditions. Applied below the specified thickness, over a damp or contaminated substrate, or outside the permitted temperature and humidity range, it will not achieve its design performance irrespective of the product datasheet. Wet-film thickness control is the principal variable determining service life, and it is a function of workmanship and supervision rather than of product selection.

Indicated where: the area has complex detailing, is of moderate size, and requires membrane continuity around numerous penetrations. That description covers the majority of residential balconies.

PMMA (methyl methacrylate) systems

System description: a fast-curing liquid resin system, generally reinforced with fleece.

Applications: rapid cure and cold-weather application. Cure times are a fraction of those for polyurethane, which is significant where the work occupies a resident’s only balcony or a trafficked area that must be returned to service. Detailing capability is high, and chemical and ultraviolet resistance are good.

Limitations: cost and odour. The system is materially more expensive than polyurethane, and application produces a strong odour that will generate complaints in an occupied building unless resident notification and ventilation are planned in advance. Working time is short, which requires applicators experienced with the system.

Indicated where: the programme is compressed, the available weather window is poor, or the area must return to service quickly enough to justify the cost premium.

Sheet membranes (torch-on and self-adhesive)

System description: factory-manufactured sheet bonded to the substrate, either modified bitumen applied by torch or self-adhesive.

Applications: consistent thickness. The membrane is manufactured rather than mixed and applied on site, so the specified build is achieved uniformly across the area, which is an advantage over large and geometrically simple areas such as podium decks and flat roofs. The systems are robust, long established and well understood by installers.

Limitations: laps and complexity. Every lap is a potential failure point, and complex geometry increases the number of laps. Torch-applied systems require hot works, with the associated permits, fire-watch provisions and risk management on an occupied building.

Indicated where: the area is large and geometrically simple, and consistency of thickness is of greater consequence than seamless detailing.

Cementitious membranes

System description: polymer-modified cementitious slurry, applied by brush or trowel.

Applications: adhesion to damp substrates and to green concrete. The systems are vapour permeable, which suits situations where trapping moisture within the substrate would create a secondary problem. Material cost is low. They are frequently the appropriate selection for internal wet areas, planter boxes and below-grade tanking applied to the positive side.

Limitations: movement accommodation. Flexibility is limited relative to elastomeric systems, so cracks subject to continuing movement will telegraph through the membrane. The systems are not a general-purpose solution for exterior decks.

Indicated where: the substrate is damp, anticipated movement is minimal, and vapour permeability is an advantage rather than a risk.

Injection systems

Injection is not a membrane, but it is frequently the correct remedy for a leak: polyurethane or epoxy resin is injected into cracks and joints. Polyurethane resins react with water and expand, which suits actively leaking cracks; epoxies are structural and bond the crack faces, which suits non-moving cracks where the restoration of strength is required.

Injection treats the entry point rather than resurfacing the entire element. Where the leak is a defined crack or construction joint, as in basement walls, planter walls and lift pits, it is substantially less disruptive than membrane replacement and does not require removal of finishes across the element. Where the membrane has reached the end of its service life across the area, injection addresses a symptom and defers the replacement rather than avoiding it.

Factors determining installation performance

Falls. Where water ponds, the membrane is required to perform as a permanent pond liner, a duty for which trafficable deck systems are not specified. Correcting falls with a screed before membrane installation is not an optional additional item on a flat balcony; it is a necessary component of the repair.

Substrate preparation. Laitance removed, contaminants removed, moisture content within the tolerance stated by the system manufacturer, and cracks treated in accordance with the specification. The work is not visible in the completed installation, and it is the principal determinant of whether the installation achieves ten years of service or two.

Detailing at terminations. Upturns carried to the correct height above finished floor level, membrane dressed into drainage outlets, penetrations sealed in accordance with the system specification, and terminations resolved at thresholds. A greater proportion of failures originates at these locations than in the field of the membrane.

Testing before covering. Flood testing after installation and before finishes are reinstated is the final opportunity to identify a defect while it remains accessible. Where testing is omitted to recover a day in the programme, the defect is instead identified after the tiles have been laid, at which point the rectification cost includes the finishes.

Selection summary

The system should be selected for the substrate condition, the geometry, the exposure and the programme constraints. Once selected, the specification, supervision and inspection effort should be directed at preparation, falls, detailing and testing, because those four factors determine the service life achieved by any of the systems described above.

General information only. Product selection for a specific building should follow site-specific investigation, the relevant Australian Standards and the manufacturer’s specification.