Waterproofing Membrane

The Full Picture

A waterproofing membrane is a barrier designed to resist water, including water under hydrostatic pressure. It is distinct from dampproofing, which resists moisture and vapor but not pressurized water. Membranes are used on foundation walls, under slabs, on elevator pits and tunnels, on plaza and parking decks, balconies, green roofs, and in showers and other wet areas.

Sheet-applied membranes arrive as rolls. Examples include self-adhered rubberized asphalt and polymer-modified bitumen sheets, thermoplastic sheets such as PVC or TPO, and bentonite-based composites. They offer controlled, factory-made thickness but need careful lapping, sealing, and detailing at corners and penetrations. Fluid-applied membranes, such as polyurethane, rubberized asphalt, or other coatings, are sprayed, rolled, or troweled and cure into a seamless layer that conforms to complex shapes, but the applied thickness must be controlled and verified.

Most waterproofing failures occur at details rather than in the field of the membrane: terminations, penetrations, joints, drains, transitions to windows and walls, and damage during backfill or the next trade's work. Good practice includes mockups, manufacturer-approved substrates and primers, protection boards, and testing such as flood testing on horizontal surfaces or electronic leak detection where specified.

Because the membrane is hidden once covered, coordination and inspection timing matter. Specifications typically require manufacturer-trained installers, field inspections before covering, and warranty terms. Quantities are usually measured by area, with flashings, terminations, and accessories counted separately, so reading the details on the drawings carefully is part of pricing the work.