A roof coating is a liquid membrane rolled or sprayed over an existing roof. It is often sold as a cheap alternative to replacement, and on the right roof it is a good one. On the wrong roof it is money spent on hiding a problem for a season or two. Here is how to tell which kind of roof you have.
What a coating actually does
A coating system is more than paint. The roof is cleaned, open seams and failed flashings are repaired, weak details are reinforced with fabric, and then the coating is applied in one or more coats to a specified dry thickness. The result is a seamless surface that protects the old membrane from sun and water. It doesn't add structure, fix slope, or dry out what is underneath.
Types of flat roof coatings
| Coating | Strengths | Watch-outs |
|---|---|---|
| Silicone | Tolerates ponding water, holds up to UV, usually white | Gets dirty; later recoats generally need silicone too |
| Acrylic | Lower cost, reflective, easy to apply | Doesn't hold up under standing water |
| Urethane | Tough, resists foot traffic and impact | Higher cost, stronger odor during application |
| Rubber / EPDM coatings | Made for EPDM and other rubber surfaces | Quality and thickness vary widely by product |
"Elastomeric" just means the coating stretches and recovers with the roof as it heats and cools. Silicone, acrylic and urethane coatings are all elastomeric.
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Which roofs qualify
- Dry underneath. The decking and insulation must be dry. A moisture check or core cut confirms it.
- Sound membrane. Weathered or chalky is fine; split, shrinking or delaminating over large areas is not.
- Fixable details. Seams and flashings must be repairable before coating.
- One roof, not three. A coating over stacked, water-logged layers just seals the problem in.
Commercial roofs with large areas of dry, aging single-ply, modified bitumen or metal are often good candidates. Many Pittsburgh row-house and porch roofs are not, because they have several old layers or soft decking. Those usually need a tear-off. See row-house roofs.
Coatings in a Pittsburgh climate
Most coatings have minimum temperature and dry-time requirements, so they are applied in the warmer, drier months. A coating needs to cure before rain or dew, which limits the season here. Ponding water rules out acrylic. And a roof that freezes and thaws all winter needs the coating at full specified thickness at seams and transitions, where movement is greatest.
How long a coating lasts
Typically 10 to 20 years, depending on the product, the thickness applied and the warranty it is designed for. Many coatings can be cleaned and recoated at the end of that term rather than tearing off the roof.
The bottom line
A coating is a good investment on a roof that is aging but dry, and a waste on one that is wet or layered. Get a moisture check first. See coatings and inspections and ponding water.