How to coat a flat roof

Is your roof ready to be coated?

A coating is a protective skin, not a new roof. It can add years to a sound membrane, but it can’t hold together a roof that’s failing underneath. Before you buy a single pail, check four things:

• Is it dry inside? Coating over wet insulation traps the water. Henry’s Pro-Grade instructions say not to coat over saturated insulation and list infrared, nuclear and capacitance scans or core cuts as ways to check. GAF’s acrylic-over-foam spec requires a moisture survey and removal of wet areas.
• Does it drain? Many coating warranties, including Johns Manville’s, exclude water that stands more than 48 hours after rain. Silicone tolerates ponding better than acrylic, but fixing drainage is still the better plan.
• Is it attached and sound? Loose seams, rotted decking, soft spots and backed-out fasteners all need repair first.
• Is it old enough? New membranes can be too slick for a coating to grip. JM wants EPDM, TPO and PVC at least a year old, and new asphalt usually needs weeks to a season to weather.

Pick your coating next, using our compatibility guide, and download its data sheet. Every number below should come from your product’s sheet first.

How do you clean a flat roof for coating?

Coatings stick to clean, dry roofing, not to dirt, chalk, algae or oil. Henry’s 887 instructions call for a pressure wash with a non-filming detergent such as TSP or a TSP substitute. Its Pro-Grade instructions add warm water and mild detergent for grease, household bleach and water for algae, and a full rinse so nothing left on the surface interferes with adhesion.

Keep the spray moving and angled away from seam edges and flashings so you don’t drive water under the membrane. On EPDM, use an EPDM-specific cleaner. On aged TPO and PVC, the weathered surface layer needs scrubbing. Then let the roof dry completely, which can take a full day or more in humid weather.

What repairs come first?

Anything that leaks or moves will crack the coating over it, so repair first:

• Splits, punctures and blisters: cut out and patch using materials that match the coating (see our patching guide).
• Metal roofs: wire-brush or grind rust, replace badly corroded panels, and replace loose fasteners with larger-diameter ones, as Henry’s instructions call for.
• Asphalt roofs: remove loose gravel completely, and replace wet insulation with like materials.
• Single-ply: patch with matching membrane, never asphalt.

If you plan to coat soon, avoid asphalt roof cement for these repairs. It needs time to cure and can bleed through white coatings.

How do you reinforce seams and flashings?

Seams, walls, curbs, pipes and drains are where roofs move and where they leak, so they get extra material. The pattern is the same for most systems, a three-course detail in coating chemistry: a heavy layer of the coating maker’s sealant or base coat, polyester fabric pressed in, then a layer that completely covers the fabric.

Specs from the manufacturers give a sense of scale:

• Henry Pro-Grade puts sealant on seams about 1/8 inch thick (125 wet mils), at least 2 inches each side.
• Its fabric-reinforced option centers 6-inch polyester fabric over the seam in a base coat that runs at least 4 inches each side.
• GAF three-courses loose seams on asphalt roofs with silicone mastic and 6-inch fabric.
• Henry has drains sealed at least 12 inches around the opening.

On metal roofs, fully encapsulate every fastener head.

Our flashing and seam repair guide covers these details by roof type.

Do you need a primer?

Primers either help the coating grip or stop something underneath from coming through it. Use what your coating’s maker specifies for your roof:

Roof surface	Common primer requirement	Why
Asphalt (mod bit, BUR, roll roofing)	Bleed-blocking primer under white silicone; smooth or hot-mopped asphalt often needs a primer under acrylic	Asphalt oils stain white coatings
EPDM	EPDM cleaner, often an EPDM primer	EPDM is slick and carries surface residues
TPO / PVC	Single-ply primer, especially on newer membranes	Smooth thermoplastic resists adhesion
Bare or rusty metal	Rust-inhibitive primer; epoxy primer under urethanes	Stops rust and improves grip
Spray foam	Per the foam-coating spec; foam left exposed to sun, or new foam tied into old, may need a primer	Sunlight degrades bare foam
Existing silicone	None; silicone bonds to clean silicone	Only silicone will stick
Existing acrylic	Per the new coating’s spec; wash and test adhesion	Chalk and loose coating must come off

Adhesion testing settles it. JM’s procedure embeds a strip of polyester fabric in the primer or base coat at several spots, lets it cure 48 to 72 hours and pulls it with a fish scale. It passes at 2 pounds per inch of fabric width. Henry’s Pro-Grade spec uses the same 2.0 pli mark and adds primer if a test fails.

How thick should the coating be?

Gallons are how you buy coating, but mils are what protect the roof (a mil is a thousandth of an inch). The math is simple:

• Wet mils: 1 U.S. gallon spread over 100 sq ft is about 16 wet mils. So 1.5 gallons is about 24 wet mils, and 2 gallons is about 32.
• Dry mils = wet mils × volume solids. A 54%-solids acrylic applied at 24 wet mils dries to about 13 mils; a 91%-solids silicone at the same rate dries to about 22.

Coating	Typical spread per coat (gal/100 sq ft)	Typical coats	Notes from data sheets
Silicone	1–2.5	1–2	Henry 887: at least 1.5 gal for about 22 dry mils
Acrylic	1–1.5	2–3	APOC 252: two full coats at 1.5 gal each
Polyurethane	1–1.5	2–3	Base coat plus aliphatic topcoat
SEBS	1–1.5	1–2	Solvent-based; pro product
Fibered aluminum	1.25–2	1–2	Asphalt and metal roofs only
Asphalt emulsion	3–4	1–2	Black base coat; not reflective

Rough and granulated roofs take more. Henry calls for 2–3 gallons of its acrylic per 100 sq ft on mineral surfaces. Use the calculator on this page, then grid the roof into 100 sq ft squares and put the right number of gallons in each one. Press a wet-mil gauge into the fresh coating every few squares and adjust before you move on.

Apply each coat at a right angle to the last. Henry’s 887 data sheet says to finish any added silicone coats within 48 hours, or the first coat may need washing again.

What weather do you need?

	Acrylic	Silicone
Minimum temperature	50–55°F and rising, per most labels	About 35–40°F and rising
Surface too hot	Several labels warn above about 140°F	Similar warnings; work early
Dew and fog	Not on a damp surface; no heavy dew expected for 24 hrs	Dry surface; Henry Pro-Grade wants 6°F above dew point
Rain-free time	24 hrs on most labels; some say 48–72	15 minutes to a few hours, by product
Cure speed	Slower in cool, humid air	Faster in humid air

Florida and other hot, humid climates: start when the dew is gone and finish well before the typical afternoon storm build-up. A dark roof in summer sun can climb past 150°F, so begin early and save the sunniest sections for shade or the next morning. Avoid scheduling a coating job right before a forecast tropical system; a coating that hasn’t cured can’t do its job. Permit rules for roof coating vary by jurisdiction, so check with your local building department before you start.

Cooler climates: the night matters as much as the afternoon. If temperatures will fall below the label minimum before an acrylic cures, wait.

Safety: coat with a partner, keep a clear path to the ladder, and never walk backward toward an edge while rolling. Wet coating is slick, so plan your route so you finish at the ladder. Solvent-based primers and urethanes need ventilation, no flames nearby and the respirator listed on the safety data sheet.