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PART 1

INTRODUCTION

The terms “drywall” and “gypsum wallboard” are commonly used in the construction industry to describe gypsum board.  The CSA and ASTM standards and Gypsum Association (GA) specifications however refer to “Gypsum Board” as the correct generic name for a family of sheet products consisting of a non-combustible core primarily of gypsum with paper surfacing.  The term “gypsum board” has therefore been incorporated throughout these standards in lieu of “drywall” or “gypsum wallboard”.

Special core formulations are available to provide fire and/or water resistant base.  Special surfacings are also available including water repellant facing, vinyl film laminated to gypsum board to provide predecorated facing and aluminum foil laminated to back surface of gypsum board to provide increased thermal resistance and resistance to vapour diffusion.  A general description of the following gypsum board products is included under this introduction:

Gypsum boards conform to the following standards as applicable:

Standard Gypsum Board

Gypsum board consists of a noncombustible core of not less than 65% gypsum as determined by the method for analysis in CAN/CSA-A82.20.1 and is surfaced with paper bonded to the core.

Fire-Resistant Gypsum Board

Type ‘X’ and Type ‘C’ are the generic terms used to designate gypsum board with a specially formulated core that provides a greater fire resistance than standard gypsum board of the same thickness.  Type ‘X’ and ’C’ gypsum board assemblies, when tested in accordance with CAN/ULCS101 or ASTM E119, in conjunction with non-load bearing steel studs, can provide 1 and 2 hour fire resistance ratings.

Water-Resistant Gypsum Board

Water-resistant gypsum board has a water resistant core such that when tested in accordance with CAN/CSA-A82.20.3, the average absorption shall not exceed 5% by mass after 2 hours of immersion; and shall have a surfacing such that when tested in accordance with CAN/CSA-A82.20.3, the average absorption shall not exceed 1.6g on either the face or back surface.

Foil-backed Gypsum Board

Foil-backed gypsum board has aluminum foil bonded to the back surface so that the foil-backed gypsum board has a vapour permeance not exceeding 15 ng/(Pasm²) when tested in accordance with ASTM E96 for the condition of 50% relative humidity on the face surface and 0% on the foil surface.  Due to its very limited use, foil-backed gypsum board has NOT been included in this section of the AWCC Specification Standards Manual.

Vinyl-Faced Gypsum Board

Vinyl-faced gypsum board has a vinyl fabric film bonded to the face surface, and meets the requirements of CGSB 41-GP-30M or ASTM C960.

Vinyl-faced gypsum board panels are normally installed as part of a demountable partition system and as such are not part of the AWCC Specification Standards Manual.

Gypsum Sheathing Board

Gypsum sheathing board has a surfacing such that when tested in accordance with CAN/CSA-A82.20.3, the average absorption shall not exceed 1.6 g on either the face or back surface.  Core-treated gypsum sheathing when tested in accordance with CAN/SA-A82.20.3 shall have an average absorption not to exceed 10% by mass after 2 h. of immersion.

Gypsum sheathing board is primarily used as a sheathing board on exterior face of exterior stud walls or exterior suspended acrylic stucco finish soffits.  However, gypsum sheathing board is subject to damage when exposed to severe moisture conditions and its use for such applications is NOT recommended by these Standards.  Glass mat gypsum sheathing is recommended for such applications.  Glass mat gypsum sheathing board for wall application is included in this section and also under Section 9.1 Lathing and Metal Furring for exterior soffit application.

Specialty Boards

In addition to the various gypsum products listed above, specialty gypsum and cementitious boards are also available including:

In lieu of water resistant gypsum board as a backer board for ceramic tile at showers and bathtub surrounds two alternative boards are available.  These backer boards are superior to moisture resistant gypsum board and are recommended behind ceramic tile in showers, bathtub surrounds and other areas where excessive moisture may occur.  (See Note 1)  The standards include the following alternative tile backer boards:

Glass Mat Water-Resistant Gypsum Backing Panels or Tile Backer Board

Glass mat water resistant gypsum backing panels conform to ASTM C1178 and have a silicone treated core with glass mat moisture protectant coating and embedded glass mat, both sides and with face side surfaced with heat cured copolymer water and vapour retardant coating.

