Drywall construction may perhaps be best defined by what it's not—it is any method of finishing the interior walls and ceilings of a house that's not traditional plastering.
A plastered wall has a great many virtues. It provides a smooth, even surface that's hard and strong, that can breathe, that takes paint and wallpaper extremely well, that's sound-resistant and very fire-resistant, and that can be patched easily if it should crack or be damaged. But, for a builder, plastering has many disadvantages. It makes for a wet, messy job that takes a considerable amount of time between application and final drying, which slows down the overall pace of the construction; the general dampness produced by the wet plaster may cause studs and joists to swell, which may result in the dried plaster’s cracking when the wood finally shrinks back down again; plastering is especially difficult and time-consuming in winter because drying times are longer and the house must be fully enclosed and heated before the plastering can begin—freezing plaster is weak plaster; and , finally, plastering is expensive, a cost that's , of course, borne ultimately by the homeowner.
So drywall construction was invented. Drywall construction may be thought of as a kind of portable gypsum-plaster wall that's manufactured elsewhere in the form of large, paper- covered panels. These panels are carried to the jobsite where they are nailed or screwed (or nailed and screwed), and sometimes glued to the studs and joists as well, without any of the messiness or bother attendant upon a wet plastering job, though drywall does have faults that will be described in the course of this article.
By a natural extension of meaning, drywall has come to mean the installation of materials other than gypsum wall- boards to finish a room, including plywood, wood paneling, fiberboard or insulation board, hardboard, particleboard, asbestos-cement panels, and so on. For the purposes of this article, however, drywall construction—which is also known as plasterboard construction—will mean a wall constructed of gypsum wallboard. For a description of the manufacture of the panels themselves as well as the range of sizes and types that are available, see GYPSUM PANELS.
The gypsum wallboards used in drywall construction are usually 4 x 8 ft in size, though panels from 6 to 16 ft long are available. The panels are made in thicknesses that range from 1/4 in. to 5/8 in., with thicker panels available for use in special construction.
The ¼ in. panels are used mainly for remodeling over an existing wall or ceiling, when for some reason the wall or ceiling no longer provides a satisfactory surface. The ¼ in. thick panels may also be used to form a curved surface—to form an arched doorway, for example—for which the thicker panels are too rigid. Other than this, however, 1/4-in, panels are never used for an original wall construction.
The 3/8-in.-thick panels are also used for repair and remodeling jobs and for double-ply construction, as described below. The ½-in. panels are the size most often used in new construction, and the ½-in.-thick panels are used where a more solid wall is needed or where a more fire-resistant construction is desired. The most important determinants of which thickness panel should be used are, in fact, the stud and joist spacing in wood-framed houses and whether the panels are attached parallel or crosswise to the studs and joists.
The panels have tapered edges on their long sides that allow them to he joined together without the joints showing using the technique described below.
The gypsum core is protected against moisture and minor injuries by a layer of hard, smooth-surfaced paper, which also provides a base to which paint and wallpaper can be applied. (There are also available gypsum wallboards that are used as lathing, to which plaster is applied; these wallboards are covered by a porous, rough-textured paper that offers a good grip to the plaster. These wallboards should never be used for ordinary drywall construction in which the surfaces of the panels remained exposed.)
Before the panels are installed, certain preliminary matters must be taken care of. For one thing, the builder must make certain that the studs and joists provide a flat plane to which the panels can be attached. In a plastered wall, by contrast, the plasterer need not be as concerned how wavy or uneven the supporting surface is, within limits, of course. It is the plasterer’s job to make the surface of the plaster flat no matter what the support is like. In drywall construction, however, the panels are relatively stiff and inflexible. If the supporting studs and joists aren't aligned with each other, the panels will not be aligned with each other, and in a gypsum-wallboard wall nothing looks worse than the wall’s weaving in and out every four feet—unless it's having the joints between panels clearly visible.
Unevenly installed studs and joists may have to be reinstalled. Warped or bowed studs and joists may have to be replaced, or forced into alignment by the installation of special bracing cut into the lumber. Another technique that's used to achieve a flat supporting surface is to nail long strips of wood, suitably wedged to align and level them, to the edges of the studs and bottoms of the joists.
