PU Waterproofing Coating for Terraces, Podiums and Wet Areas

What it is

A PU waterproofing coating is a liquid polyurethane, single or two-component, applied by roller, brush, trowel or spray. As it cures it forms a continuous rubber-like membrane that bonds to the substrate and follows its shape. It is used on terraces, podium decks, balconies, planters, wet areas under tiles, and roof details too awkward for sheets.

The commercial argument for PU is that it has no laps, which is where sheet membranes most often fail. The catch is that the thickness is made on site by a roller, not in a factory. A terrace coated at half the specified thickness looks exactly like one done properly, until the first monsoon. Leaks found after the screed, tiles and rooftop equipment are in place mean breaking all of them out. Thickness control and detailing decide the outcome far more than the product brand.

Aromatic vs aliphatic

The misconception is that "PU coating" means an exposed, weatherproof finish. Most base coats are aromatic and are meant to be covered. If the terrace will stay exposed, the system must include a UV-stable top coat, and if people walk on it, a wearing coat rated for that traffic.

Hybrid chemistries, such as PU-acrylic and polyurea-based coatings, are also sold as "PU waterproofing". Each has its own system data; compare them on tested properties, not on the label.

Single or two-component. Single-component PU cures by reacting with moisture in the air, so it is simple to use but cures slowly and can bubble if laid too thick or in very humid conditions. Two-component PU is mixed on site in fixed proportions and cures more predictably, but the mix ratio, mixing time and pot life must be controlled, because an off-ratio batch may never cure properly.

Detailing upstands, outlets and corners

A liquid membrane is only as good as its edges.

• Substrate. Sound, dry, clean concrete or screed, laid to falls, with laitance removed and the manufacturer's primer applied. Moisture limits are given in the product data.
• Corners and cracks. Internal corners get a fillet and a strip of reinforcing fleece embedded in the first coat. Static cracks are treated and reinforced; moving joints get a joint detail, not just more coating.
• Upstands. The coating runs up parapets, door thresholds and kerbs above the finished level and terminates into a chase or under a flashing.
• Outlets and penetrations. Drains, pipes, railing posts and HVAC supports get reinforced collars, and outlets are set low so water does not pond.
• Crack bridging. A liquid membrane's ability to bridge a crack under cold conditions is measured under ASTM C1305.

Specifications for liquid-applied membranes under a separate wearing course, such as podium decks over occupied space, are covered by ASTM C836, with design and application guidance in ASTM C898. Coatings that protect concrete surfaces are classified under EN 1504-2, and liquid-applied membranes beneath tiling in wet areas under EN 14891.

Thickness, testing and protection

Wet film checks. The applicator checks thickness with a wet film gauge during each coat. Methods for wet and dry film measurement are in ISO 2808. Material used against area covered is a second check: if a terrace of known area used less material than the system requires, it is under thickness.

Flood test. Plug the outlets and flood the area for the period the specification sets before any screed or tile goes on.

Protection. Protected systems receive a separating layer and a protection screed laid to falls, then the finish. Exposed systems receive the approved top coat.

Maintenance. Exposed coatings are inspected periodically, especially at upstands and around equipment, and top coats renewed when worn.

For sheet alternatives, see bituminous waterproofing membrane; for treating the concrete itself in basements and tanks, see crystalline waterproofing.

Common mistakes

• Coating a damp slab, then blistering in the first hot week.
• Aromatic coat left exposed because the protection screed was cut.
• No fleece at corners and outlets, where movement tears the film.
• Single thick coat instead of the specified coats.
• Later trades drilling railing and HVAC fixings through the finished membrane.
• No flood test before tiling.

What to ask your contractor or supplier

• Is the system aromatic, aliphatic or hybrid, and is it rated for exposure?
• What minimum dry thickness and consumption per square metre are specified?
• How will wet film thickness be checked and recorded?
• How are corners, upstands, outlets and penetrations reinforced?
• What protection or top coat goes on, and when is the flood test?

Standards referenced

Liquid-applied elastomeric membranes with a separate wearing course under ASTM C836, with guidance in ASTM C898. Crack bridging under ASTM C1305. Surface protection systems for concrete under EN 1504-2. Liquid-applied membranes beneath tiling under EN 14891. Film thickness measurement under ISO 2808. The system, thickness and detailing for a particular roof, podium or wet area must be confirmed by the project's designer and engineer.