Reinforced Cementitious Board or Cementitious Backer Units

Reinforced cementitious board or cementitious backer unit (CBU) conforms to ANSI A118.9 and is an incombustible panel formed in a continuous process of aggregated Portland Cement slurry with polymer-coated glass fiber mesh completely encompassing edges, back and front surfaces.

Abuse-Resistant Gypsum Board

Abuse-resistant gypsum board conforms to CAN/CSA A82.27 and ASTM C1278 and is engineered to provide increased resistant to abrasion, indentation and penetration for interior walls and ceilings and are designed to outperform standard gypsum board.  Abuse-resistant board has a dense gypsum core (incombustible) with high strength glass fiber reinforcement and wrapped with heavy duty paper facing.

Flame Spread

All gypsum board, except vinyl faced gypsum board, has a flame spread rating of not more than 25 when tested in accordance with CAN/ULC-S102.

Gypsum Board Application

Gypsum board application should be in accordance with CSA A82-31, ASTM C840, or Gypsum Association GA216 specification.

Gypsum board over wood and steel stud framing provides the Design Authority with a variety of versatile fire and sound resistance rated systems for partitioning, suspended ceilings, cladding of interior walls, and protection of beams and columns. (Refer to Gypsum Association GA-600 Fire Resistance and Design Manual)

Gypsum board is NOT suitable as a plaster base.

Standard gypsum board materials are for interior use only in areas not exposed to prolonged excessive moisture or humidity.  Gypsum board cannot be made waterproof; the paper and the core can only be made water resistant.  Therefore, standard gypsum board should NOT be used behind ceramic tile in showers or at bathtubs. (see Note 1)

Gypsum board may be applied using screw, nails or adhesive, depending on the surface to which board is applied.  Screws are recommended for all applications of gypsum board.  The use of nails contributes to nail popping problems.  The recommended nails are those specially manufactured for gypsum board installations.  Of the types recommended, the annular ring nail attains the maximum holding power with the minimum penetration.

There is no advantage to single nailing over double nailing; it is not cheaper and no faster.  By counting the recommended nails per bearing on a 1200mm (48”) width of gypsum board, single nailing required 7 nails and double nailing requires 8.  Double nailing insures proper nailing by the applicator.  The first nail holds the board in position and the second nail draws the board up tight.  The first nail is then reset by driving it home.  Nail heads should be driven slightly below the surface of the boards, in a dimple which does not break the face paper.  The recommended sequence for double nailing should be followed.  It should be noted that neither double nor single nailing will eliminate nail popping.

There are basically two specific kinds of screws for attachment of gypsum board, one for wood framing and one for steel framing.  Screws lessen the possibility of nail popping.  Only one screw is required per fixing point.  Screws used in wood have a greater holding power than any of the recommended nails.  Only screws should be used in steel stud framing.  The type and size of screws for each type of framing should be those made specifically for use with gypsum board.  There are other additional types of screws for specific applications and installations, i.e. gypsum to gypsum and metal to metal.

Adhesive Applications

The only areas where adhesives may be used as the only method of fixing is when one gypsum board is fixed to another gypsum board (board to board) or when gypsum board is applied to vertical concrete surfaces where surface and tolerance of the concrete are acceptable for gypsum board application.

Horizontal and Vertical Applications

Gypsum board may be applied horizontally or vertically.

In horizontal application, the long dimension of gypsum board (the paper bound edge) is applied at right angles to bearings.  By job planning and layout, horizontal application has the following advantages:

In vertical application, the long dimension of the gypsum board (the paper bound edge) is applied parallel to bearings.  The following points should be considered as advantages and/or disadvantages to the vertical application methods:

Joints in Gypsum Board

Joints in gypsum board are taped and filled flush. 

