The many benefits of gypsum sheathing is Walker's topic this month.

Gypsum sheathing is a versatile, economical and water-repellent panel product intended for use as a substrate sheathing under a variety of exterior wall claddings. It is designed to be mechanically attached to the outside surface of exterior wall framing spaced up to 24 inches (610 mm) on center using either nails, screws or staples. Manufactured in a wide range of lengths to meet the demanding needs of the construction industry, gypsum sheathing is easily cut to size and installed. Gypsum sheathing provides long-lasting stability to a structure’s vital framing elements. Inspections of buildings more than 25 years old have shown the original gypsum sheathing to be in excellent condition.

The natural non-combustible core of gypsum sheathing protects the framing from the hazards of fires that may occur outside the building, even when the siding material is combustible. When exposed to high temperatures, chemically combined water in the gypsum is gradually released, providing protection until all of this water is completely driven off. In addition to its fire-resistance properties, gypsum sheathing has a low flame spread rating of less than 25 (tested in accordance with ASTM E 84 or CAN/ULC-S102). Where fire-rated and/or sound control systems are needed, the applicable test report should be reviewed for complete information on systems and component rating. Specific system details are available in the Gypsum Association’s Fire Resistance Design Manual (GA-600-00).

The many faces of gypsum

Gypsum sheathing can be used with virtually all exterior wall claddings, including wood siding and shingles, panel siding, vinyl or metal siding, masonry or brick veneer, Portland cement stucco, and EIFS. Mechanically attached exterior wall claddings are attached through the sheathing and into the wall framing. EIFS that incorporate gypsum sheathing are installed in accordance with the EIFS manufacturer’s recommendations.

Gypsum sheathing is an economical material that generates a minimum of waste during application. Gypsum sheathing panels are applied quickly and easily. In some cases, fewer fasteners are required to cover a given area than with other sheathing materials. Low material and labor costs, plus possible elimination of the need for building felt, are important economic advantages gained by the use of this water-repellent panel sheathing.

When properly applied, gypsum sheathing provides a barrier to reduce wind and water infiltration. The surface of gypsum sheathing is water repellent. This special feature makes the use of a weather-resistive barrier unnecessary in some cases. A weather-resistive barrier may be required when sheathing is used in exterior stucco curtain walls, with EIFS, when square-edge gypsum sheathing is applied horizontally, or when regular (non-water resistant) core sheathing is used. Consult the local building code or cladding manufacturer to determine when a weather-resistive barrier is required. Rough openings should be properly protected by such means as flashing, weep holes or caulking so as to avoid trapping water. Window and door manufacturers’ recommendations should be followed for proper installation.

Gypsum sheathing should be protected from prolonged weather exposure during storage. It may be stored outside for up to one month as long as it is stacked off the ground under a protective covering. Upon installation, gypsum sheathing should be covered immediately by building felt, or equivalent, if exposure time will exceed one month or if weather conditions will be severe.

Gypsum sheathing is manufactured to meet the requirements of ASTM C 79 (or C 1396) and CAN/CSA A82.27-M (Canadian National Standard); major characteristics are detailed in the table.

Many building codes allow the use of vertically applied 4-foot (1220 mm) wide panels of 1/2-inch (12.7 mm) gypsum sheathing in place of continuous diagonal wall bracing. In areas where potential wind or seismic forces require shear walls to resist lateral forces, most building codes provide allowable shear values for walls having gypsum sheathing applied to wood framing. Specific values with construction requirements and limitations are contained in the major model building codes (National Building Code, Standard Building Code, Uniform Building Code, and the National Building Code of Canada). Gypsum sheathing permeability provides for the escape of normal water vapor in walls with vapor retarders.

Gypsum sheathing is the sheathing of choice wherever fire-resistance ratings or noncombustible materials are desired. W&C