Joists are less likely to bow or warp because of their larger size, but when they are out of shape they are more difficult to straighten. Furring the joists with 1 x 3 in. lumber may be necessary. Sometimes, when the tops of the joists form the floor of an unfinished attic, out-of-line joists can be forced into alignment by the installation of a strong back, as shown in Fig. 1.
Another thing the builder must do beforehand, even before the panels have been ordered, is make a careful to-scale layout of the walls and ceilings, including the positions of the doors and windows in the walls. The purpose of this layout is to discover how best to install the panels to minimize both the number of visible joints and the amount of cutting and fitting that will be required. Making an advance layout will probably reduce the overall cost of the installation as well.
The panels can be installed on the walls either vertically or horizontally (refer to Fig. 2). When only one ply of wallboard is being installed, the preferred direction of installation is with the long sides of the panels horizontal. This construction is stronger, since a horizontal panel ties more studs together and , in addition, as much as 25 % less jointing is necessary between panels. Horizontal joints are also less obvious to the eye than vertical joints.
These factors also apply to the ceiling panels. The preferred direction of panel installation is crosswise to the direction of the joists. If this also happens to be the long dimension of the ceiling, good. If not, the builder will have to choose between installing the panels crosswise or parallel to the direction of the joists. On the whole, it's preferred that the long sides of the panels run crosswise to the joists for the sake of the stronger construction.
As much as possible, single panels should run from one corner of a room to another. Since panels 16 ft long are available, this should be possible for most of the walls in a house. If not, the builder will have to calculate where best to make the breaks or whether to install the panels vertically. The least conspicuous breaks are over the centers of doors and windows.
A layout will also enable the builder to locate the places in the walls and ceilings where he will have to install 2 X 4 in. blocking, since all the edges of all the panels must have a solid backing into which the fastenings (i.e., nails, screws, or staples) can be driven and /or adhesive applied.
There are two general methods of installing gypsum wall- boards. One method, called single-ply construction, consists of attaching a single layer of wallboard to the walls and ceilings. The other method, called two-ply or laminated dry wall construction, consists of attaching two layers of wallboard to the walls and ceilings. Two-ply construction results in a wall that's much stronger and more resistant to any structural stresses that might crack or crush the panels of a single-ply installation, It is also a more fire- and sound-resistant construction than single-ply, and it enables the installers to lay a flatter wall surface, which gives the finishers a better chance to disguise the joints between panels. On the other hand, two-ply construction is twice as expensive to install as single-ply construction.
In single-ply construction the panels are first installed on the ceiling. The panels should butt loosely against each other and against the adjacent wall studs. The panels should never be jammed uptight against each other or against the studs as they tend to swell slightly when they become damp, which may cause the drywall to crack or buckle.
Once the ceiling panels are in place, the wall panels can be installed. If these panels are being installed horizontally, the row of panels adjacent to the ceiling panels should be installed first. All the panels can either be nailed or screwed into place, or a combination of these fastenings plus adhesive can be used.
Nailing and Nail Popping
Nailing is the usual method of attaching panels; the spacing between nails on both ceilings and walls is shown in Fig. 3. One should always nail from the centers of the panels out ward, leaving the edges that abut the wall and ceiling junctures unnailed. This is thought to provide a slight amount of give to the construction, in case the framing should settle or shrink, that will help prevent stressing the panels.
The nailing-on of gypsum wallboards has had an unforeseen and unfortunate consequence—the phenomenon of nail pop ping. Assume a gypsum panel has been nailed in place. As the wood to which the panel is attached dries out, it shrinks, which is normal enough. But as the wood shrinks down, the nails stay where they are in relation to the central axis of the wood. Consequently, the nails will appear to have popped out of the wood by 1/16 to 1/8 in. (If the panel should follow the popped nails away from the wood, this will result in bulges appearing here and there instead of nail popping.)
Nail popping is likely to occur whenever the wood framing has an inordinate amount of moisture in it, which is, roughly speaking, any moisture content in excess of 19 % . To reduce the probability of nail pops, therefore, the builder must either order kiln-dried lumber, which will increase the cost of the construction, or he must order the lumber long enough in advance of construction to enable him to store the lumber under suitably dry conditions until it does dry out to a moisture content of 19 % or less. Or he can put up the wood framing and then go away until the framing has had a chance to dry out. Or he can install the panels using one of the adhesive techniques described below, which does away with the need for most of the nails altogether.
