Section II – Supporting Construction
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Table of Contents
Gypsum board cannot compensate for defective, improperly installed, or misaligned framing. All wood framing and furring must be accurately aligned in the same plane so that the gypsum board fits flat against it at all points (Fig. 11). Following proper framing practices ensures a firm, even structure for the application of gypsum boards.
To ensure wood framing is properly installed, do not allow the fastening surface of any framing or furring member to vary more than 1/8 -inch from the plane of the faces of adjacent framing, bridging, or furring members, and do not exceed the maximum recommended spacing between framing members for the specific gypsum board thickness. See Table II for single-ply construction and Tables III and IV for multi-ply construction. Avoid placing and stapling paper flanges of insulation batts over framing faces because the presence of the paper may cause poor nailing, which in turn can eventually result in joint ridging and nail popping problem.
Take care to use only properly cured lumber for wood framing and furring. Excessive moisture in wood causes warping as the wood dries. Replace warped or crooked studs and joists with straight lumber. Provide headers or lintels over openings to support structural loads, and provide special construction wherever necessary to support wall-hung equipment and fixtures. Bathand shower areas are examples where special framing must be provided for grab bars and heavy fixtures. Cabinets or wall-hung appliances also require additional framing for support.
Fig. 11 – Improper Board Attachment Due to Faulty Stud Alignment
Table II – Maximum Framing Spacing Single-Ply Application
Table III – Two-Ply Application without Adhesive between Plies
Table IV – Two-Ply Application with Adhesive between Plies
Use cross furring to correct surface unevenness in existing framing. Make sure that the fastening surface of wood furring strips is no less than 1-1/2 -inch actual dimension. When using nails to attach gypsum board to wood framing, use nominal 2 x 2 wood furring over the underlying wood framing to provide a rigid support during nailing. Where wood furring strips are attached to concrete or masonry walls or where screws are used to attach the gypsum board over wood framing, use nominal 1 x 3 wood furring (3/4 -inch minimum thickness). Maintain the same maximum spacing between furring strips as is shown for framing members in Table II or Table III. In wood framed construction where a higher degree of sound control is desired, use screws to attach gypsum board to resilient metal furring channels, laminate another layer of gypsum board, or follow the directions for board attachment in specific sound control assemblies.
Wood studs in load-bearing partitions are typically nominal 2 x 4, or larger, and can be 2 x 3 in double walls. In non-load-bearing or non-fire rated single row stud or in staggered stud partitions, 2 x 3 wood studs may be used (Fig. 12).
Provide back-up framing or special clips at all interior corners for support or as a nailing base for the gypsum board.
Fig. 12 – Staggered Stud Partition
Place ceiling joists with faces aligned in a level plane. Space joists evenly. Do not use excessively bowed or crooked joists. Install joists with a slight crown with the crown up. Slightly crooked or bowed joists can sometimes be aligned by nailing bracing members (strong-back) across the joists approximately at mid span (Fig. 13).
Fig. 13 – Stringers Used to Straighten Crooked Joists
Wood trusses, to which a ceiling is to be attached, might have irregularities in spacing and leveling. When wide variances are found, use cross furring to provide a level surface to support the gypsum board.
When trusses change direction in the middle of a room, install blocking so that the spacing of the framing does not exceed the requirements indicated in Tables II, III and IV. With truss roof construction, the exterior walls and ceilings often are finished before interior partitions are erected and finished. However, if substantial roof or other loads are introduced after partitions are installed, the ceiling may be forced down against the partitions, which in turn can result in distortion of the ceiling or the partitions or both. To avoid these distortions, install the roofing and all other construction elements that increase roof loads before erecting the interior partitions (Fig 14).
Fig. 14 – Ceiling Truss Movement
The advantages of metal framing are noncombustibility, uniformity of dimension, lightness of weight, freedom from rot and moisture problems, and relative ease of installation. The components of metal frame systems fit together easily and are generally friction fit (requiring no fasteners) except at doors, windows, and other openings. There is a wide selection of metal framing systems, both load-bearing and non-load-bearing. Some non-load-bearing gypsum partition systems are designed to be demountable or movable and still meet requirements for sound isolation and fire resistance.1 Certain metal stud partition systems meet requirements for non-combustible partition framing and furring in high-rise and other buildings (Fig. 15). Metal framing systems are beginning to enjoy increased use in residential construction.2 Ceilings in commercial and institutional structures are typically attached to metal furring members that are secured to or suspended from steel open-web joists or light beams.3 Note: See Tables II and III for the recommendations for spacing of metal framing to receive gypsum board in single and multi-ply construction.4
Fig. 15 – Metal Studs Being Fastened to Track
Typically, metal studs used in non-load-bearing partition framing systems are “C” shaped. (Fig. 16).They are available in a range of widths, including 1-5/8-, 2-1/2-, 3-5/8-, 4- or 6- inches, and lengths from 6 feet. to 16 ft.4 Steel studs may be spaced up to a maximum of 24 inches o.c.
