Restoring roofs

Stained, deteriorated surface of a granulated modified bitumen membrane system.

Most bituminous restoration coatings are applied cold and used at ambient temperatures. They consist of a bituminous resin, a solvent or water carrier, reinforced fibers or fillers,and sometimes a reflective pigment.

The two principle forms of bituminous roofing are built-up or polymer (APP or SBS) modified membranes. Systems that combine both are referred to as composite or hybrid. Three surfacings are available: granulated, flood coat with gravel or smooth (with or without reflective coating).

Type of bitumen

Before restoring a bituminous roof, it's necessary to determine whether the materials of construction are asphalt or coal tar. Here are some techniques to identify the materials.

Odor. When heated, asphalt smells like fuel oil. By comparison, coal tar smells like creosote wood preservative.

Solubility. Asphalt dissolves in petroleum solvents, such as gasoline, kerosene and lighter fluid. Coal tar does not. Take a cloth soaked in petroleum solvent and rub the roof vigorously. Asphalt transfers a heavy residue of asphalt to the cloth and feels tacky. The residue will be dark brown in the center and light brown at the edges. A similar cloth rubbed on coal tar becomes discolored, but does not feel tacky or transfer a heavy film. The cloth will be light to medium brown at the center and yellow at the edges.

Laboratory identification. Coal tar and asphalt can be differentiated using mineral spirits. Place a sample into a beaker of mineral spirits (the ratio should be 2 grams of material to 20 grams of spirits.) Asphalt quickly turns mineral spirits to deep brown. Coal tar gradually turns it yellow.

Asphalt and coal tar compatibility

Always apply coating evenly with a flat squeegee.

Five rules apply when bringing asphalt and coal tar together:

1. Don't try to heat or mix asphalt and coal tar in the same kettle.
2. Don't use the same tools to apply asphalt and coal tar unless they are cleaned between uses.
3. Don't "gravel-in" a coal tar and felt roof with asphalt.
4. Don't use asphalt as the interply bitumen in a coal tar/felt membrane construction. Don't use coal tar as an interply bitumen with an asphalt saturated organic felt in a roofing or waterproofing system.
5. Repair coal tar roofs with coal tar based repair compounds and asphalt membranes that use asphalt-based materials.

Substrate preparation

Follow these steps to prepare the existing membrane and substrate before applying the restoration coating.

Power broom or vacuum the membrane surface to remove loose aggregate, dirt and debris. If the membrane is installed over an easily compressible substrate, such as fiberglass insulation, vacuuming is recommended. Vacuuming does less damage to the existing membrane. If a power broom is used, remove any remaining dirt with a high-velocity leaf blower.

Conduct a roof moisture survey with a nuclear or infrared scan to identify any wet areas. Remove and replace wet insulation. Provide a smooth transition to the surrounding roof membrane.

Remove blisters, ridges, splits, punctures, loose felt plies, gravel stop splits or other membrane irregularities with appropriate procedures and compatible materials.

Remove mechanical equipment and other obstructions that prevent proper installation of repair materials. Don't start repairs unless they can be made weather-tight before the end of the workday.

Correct substrate defects before applying restoration coating. Major repairs using a cold process method should be allowed sufficient time to set firm before being exposed to foot or equipment traffic.

Restoration coating selection is based on the type of existing surfacing and the preferred application method. A coal tar pitch built-up gravel surface roof, for example, can be restored using either a cold or hot method. The cold process method typically uses a solvent-borne coal tar resaturant. The unique penetrating qualities of solvent-borne resaturant improve flexibility and add new life to an aging system.

Restoring coal tar roofs

After the preparation, membrane replacement and flashing repairs, verify that the surface is dry. Apply the solvent-borne coal tar resaturant evenly at a rate of seven gallons per 100 sq. ft. Apply the new roofing aggregate immediately at a sufficient rate to cover the coating completely.

If the hot coal tar is preferred, heat it to the manufacturer's recommendation. Apply an even top pour at a rate of 85 pounds per 100 square feet. While the coal tar pitch is still molten, apply immediately a uniform layer of new roofing aggregate at the rate of 400 pounds per 100 sq. ft.

Restoring asphalt built-up gravel roofing

The cold process method usually is used to restore an asphalt built-up gravel surface roof. Follow the same procedure as with a coal tar built roof, but use a solvent-borne or water-borne asphalt coating. While coal tar built-up roofs always have an aggregate surface, asphalt built-up roofs can have a smooth surface. Polymer (APP or SBS) modified membrane systems, standard or hybrid, have either a smooth or granulated surfacing. These surfaces are the easiest to inspect and prepare.

After the preparation, membrane replacement and flashing repairs, verify that the surface is dry. Treat the surface with a recommended asphalt-based primer to enhance adhesion of the restoration coating to the granulated surface.

Allow the primer to dry. Apply a compatible restoration coating evenly. Coverage rates vary depending on surface conditions. A rate of four to five gallons per 100 sq. ft. is reasonable for a smooth surface. If using a reinforcing membrane with the restoration coating, apply one-half of the coating approximately 42 in. wide as an undercoat and roll the membrane immediately into the wet coating. Broom the membrane to eliminate wrinkles and provide a positive embedment. Make side and end laps at least 2 in. wide. Apply the remaining restoration coating as a top layer.

Smooth surface restoration coatings may include a reflective pigment that acts as a barrier against ultraviolet light and its photo-oxidizing effects. If the coating is not pigmented, refer to manufacturer's recommendations for the allowed curing time before adding a reflective coating.

The newest restoration coatings can be applied to any type of substrate-asphalt or coal-tar gravel. They are formulated with a variety of synthetic resins, such as styrene butadiene and urethane. The synthetic resins provide enhanced performance properties that include elasticity, chemical resistance and low-temperature flexibility.

Bob Tomlinson is an applications technology leader at Honeywell Commercial Roofing Systems. He can be reached at [email protected]