Silicone vs Acrylic vs Polyurethane, Coverage & Prep
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A well-chosen roof coating can buy a tired flat or metal roof another decade, and the wrong one peels off within two summers. Silicone handles ponding water and suits flat recover work; acrylic elastomerics give strong reflectivity on sloped roofs in dry sunny climates but erode under standing water; polyurethane resists traffic and hail; aluminium-fibred asphalt suits old asphalt and mobile-home metal. This guide compares each family with its trade-offs, explains coverage in terms of dry film thickness rather than square footage alone, and walks the prep sequence of cleaning, seam reinforcement, priming and two cross-directional coats that decides whether the coating lasts.
A roof coating is a liquid-applied membrane over an existing low-slope or metal roof. Over a sound substrate it adds years cheaply; over a failed one it wastes the money. Choosing between the chemistries is mostly about water and sunlight.
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What to look for
Silicone where water ponds. It is the only common chemistry that tolerates standing water well.
Acrylic for reflectivity on a well-drained roof, at the lowest cost — accepting that it erodes and needs recoating sooner.
Polyurethane where the roof takes foot traffic, because it is the toughest underfoot.
A stated coverage rate per gallon per coat, and a stated dry film thickness the warranty is written against.
Substrate compatibility on the data sheet — the same product is rarely right for metal, EPDM and modified bitumen alike.
Reinforcing fabric and mastic for seams and splits, which are a separate part of the system.
What is not worth paying for
Coating sold as a fix for a failed roof. It is maintenance, not a replacement.
Estimating on the smooth-surface coverage rate for a weathered roof, which can halve it.
Buying without the matching primer where the data sheet calls for one — the warranty depends on it.
Common mistakes
Applying over a dirty surface. Most coating failures are adhesion failures and most adhesion failures are dirt.
One heavy coat instead of two. Warranties are written against a dry film thickness reached in the stated number of coats.
Coating over ponding water without fixing the fall first, so the coating fails exactly where the roof already failed.
Ignoring the weather window. Rain or heavy dew inside the cure time ruins the work.
Coating asphalt shingles, which traps moisture in the mat and voids the shingle warranty.
Frequently asked questions
Which roof coating is best for ponding water?
Silicone. It is the only common chemistry that tolerates standing water well — acrylic in particular erodes under it.
Can I coat a shingle roof?
Generally no. Coatings are made for low-slope membranes, metal and modified bitumen. Coating asphalt shingles traps moisture in the mat and voids the shingle warranty.
How many coats does a roof coating need?
Usually two, applied in opposite directions, hitting the dry film thickness the warranty is written against. One heavy coat is not equivalent.
Will a coating fix a leaking roof?
It seals pinholes, fastener heads and surface weathering. It cannot bridge an open seam or a split, and over wet insulation it seals the moisture in and the deck keeps rotting.