1The Gypsum Association GA-600, Fire Resistance Design Manual, contains a wide variety of designs tested for fire resistance and sound control.
2See ASTM C 754 for detailed application procedures for metal framing.
3See Gypsum Association GA-216, Recommended Specifications for the Application and Finishing of Gypsum Panel Products, and ASTM C 840 for detailed specifications for application of gypsum board to metal framing.
4Ensure that studs are manufactured in conformance with ASTM C 645.
Fig. 16“C” – Shaped Steel StudsPlaced in Runners Support Multi-Ply Partition
Open-web steel bar joists are not designed to receive wallboard
directly, and they are often spaced more than 24 inches o.c. Hence, theyrequire suitable furring or suspension systems, installed with appropriatespacing, to support gypsum board ceilings.
Channels – There are two major types of metal furring channels used in gypsum board construction (Fig. 17):
(1) Rigid furring channels
(2) Resilient furring channels
Rigid furring channels are 18-mil thick (minimum) galvanized steel and are generally “hat” shaped. Rigid furring channels are designed for gypsum board attachment with gypsum drywall screws. Install furring channels at right angles to the furring supports (Fig. 18). Fasten rigid furring to wood framing with 1-1/4 -inch long screws or 1-3/4 -inch 5d nails, and to metal framing with screws, wire ties or clips.
Fig. 17 Varieties of Metal Furring
Fig. 18 Furring Channel Details
Ridgid furring channels can be installed over masonry or concrete surfaces either horizontally or vertically. Where necessary, metal studs can be used as furring channels. Follow the spacing recommendations shown in Table I or Table II for the proper placement of furring.
Grid-type furring systems allow the use of suspended grid systems below both metal and wood framing. They include directly suspended inverted T-shaped main runners (beams), typically 4′ o.c., and inter-connecting cross channels spaced either 16 inches or 24 inches o.c.
Resilient furring channels provide a sound absorbent mounting framework for gypsum board over both metal and wood framing. Attach resilient furring to framing per the manufacturer’s specifications. Pay particular attention to the orientation of the channel so that the open edge faces up so that the system performs as intended. Resilient furring channels not only improve sound isolation, but they also help to isolate the gypsum board from structural movement, thus minimizing the possibility of cracking. Resilient furring channels are also suitable for the application of gypsum board over masonry or concrete walls.
Other materials used as furring channels:
Steel studs may also be used as furring channels for spans exceeding 48 inches. The minimum width of any metal furring channel to provide a sufficient fastening surface for properly attaching abutting edges or ends of gypsum board panels is 1-1/4 inches.
Cold-rolled channels may be used in furred partitions and in most types of suspended ceiling assemblies. Ordinarily, they are suspended by wire or rods with the furring channels tied or clipped to them. These channels are usually composed of 54-mil thick steel with either a galvanized or black asphaltum finish. They are available in standard widths of ¾, 1-1/4, or 2 -inch, with lengths ranging up to 20 ft.
Nails, screws, and staples are the fasteners most commonly used for attaching gypsum board to various substrates. Use nails and screws to attach gypsum board in both single and multi-ply installations; use staples only to attach the base layer in multi-ply construction.
Use special drywall adhesives to improve the bond and reduce the number of fasteners required to secure single-ply gypsum board to framing and furring, masonry and concrete, or to laminate face plies to base layers of gypsum board or other base materials. Adhesives require the use of supplemental mechanical fasteners, either temporary or permanent, depending on the application.
Install fasteners at least 3/8 -inch from board edges and ends. Begin installing fasteners in the middle of the board and proceed outward toward the perimeter. Drive fasteners as near to perpendicular as possible while holding the board firmly against the supporting construction. Drive nails using a crown-headed hammer, which forms a uniform depression or “dimple” not more than 1/16 -inch deep around the nail head (Fig. 19). Take particular care not to break the face paper or crush the core with too heavy a blow.
