Cleaning Is Not Optional. It’s the Job.

Roof Riffs is presented by Division 7 Sales, Inc. — independent Karnak manufacturers representative since 1990. Visit division7sales.com or subscribe to the Roof Riffs newsletter for industry insights delivered straight to your inbox.

Q: Why do roof coatings peel off after application?
A: Roof coatings peel when the substrate is not properly cleaned before application. Dirt, chalk, carbon residue, and loose particles break the chemical bond between the coating and the roof surface. The coating appears to adhere initially but delaminates over time. To test surface cleanliness, wipe a hand across the cleaned surface — if it comes back chalky, the roof is not ready for coating. Proper cleaning requires a chemical cleaning agent appropriate to the substrate, not just water or a leaf blower.

John McDermott was sent a photo. On the roof in the picture, pieces of coating were peeling up and flopping over — big floppy ears, he said, like an elephant. Someone was asking what they should do about it. John looked at the back side of the coating in the photo and saw it was covered in dirt. Not a little dirt. Completely covered.

“What was the cleaning process?” he asked.

“Oh, we cleaned it,” they said.

They did not clean it.

Cleaning is a chemistry problem, not a cosmetic one

Liquid roof coatings form a chemical bond with the substrate. That bond requires a clean, dry surface. Dirt, chalk, carbon residue, loose particles — all of these are bond breaks. The coating goes down, it looks fine, and then it peels.

John shared a test he’s used in the field: after cleaning, wipe your hand across the roof surface. If your hand comes back chalky, the roof is not ready for coating. That’s it. That’s the test. Chalky residue means the cleaning process wasn’t adequate, and anything applied over it will not bond properly.

What cleaning actually requires

Water alone is not enough. Hot water is better than cold. But what actually works is a cleaning agent appropriate to the substrate:

• On EPDM membranes, you need a product specifically designed to draw loose carbon out of the surface. Most major manufacturers, including Karnak, specify the same EPDM cleaner for this reason.
• On other substrates, a TSP substitute-type product will loosen contaminants and speed up the process.
• Pressure washing is effective — but only in the right sequence. Pressure washing before repairs means you’re driving water into the roof system before the problems are sealed. Repairs first, then cleaning.
• A leaf blower is not a cleaning method. Neither is a broom. Both have their place in moving dirty water to drains after washing, but they are not substitutes for actual cleaning.

Order of operations matters

John made the sequence explicit: assess the roof, do your repairs, then clean, then coat. Skipping or reordering any of those steps produces failures that are entirely predictable in hindsight and entirely preventable in advance.

Danny added that after walking a chalky roof himself and pointing out it wasn’t clean, the crew had to go back and do it again. He joked that he wasn’t coming back to check a second time. Of course he did, but the second trip could have been prevented. 

Division 7 Sales works with contractors before the job starts to make sure the prep process is right. If you have questions about cleaning protocols for a specific substrate or Karnak product, visit division7sales.com or reach out directly. Getting this right at the front end is a lot less expensive than fixing it after the coating is already on the roof.

Hear John and Danny’s full cleaning conversation on Episode 1 of Roof Riffs.