What Are Roof Coatings? A Quick Explainer

A roof coating is a liquid that a crew applies over an existing commercial roof. It cures into a seamless, flexible, waterproof membrane that is bonded to the roof underneath. Done right, it stops leaks, reflects heat and adds ten to twenty years of service to a roof that would otherwise be torn off and replaced.

That is the whole idea. Everything else in this guide is detail.

What a coating actually is

Think of it as a new roof surface poured over the old one. The crew sprays or rolls the liquid across the entire roof, usually in two coats, at a thickness measured in mils. A mil is one thousandth of an inch, and a finished system is typically 20 to 40 mils thick, about the thickness of a credit card or two. Seams, fasteners, curbs and drains get extra material first, because those are the places roofs leak.

Because the membrane is applied as a liquid, it has no seams of its own. Water has nowhere to get in.

What it does for the roof

• Stops leaks. Seams, fastener heads and flashings are sealed under one continuous layer.
• Reflects heat. Most coatings are white or light. A reflective roof runs cooler in summer, which lowers cooling costs and strain on rooftop units.
• Protects the old roof. UV, rain and temperature swings now hit the coating instead of the aging membrane or metal.
• Buys time, repeatedly. When the coating wears, it can be cleaned and recoated. The roof does not have to come off.

What it is not

A coating is not paint, even though it goes on like paint. Paint is a thin decorative film. A coating system is a thick, engineered membrane with a manufacturer warranty behind it.

A coating also cannot save a roof that has already failed. If the insulation under the roof is soaked, if the deck is rotten or badly rusted, or if the membrane is splitting apart, a coating locks those problems in. That is why every honest contractor inspects the roof first and sometimes says no.

The main types

Three chemistries do most of the work in commercial roofing.

• Acrylic is water-based, reflective and the most economical. It is the standard choice on metal roofs and on roofs that drain well. Standing water is its weakness.
• Silicone handles ponding water and is the usual pick for flat roofs. It costs more per gallon and is harder to recoat with anything but more silicone.
• Polyurethane is the toughest against foot traffic and impact, often used as a base coat under silicone or acrylic, or on roofs that get walked on.

Specialty products fill the gaps: primers that help coatings stick, mastics for seams and flashings, and aluminum coatings for asphalt roofs. Our page on types of roof coatings compares them side by side.

Where coatings are used

Coatings go on most commercial roof types: metal panels, single-ply membranes such as TPO, EPDM and PVC, modified bitumen and built-up asphalt roofs, spray foam, and concrete decks. Each surface needs its own prep and often its own primer, which is why the roof type matters as much as the chemistry. Our flat and low-slope roof guide covers the membrane roofs.

How a project runs

1. Inspection. The contractor walks the roof, checks for trapped moisture and confirms the coating will stick.
2. Cleaning. The roof is pressure washed. Adhesion depends on it.
3. Repairs and details. Seams, fasteners, penetrations and flashings are repaired and sealed.
4. Primer, if the roof needs it.
5. Coats. Usually two, applied to the thickness the warranty requires.
6. Inspection and warranty. Thickness is checked and the manufacturer warranty is issued.

Most projects take days, not weeks. There is no tear-off, no dumpsters, and the building stays open.

Cost and warranty, briefly

A coating system typically costs well under what a tear-off and replacement would, because there is no demolition, no disposal and far less labor. Manufacturer warranties commonly run 10, 15 or 20 years, with the longer terms requiring thicker systems. Our page on coating versus replacement walks through the numbers.

Is a coating right for your roof?

If your roof is structurally sound, mostly dry inside, and leaking at seams and details rather than everywhere, it is probably a good candidate. Start with the qualifying checklist, then get free estimates from contractors who work with coatings every day.