Fig. 19 Cross Section of Nail-Attached Gypsum Board to Wood Framing
See Fig. 20 below for examples of nails that are acceptable for gypsum board application. Nails used for gypsum board installation preferably have heads that are flat or concave and thin at the rim. Use nails with heads at least 1/4 -inch in diameter and not more than 5/16 -inch in diameter to provide adequate holding power without cutting the face paper when the nail is dimpled. Avoid using casing nails and common nails because their heads are too small relative to their shanks; consequently, they easily cut into the face paper. Nail heads that are too large are also likely to cut the paper surface if the nail is driven incorrectly at a slight angle. Use nails long enough to go through the wallboard layers and far enough into supporting construction to provide adequate holding power. Be sure that nail penetration
into framing members is 7/8 -inch for smooth shank nails but only ¾ -inch for annular ringed nails, which provide more withdrawal resistance. Fire-rated assemblies require greater nail penetration, generally 1-1/8 -inch to 1-1/4 -inch for one hour assemblies.
Fig. 20 Nails Used in Drywall Construction
The drywall screw is the most commonly used type of fastener for attaching gypsum board to either framing or existing gypsum board. Drywall screws have Phillips’ heads that are designed to snugly fit the tip of a powered drywall screwdriver. Drywall screws pull the board tightly to the supports without damaging the board, and they minimize surface defects caused by loose boards and shrinking lumber. The specially contoured head, when properly driven, makes a uniform depression free of ragged edges. Correct setting of the nosepiece on the power screwdriver permits correct countersinking of the drywall screw head for proper fastener concealment with joint compound. See Figure 21 for the three basic types of drywall screws, Type W for wood, Type S for light gage steel studs, and Type G for solid gypsum construction. Also shown is a sample of a Type S-12 screw, which is used for attaching wallboard to heavier gauge steel framing.
For best results:
- Use Type W gypsum drywall screws for fastening gypsum board to wood framing or furring. Diamond-shaped points on Type W screws provide efficient drilling action through both gypsum and wood, and a specially designed thread gives quick penetration and increased holding power. Type W screws must penetrate into supporting construction at least 5/8 -inch.However, in two-ply construction where the face layer is screw attached, additional holding power is developed in the base ply. In multi-layer applications, screws need only penetrate ½-inch into supports. Type S screws may be substituted for Type W.
Fig. 21 Screws Used for Attaching Gypsum Board
Use Type S gypsum drywall screws for fastening gypsum board to steel studs or furring. They are self-drilling, have a self-tapping thread and generally a mill-slot or hardened drill point for penetrating metal with little pressure. (Easy penetration is important because steel studs often are flexible and tend to bend away from the screws.)
Use Type G gypsum drywall screws for fastening gypsum board to gypsum backing boards. Type G are similar to Type W screws, but they have a deeper special thread design. They are generally 1-½ -inch long, but other lengths are available. Type G gypsum drywall screws require penetration of at least ½ -inch of the threaded portion into the supporting board. Do not use Type G drywall screws to attach wallboard to 3/8-inch backing board because the backing board will not support a face layer using screws alone. Drive nails or longer screws through both the surface layer and the 3/8 -inch base ply to penetrate sufficiently into the supporting wood or steel framing members.
Staples are suitable for attaching the base ply to wood members in multi-ply construction. Preferred staples are 16-gauge, flattened, galvanized wire with a minimum 7/16-inch – wide crown and spreading points. Staples must penetrate at least 5/8 -inch into framing or supports. Do not use staples for attaching face layers.
Pneumatic pins attach gypsum board to steel framing by using forced air to drive the pin through the board into the steel framing member. The pin has ridges or ribs that grip the framing member and prevent it from working loose. Using pneumatic pins is much faster than using screws.
Wallboard adhesives bond single layers of gypsum board directly to framing, furring, masonry or concrete. They also bond face layers of gypsum board to base layers of backing board, sound deadening board, rigid foam, or other rigid insulation boards. Follow the recommendations of the adhesive manufacturer. Do not use adhesives without nails, staples, or screws to provide supplemental support.
