# PU Waterproofing Coating for Terraces, Podiums and Wet Areas

> A PU waterproofing coating is a liquid polyurethane applied by roller, brush or spray that cures into a seamless, elastic membrane bonded to the substrate. Aromatic grades must be covered or top-coated because sunlight degrades them; aliphatic grades resist UV. Its performance depends on a dry, primed substrate, reinforced corners and outlets, and reaching the specified thickness everywhere.

## 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

| Property | Aromatic PU | Aliphatic PU |
| --- | --- | --- |
| UV resistance | Poor: chalks, yellows and degrades in sunlight | Good: colour and properties stable |
| Typical role | Base waterproofing coat under protection | Exposed top coat, or full exposed system |
| Relative cost | Lower | Higher |
| Use under screed and tiles | Common | Not needed |
| Use exposed | Only with an aliphatic top coat | Yes, in a system rated for exposure |

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](/glossary/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](/materials/bituminous-waterproofing-membrane);
for treating the concrete itself in basements and tanks, see
[crystalline waterproofing](/materials/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.

## Frequently asked questions

### Can PU waterproofing be left exposed on a terrace?

Only if the system is designed for exposure. Aromatic polyurethane is the common, lower-cost base coat, but sunlight makes it chalk, yellow and lose properties, so it must be covered by a screed and tiles or protected by a UV-stable aliphatic top coat. Exposed systems that take foot traffic also need a wearing top coat recommended by the manufacturer for that use.

### What thickness of PU coating should be applied?

The thickness comes from the manufacturer's system for that use, typically stated as a minimum dry film thickness and a minimum consumption per square metre, usually built in two or more coats. Exposed, trafficked or hydrostatic situations are specified differently from a tiled bathroom. The specification should state the minimum thickness, not just the product name, so it can be checked on site.

### How is the thickness checked on site?

During application, with a wet film gauge pressed into the fresh coat, and by checking material consumption against the area covered. Dry film thickness can be checked later by cutting small samples, which are then repaired. Methods for measuring wet and dry film thickness are set out in ISO 2808. Thin spots usually appear on upstands, edges and around outlets.

### Is PU better than a bitumen membrane?

Neither is better everywhere. PU is seamless and easier to detail around many penetrations, odd shapes and upstands, with no laps to fail. Bitumen sheets give a controlled factory thickness and tolerate a less perfect substrate. On a terrace crowded with HVAC supports and pipes, a liquid system is often easier to detail well; on a large simple basement, sheet systems are common.

### Why does PU waterproofing blister or bubble?

Usually moisture or air in the substrate. A damp slab, rising moisture, or air escaping from porous concrete as it warms in the sun pushes up through the curing coat. Applying on a dry, primed substrate, in the cooler part of the day when the slab is not heating up, and within the manufacturer's humidity limits prevents most blistering.

### How long does PU waterproofing last, and can it be recoated?

Life depends on exposure, traffic, thickness and maintenance. Protected systems under screed last much longer than exposed ones. Exposed aliphatic top coats wear and are renewed periodically, which is simpler than replacing the membrane, provided the surface is cleaned and prepared as the manufacturer specifies and the recoat is compatible with the existing system.

## Sources

- [ASTM C836](https://www.astm.org/products-services/standards-and-publications.html) — High solids content, cold liquid-applied elastomeric waterproofing membrane for use with separate wearing course
- [ASTM C898](https://www.astm.org/products-services/standards-and-publications.html) — Guide for use of high solids content, cold liquid-applied elastomeric waterproofing membrane with separate wearing course
- [ASTM C1305](https://www.astm.org/products-services/standards-and-publications.html) — Crack bridging ability of liquid-applied waterproofing membrane
- [EN 1504-2](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Products and systems for the protection and repair of concrete structures — surface protection systems for concrete
- [EN 14891](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Liquid-applied water impermeable products for use beneath ceramic tiling bonded with adhesives
- [ISO 2808](https://www.iso.org/standards.html) — Paints and varnishes — determination of film thickness

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Source: https://hagerstone.com/materials/pu-waterproofing-coating