Joint reinforcement incorporates the use of a perforated paper tape fixed into position by the use of jointing compounds.  Cotton tape used in the past as reinforcement has proved to be unsatisfactory.  Lack of shear strength results when the jointing compound dries, causing deterioration and/or rot in the cotton tape.

Joint compounds vary according to use and drying time.  Most types of jointing compounds dry rather than “set”.  Fast setting joint compounds should not be used with mechanical finishing tools.  The rapid setting action of joint compounds is the cause of unsatisfactory joints.  Major problem areas are:

The recommended joint compounds are the slow drying type.  The bases for these compounds are either vinyl or latex bases.  All of these compounds dry from the outside towards the inside through the evaporation of moisture from the surface.  These drying compounds require a minimum one full day (24 hours) between coats, under normal conditions, or until completely dry (Refer to Best Drying Envelope Table).

Joint or all-purpose compound should be used for embedding tapes.  All purpose or topping compound should be used for finishing.

Various trim and accessories are applied to gypsum board and serve to protect and finish gypsum board around openings and at intersections with other materials.  Typical accessories are corner beads, control joints and casing beads.  Galvanized metal, PVC and paper covered metal accessories are available and are satisfactory for use with gypsum board.

Problem Areas

Problem areas of gypsum board application are outlined under the Quality Assurance Procedures Section of the AWCC Standards.  The most common problem being nail popping.  Nail popping may occur with any type of material secured to wood with nails or with screws.  With gypsum board it is a product of improper application, lumber shrinkage, or a combination of both.  Loose gypsum board caused by nails being set too forcefully resulting in a powdered gypsum core should not be confused with nail popping.  Correct double nailing method of fixing, in which the first nail is set again after installing the second nail, will tend to minimize nail popping.  Screw attachment to wood framing may also alleviate some of the nail popping problems.  However, warpage of framing members may still cause popping.

Moisture content of lumber resulting in lumber shrinkage and nail popping may be minimized by using lumber having 15 percent maximum moisture content.  Nail popping or similar defects are almost eliminated with steel framing and screw fixing.  However, improper application is still a factor.  The most successful remedy for a “true nail pop” is to install a second nail, drawing the board up tight, then to set the original nail and fill both holes with joint compound.

Site applied sprayed textured finish can be applied to gypsum board walls and ceilings using a low pressure spray gun.  These sprayed textured finishes are covered under Section 9.5 of the AWCC Standards.  All joints should be taped and filled and all fasteners should be filled level.  The complete face surface of the board should be primed using a primer as recommended by the manufacturer.  One of the major problems of textured finishes is known as “joint flashing”.  This is caused from the jointing compound showing through the sprayed texture.  To alleviate this problem adequate drying time should be allowed for the joint compound and primer.

Site Conditions

Cold, damp weather contributes to gypsum board sagging and shrinkage, joint compound bond failures, nail pops and joint flashing.  It is critical that the best site conditions possible be maintained before gypsum board is applied and finished.

Gypsum board must be handled correctly, is best stored off the ground on properly spaced risers and is weather protected.  It should never be placed directly on concrete floors.  Deliveries should be inspected for damaged corners, scuffed edges and dropping impacts to ensure core integrity.

The building must be weather-tight before board application takes place.  Unfinished roofs, standing water, absence of doors or windows will severely compromise the integrity of the gypsum board and finishing, and will void manufacturers warranties.

Gypsum board, joint treatments and texture should not be applied until a temperature of at least 10°C (50°F) has been maintained for 24 hours.  This minimum temperature must be maintained during application and until occupancy.  The board should be dry for 48 hours before application of joint compounds, texture or paints.

Ceilings adjoining attic space must be insulated the same day as the gypsum board is applied.  Without the insulation, condensation will form under the ceiling vapour barrier, causing the board to sag.