In this connection, it should also be remembered that, in most sections of the country, the structural framework of a house will stabilize at a moisture content of around 10 % after the house has been occupied for a while, so even the 19 % moisture Content that's considered a minimum to prevent nail popping means the wood will still shrink down a bit after the house has been completed.
Manufacturers of gypsum products have spent considerable time and money investigating the phenomenon of nail popping and one of the conclusions that has been reached is that the less distance a nail is driven into wood, the less it will protrude when the wood does shrink down. It follows, therefore, that the shorter the nail, the better. As a consequence, special ringshank nails 1/8 in. shorter than ordinary finishing nails but having an equal holding ability are recommended for use with gypsum wallboards.
The nails should always be driven slightly below the surface of the panels. This requires the use of a hammer having a crown head, so that when a nail is struck that one last blow, it will be driven below the surface of the panel and a small indentation, or dimple, about 1/32 in. deep will be left in the surface. These dimples will be covered later with joint com pound to hide the nails.
Other Kinds of Fastenings
A contractor specializing in the installation of gypsum wall- boards will use power screwdrivers to drive special Phillips- head screws that secure the panels in place. These screws are double-threaded and have a diamond point. They are reputed to hold the panels more firmly against the support than nails do. The labor output is also higher and the screws do reduce the amount of nail popping.
Staples aren't recommended for single-ply construction, as the panels aren't as securely fastened as when they are nailed or screwed in place.
Special adhesives compounded by manufacturers of gypsum wallboards can also be used to attach the panels to wood framing. These adhesives are of the rubber-latex type, and they are applied to the studs and joints with a caulking gun as a continuous ribbon. Use of an adhesive does not eliminate the necessity for nails or screws; it merely reduces the total quality of these fastenings required. The idea behind using an adhesive is that the fewer the fastenings, the less chance of nail pops appearing. When an adhesive is used, therefore, the nail spacing need be only 12 in. on ceilings and 16 in. on walls, while screw spacing may be reduced to 16 in. on ceilings and 24 in. on walls.
Double nailing is also practiced. The spacing between nails is also shown in Fig. 3. The first set of nails must be completely installed before the second set is installed. Double nailing does not stop or reduce nail popping. What it does is secure the panels more firmly to the wood, which of course makes the construction that much stronger.
All joints, including those in the corners of a room, are covered by joint compound, which is spread over the tapered edges of the panels with a 5 to 6-in.-wide flexible broad knife (see Fig. 4). Joint compound is a gypsum product formulated by wall board manufacturers for this purpose, and it can be purchased either ready-mixed and ready for application or as a dry powder.
A strip of paper tape is used to reinforce and strengthen the joint. The tape is 2 to 2¼ in. wide and it's usually purchased in 250-ft-long rolls. A thin layer of joint compound (approximately 1/32 in. thick) is then spread over the tape, the edges of the cement being feathered out smoothly. The joint compound is then allowed to dry for 24 hr.
Ceiling joints are taped first, then the wall Corners, then the joints between wall panels, and finally, the corners between ceiling and wall panels. The corners are taped using the same general technique described above for ordinary joints; how ever, with interior corners, the tape must first be folded and creased before it's pressed into the compound. Special V-shaped broadknives are available that enable the plasterer to spread the compound evenly and firmly into corners.
Exterior corners, and all other cut or otherwise unprotected edges of the panels (such as at the edges of door and window openings) are first covered by metal cornerbead trim especially manufactured for the purpose (see Fig. 5). Once the trim has been installed, the exterior corners can be treated as shown in Fig. 4.
As the joint compound dries, it will shrink slightly. The joint is then sanded smooth and a second thin layer of the com pound is applied over the first. This second layer should extend beyond the edges of the first layer by about 1 in., with the edges again being feathered out smoothly. Wallboard manufacturers recommend the use of a premixed finish compound (or topping compound) for this second layer. The finish com pound has a more buttery consistency than joint compound, and it spreads very smoothly. After it has dried, it has excellent sanding properties. The finish compound should be allowed to dry for 24 hr before it's sanded.
