How a Previously Coated Flat Roof Is Prepared for New Work

A previously coated roof is prepared in four steps. Identify what the coating is and what lies beneath it, using core cuts. Establish whether the assembly is dry. Test adhesion on the cleaned surface. Then remove everything loose, blistered or incompatible before any new coating, patch or membrane goes down.

Why does an existing coating change the job so much?

Because it hides the roof and it dictates chemistry.

A coating is a thin surface film. It conceals the membrane below it, so splits, open laps, blisters and the boundaries between different repair generations are no longer visible from the surface. It also conceals what has been done in the past, and coated roofs frequently carry two or three previous rounds of patching under the film.

Then there is compatibility. Coatings are not one product. Aluminum-fibred asphalt cutback, asphalt emulsion, acrylic, silicone, polyurethane and various hybrids all behave differently and stick to different things. Silicone in particular is difficult to overcoat with anything except more silicone, and almost nothing adheres reliably to a chalking, dirty or oxidised surface of any type.

So the preparation is not cleaning followed by application. It is investigation, then testing, then removal, then application. Skipping the middle two is why coated roofs fail early, and it is a large part of why we insist on core cuts before quoting restoration work at all, as set out on our roof restoration guidance.

How do you find out what is actually on the roof?

Nobody guesses this reliably from the surface, so it gets established physically.

• Core cuts. A cut through coating, membrane, cover board and insulation to the deck shows the number of coating layers, the membrane system beneath, how many roofs are already present, the insulation type and thickness, whether it is dry, and the deck condition.
• Building records. Old invoices, specifications and product data sheets, if the building still holds them, will often name the coating. It is worth asking before anyone climbs a ladder.
• Simple field checks. Solvent rub tests and physical behaviour tell an experienced crew a good deal: silicone feels and behaves quite unlike an acrylic, and an aluminum-fibred asphalt coating is unmistakable once it is cut.
• Manufacturer verification. Where a new system is being specified over the old one, the coating manufacturer's technical service should confirm compatibility with what the cores show. That confirmation belongs in writing, in the file.

If the cores come back with wet insulation, the preparation discussion is over. No surface treatment resolves water inside a sealed assembly, and the project becomes a repair or replacement scope instead.

What does adhesion testing involve?

Adhesion testing is the step people skip, and it is the one that decides whether the money spent on the roof stays on the roof.

The method is straightforward. Sample areas are chosen across the roof, including in the worst and dirtiest conditions rather than only the easiest ones. Each area is cleaned exactly the way the whole roof will be cleaned, not more thoroughly. The proposed coating is applied to the prepared area and allowed to cure fully. Then it is pulled, typically with an embedded fabric strip or by scoring and lifting, and the result is recorded.

What matters is where the failure occurs. If the new coating pulls away from the old surface, the preparation or the compatibility is wrong. If the old coating pulls away from the membrane below it, the old coating has to come off. If the sample tears through itself while staying bonded, adhesion is sound.

Test in several locations, because a roof is not uniform: the area under a grease exhaust behaves nothing like the clean corner near the hatch, and a south-facing field that has chalked heavily behaves differently again.

How is a coated roof cleaned properly?

Cleaning removes what stops adhesion: dirt, chalk, algae, oxidised binder, oils and loose granules. It is more involved than a rinse.

1. Sweep and remove debris first. Pressure washing debris around a roof just packs it into the drains.
2. Detergent or manufacturer-specified cleaner where there is grease, biological growth or heavy chalking. Plain water does not remove oils.
3. Pressure washing at a controlled pressure. Enough to remove chalk and loose material, not enough to drive water into laps, lift edges or cut the membrane. Pressure is set by the substrate, and a poorly aimed wand at close range will open a seam.
4. Protect and monitor the drains through the whole operation. Washing a roof puts a lot of water on it in a short time.
5. Rinse thoroughly so no detergent residue is left, since residue is itself a bond breaker.
6. Let it dry completely. This is the step that gets compressed when a schedule is tight, and it is a common cause of blistering later. Wet substrate under a new coating means trapped moisture.

On a plant roof with grease or process fallout, cleaning may not fully restore the surface, and the correct answer in that zone is often removal rather than more washing.

What has to be removed rather than cleaned?

Anything that is not soundly bonded to what is under it. New material only holds as well as the layer it is stuck to.

• Blisters. Cut them out, dry the area, and repair the membrane properly. Coating over a blister guarantees the blister comes back through.
• Loose, flaking or peeling coating. Scraped or spudded back to sound material, with the boundary feathered so the transition is not a ridge that cracks.
• Failed or incompatible coating systems. Where the cores or the adhesion tests say the existing film has to go, it goes. On some roofs that is the whole surface, and at that point the honest comparison is against replacing the membrane.
• Old sealant beads and mastic build-up. Thick mastic at details cracks, holds water and prevents proper flashing work. It comes off and the detail is rebuilt.
• Loose aggregate and granules. Anything that moves under a broom will move under a coating.

Once the surface is sound, the underlying defects get repaired in the appropriate system: laps re-welded or re-torched, splits patched with reinforcement, base flashings reinstated. A coating is a surfacing, not a repair, and every open detail should be closed properly before it goes on. That work is standard flat roof repair scope.

What conditions does the work need?

Coating and adhesive-based work is weather-dependent in ways that ordinary membrane work is not.

• Temperature. Each product has a minimum application temperature and a cure window. Applying below it does not slow the cure down, it changes the result.
• Dew point. Condensation forming on a surface as evening temperatures fall will ruin a coating that has not cured. Late-day application into a cool evening is a common cause of failure in the shoulder seasons.
• Rain. Most coatings need a rain-free window measured in hours after application. A shower on an uncured film washes it into the drains.
• Ponding. A coating applied where water stands will sit under water. Most systems are not intended for continuous immersion, so the drainage problem is corrected first with taper or additional drainage, not coated over.

Which is why coating work in Ontario has a season, and why it should be planned rather than squeezed in at the end of the year. Timing considerations across the year are covered in our post on the best time of year for roofing.

When does preparation tell you to stop?

Preparation is also a decision-making exercise. Stop and reconsider the whole approach when:

• Cores show wet insulation over any meaningful area.
• The deck shows corrosion or deterioration. That is an engineer's call and it is not solved from the surface.
• Adhesion tests fail across multiple locations, and the removal required approaches the cost and disruption of re-roofing.
• The membrane is splitting in unrelated areas across the field rather than weathering at the surface.
• The roof already carries the maximum number of layers allowed on that deck.

None of that is wasted effort. A few cores and a set of adhesion tests are the cheapest way to find out that the answer is a flat roof replacement, and they are considerably cheaper than coating a roof that could not be saved.

To have a coated roof cored, tested and assessed, call 416-456-0777 or book through the free estimate form.