Roof Coating Application

A roof coating is a fluid-applied layer that cures into a continuous, elastic film over an existing roof or new substrate. Coatings are used to protect spray foam, extend the life of ageing low-slope roofs, reflect sunlight (cool roofs) and seal metal roofs. Common chemistries include acrylic, silicone, polyurethane, polyurea, butyl, styrene-butadiene and bitumen-based coatings.

Suitability

Not every roof is a candidate. The existing system must be mostly dry and structurally sound, since coating over wet insulation traps moisture. Some coatings are not rated for ponding water; acrylics generally are not, while some silicone and polyurethane coatings are warranted for it by their manufacturers. Coating asphalt shingles and certain built-up roofs is generally discouraged, because the coating can trap moisture or may not be compatible.

Typical sequence

1. Survey. Check for wet areas (often with infrared or moisture meters), blisters, open seams and failed flashings.
2. Clean. Pressure-wash to remove dirt, chalking and biological growth, and allow to dry.
3. Repair. Replace wet insulation, reseal seams, reset fasteners on metal roofs and treat rust.
4. Prime where required for adhesion or to block bleed-through from bitumen.
5. Reinforce details. Apply fabric-reinforced coating or seam sealers at seams, flashings, drains and penetrations.
6. Apply the field coats by airless spray, roller or brush, in one or more coats to the specified thickness.
7. Check thickness. Wet film thickness is measured during application, and dry film thickness after curing.

Thickness

Coatings range from about 0.075 mm (3 mils) to more than 1 mm (40 mils) dry film thickness, depending on the product and the warranty term sought. Thickness, not the number of gallons applied, determines performance.

Weather limits

Each product has limits for air and surface temperature, humidity, dew point and time before rain. Water-based acrylics are sensitive to cold and rain during cure; solvent-based and moisture-cure products have their own restrictions. Manufacturers' data sheets set these limits.

Maintenance

Coatings are renewable: many systems are designed to be cleaned and recoated at intervals to restore thickness and reflectivity.