Technical Elements

Low-slope membrane surfacings

Surfacings can serve one or more functions for low-slope membrane roof systems. Surfacings' primary purpose is to protect membrane roof systems from the effects of ultraviolet rays and weather exposure. However, surfacings also can provide increased fire resistance and impact resistance, aesthetic enhancement and improved roof system thermal performance.

Surfacings can be an integral factory-applied element of a membrane or a separate field-applied component of a roof system. Membrane surfacings come in various forms, including aggregate, ballast, mineral granules, liquid-applied coatings, metal foils, green roof overburden and an occasional combination. Factory-surfaced membranes include metal foils, mineral granules, coatings and granule-coating combinations. Field-applied surfacings include aggregate, ballast, mineral granules, coatings and green roof overburden.

Aggregate is used to surface built-up and polymer-modified bitumen roof systems. When used in this context, aggregate is embedded on a roof surface using hot bitumen or—less frequently—cold mastic. Aggregate generally is 3/8 to 3/4 of an inch in diameter river gravel, crushed stone or slag. Gravel and crushed stone, such as Merrimac gravel, marble chips, geo rock and green garnet, may be used for decorative purposes. Mineral granules also can be broadcast into mastic as a field-applied surfacing.

Ballast also is used to surface loose-laid, ballasted single-ply roof systems and protected membrane roof systems. Ballast generally is 1 to 2 1/2 inches in diameter aggregate, as differentiated from built-up roof aggregate, or concrete roof pavers. Ballast also can be used for decorative purposes and designed to provide or enhance a roof system's wind-uplift resistance.

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