Adhesives for applying wallboard finishes are classed:
1. Stud adhesives
2. Laminating adhesives:
A. joint compound adhesives
B. drywall contact and modified contact adhesives
Stud adhesives are specifically formulated for attaching single-ply wallboard to steel or wood supports and are used with adhesive/nail-on application. Some stud adhesives are strong enough to permit the use of fewer mechanical fasteners for attaching gypsum board panels to studs; however, panels attached in this fashion do require that mechanical fasteners are installed around their perimeters. Stud adhesives must be of caulking consistency to properly bridge framing irregularities. Use an adhesive that meets the requirements of ASTM C 557, Standard Specification for Adhesives for Fastening Gypsum Wallboard to Wood Framing, for workability, consistency, open-time, wetting characteristics, strength, bridging ability, aging and freeze-thaw resistance. Apply stud adhesive with a caulking gun (Fig. 22)in a continuous or semi-continuous bead. Do not use solvent-based stud adhesives near an open flame or in poorly ventilated areas.
Fig. 22 Image of Applying Adhesive with Gun.
Joint compound, either dry powder setting-type or ready mixed drying type, is designed for laminating gypsum boards to each other or to suitable masonry or concrete surfaces. They are not intended for adhesive attachment of gypsum board panels to steel or wood framing or furring. When using dry powder setting-type compound, mix only as much compound as can be used within the working time specified by the manufacturer. When the use of water for mixing the adhesive compound is necessary, ensure that the water is room temperature and clean enough to drink. Apply compound over the entire board area with a suitable notched spreader, in spaced parallel ribbons, or a pattern of spots as recommended by the manufacturer. All boards attached using joint compound require permanent mechanical fasteners installed around their perimeters. When attaching board vertically on sidewalls, install fasteners at the top and bottom. Laminated face boards may require temporary support or supplemental fasteners until full bond is developed.
Drywall Contact and Modified Contact Adhesives serve to laminate gypsum boards to each other or to steel studs. Gypsum boards applied to walls, ceilings, and soffits using contact adhesives also require, at minimum, permanent mechanical fastener attachment along their perimeters. On ceilings, space fasteners no further apart than 24 inches o.c., regardless of the type adhesive used. Apply the adhesive by roller, spray gun, or brush, in a thin, uniform coating to both surfaces to be bonded.
Most contact adhesives require some drying or setting time before surfaces can be joined and bond can be developed. To ensure proper adhesion between mating surfaces, strike the face board, once in place, over its entire surface with a suitable tool, such as a rubber mallet. No temporary supports are needed while a contact-adhesive sets and the bond forms. One disadvantage of contact adhesives is their inability to fill in irregularities between surfaces, which leaves areas without adhesive bond between the gypsum board and its substrate. Another disadvantage is that most of these adhesives do not permit moving the boards once contact has been made, so they must be carefully applied. Take extra care when working with contact adhesives. Carefully follow the manufacturer’s recommendations.
Modified contact adhesives provide longer placement time. They have an open time of up to 1/2 hour during which the board can be repositioned if necessary. They combine good long-term strength with sufficient immediate bond to permit attachment with a minimum of temporary fasteners. In addition, a modified contact adhesive has bridging ability. Modified contact adhesives are intended for attaching wallboard to all kinds of supporting construction, including solid walls, other gypsum board, and various insulating boards, including some types of rigid foam insulation.
Gypsum board can be laminated directly to above-grade interior masonry and concrete wall surfaces, provided those surfaces are dry, smooth, clean, and flat. Gypsum board can be laminated directly to the interior surface of exterior masonry walls provided the cavities are properly insulated to prevent condensation and the interior surface of the masonry is properly water proofed.
Before applying gypsum board to an interior masonry or concrete surface, ensure that the surface is as level as possible. Remove rough or protruding edges and excess joint mortar; fill depressions with mortar to smooth out the wall surface.
Verify that all surfaces are free of form oils, curing compounds, loose particles, dust, or grease to ensure complete bond. Allow concrete to cure for at least 28 days before laminating gypsum board directly to it.
Install furring and insulation on the interior side of exterior below-grade walls or surfaces and protect with a vapor retarder to provide a suitable base for attaching the gypsum board. Follow this procedure for any similar surface that cannot be readily prepared for direct adhesive lamination.
Use supplemental mechanical fasteners spaced 16 inches o.c. to hold gypsum board in place while adhesive is developing bond. Several manufacturers offer furring systems that employ clips, runners, and adjustable brackets to make the installation of gypsum board over irregular masonry walls possible (Fig.23). Carefully follow the manufacturer’s instructions when using special clips in a furring system.
Fig. 23 Examples of Furring Systems Designed for Installing Gypsum Wallboard over Masonry Walls.