If the walls are to receive a paint finish that has a texture to it, the joints can be considered completed. But if the wall is to be papered, or if a glossy coat of paint is to be applied, which will show up the slightest unevenness in the joints, another thin layer of finish compound should be applied to the joints. This third layer should extend beyond the edges of the second layer by another 1½ in. and it, too, should be feathered out smoothly. The third layer should also be allowed to dry for 24 hr before it's lightly sanded. The joints may now be considered finished.
At the same time that the first layer or finish joint compound is being applied over the joints, the nail indentations mentioned above should also be filled with finish compound. Two coats of compound should be applied to all the indentations, with each of the coats being allowed to dry for a 24-hr period and each coat being sanded smooth after it has dried.
An alternative to cementing and taping the joints at the ceiling lines is the installation of wood cornices that will hide the juncture, in the same way that a baseboard hides the juncture between wall and floor. A variety of moldings is available, and cornices do enhance the appearance of a room just as a baseboard does.
In all essential points, the installation of two plies of wallboard is identical to the installation of only a single ply. The first ply of a two-ply installation is installed in exactly the same way as already described for single-ply construction, except that it's not necessary to treat the joints, and staples may be used to attach the panels. When laying out the panels for the second ply, however, the edges must be offset at least 10 in. from the edges of the first ply. The plies should also run crosswise to each other, if possible, since this makes the construction much stronger. Thus, if for appearance’s sake the second ply is to be installed horizontally, the first ply should be installed vertically—though there is no law that says so. If convenience dictates that both plies must run vertically, or horizontally, so be it.
Somewhat greater freedom is allowed in the placement of the second-ply panels if the second layer is laminated to the first since the installer does not have to be concerned about ending the panels along studs and joists. The second ply, of course, will be adequately supported by the first ply.
Most domestic two-ply installations use 3/8 in. wallboards for both plies, but any combination of 3/8-in., and 1/2.in thick panels is satisfactory. The thicker the panels, the stronger the walls will be, the greater the fire resistance, and the better the sound resistance.
The plies may be attached to the studs and joists and to each other in a number of different ways. Both plies can be nailed or screwed in place, with both plies having the same nail or screw spacing specified above for single-ply installation. The only difference between the two is that the fasteners used for the second ply must be long enough to penetrate into the wood the same distance as the fastenings of the first ply. Or a combination of fastener and adhesive can be used for the second ply as already described for the first ply.
But the most common method of fastening the two plies in place is to nail the first ply (as already described) and then fasten the second ply to the first ply with an adhesive. The adhesive most commonly used is joint compound, and it's applied to the back of the second ply with a special trowel that has 1/4 x 1/4 in. notches located 2 in. apart. After the adhesive has been applied to a panel, the panel is placed against the first ply and held there temporarily by nails until the adhesive sets. The nails are then removed and the nail holes filled with finish compound.
The joints between the panels of the second ply are treated as already described for a single-ply installation.
DRYWALL FOR MASONRY WALLS
Gypsum wallboards can also be used to cover masonry, that's , concrete, brick, or cement-block walls. The simplest, though not the preferred, technique is simply to nail the wallboards directly to the masonry surface, assuming the wall is above ground and the climate is temperate. This can be done most easily against a concrete wall because of the smooth surface usually offered by concrete, but even with concrete it will probably be necessary to remove projections with a hammer and chisel before the panels are installed. On any exterior masonry wall, nails driven directly into the masonry very often provide what are called cold bridges that will transmit cold outside temperatures into the building. The nails, being colder than the interior face of the wall, will enable moisture within the building to condense on the nail heads, with the result that the wallboards will become discolored, as will any paint or other finish that's applied over them.
For these reasons, it's more sensible to apply furring to the wall and attach the wallboards to the furring. Furring consists of 1 x 2 in. or 1 X 3 in. strips of wood nailed to the masonry. The wallboards are then nailed, screwed, or glued to these furring strips in the same way that has already been described for single-ply construction. This method of providing a suitable support for the wallboards has the advantage that the furring strips can be carefully plumbed and aligned with each other by placing wedges under the strips before the wallboards are installed, which means the wallboards will present a flat and even appearance.
Another advantage in using furring is that an air space is created between the masonry and the wallboards that acts as insulation and prevents dampness reaching the wallboards. If, in addition, aluminum-foil-backed wallboards are installed, the insulating value of the air space will be further increased (see INSULATION).