DRYWALL AND OTHER COVERINGS
Though lath-and-plaster finish is still used in building construction today, drywall finish has become the most popular. Drywall finish saves time in construction, whereas plaster finish requires drying time before other interior work can be started. Drywall finish requires only short drying time since little, if any, water is required for application. However, a gypsum drywall demands a moderately low moisture content of the framing members to prevent "nail-pops." Nail-pops result when frame members dry out to moisture equilibrium, causing the nailhead to form small "humps" on the surface of the board. Stud alignment is also important for single-layer gypsum finish to prevent a wavy, uneven appearance. Thus, there are advantages to both plaster and gypsum drywall finishes and each should be considered along with the initial cost and maintenance.
There are many types of drywall. One of the most widely used is gypsum board in 4- by 8-foot sheets. Gypsum board is also available in lengths up to 16 feet. These lengths are used in horizontal application. Plywood, hardboard, fiberboard, particleboard, wood paneling, and similar types are also used. Many of these drywall finishes come prefinished.
The use of thin sheet materials, such as gypsum board or plywood, requires that studs and ceiling joists have good alignment to provide a smooth, even surface. Wood sheathing often corrects misaligned studs on exterior walls. A strongback (fig. 5-1) provides for alignment of ceiling joists of unfinished attics. It can also be used at the center of a span when ceiling joists are uneven.
Figure 5-1.Strongback for alignment of ceiling joists.
Gypsum wallboard is the most commonly used wall and ceiling covering in construction today. Because gypsum is nonflammable and durable, it is appropriate for application inmost building types. Sheets of drywall are nailed or screwed into place, and nail indentions or "dimples" are filled with joint compound. Joints between adjoining sheets are built up with special tape and several layers (usually three) of joint compound. Drywall is easily installed, though joint work can be tedious.
Drywall varies in composition, thickness, and edge shape. The most common sizes with tapered edges are 1/2 inch by 4 feet by 8 feet and 1/2 inch by 4 feet by 12 feet.
Regular gypsum board is commonly used on walls and ceilings and is available in various thicknesses. The most common thicknesses are 1/2 inch and 5/8 inch. Type X gypsum board has special additives that make it fire resistant.
MR (moisture resistant) or WR (water resistant) board is also called greenboard and blueboard. Being water resist ant, this board is appropriate for bathrooms, laundries, and similar areas with high moisture. It also provides a suitable base for embedding tiles in mastic. MR or WR board is commonly 1/2 inch thick.
Sound-deadening board is a sublayer used with other layers of drywall (usually type X); this board is often 1/4 inch thick
Backing board has a gray paper lining on both sides. It is used as a base sheet on multilayer applications. Backing board is not suited for finishing and decorating.
Foil-backed board serves as a vapor barrier on exterior walls. This board is available in various thicknesses.
Vinyl-surfaced board is available in a variety of colors. It is attached with special drywall finish nails and is left exposed with no joint treatment.
Plasterboard or gypsum lath is used for plaster base. It is available in thickness starting at 3/8 inch, widths 16 and 24 inches, and length is usually 48 inches. Because it comes in manageable sizes, its widely used as a plaster base instead of metal or wood lath for both new construction and renovation. This material is not compatible with portland cement plaster.
The varying lengths of drywall allow you to lay out sheets so that the number of seams is kept to a minimum, End points can be a problem, however, since the ends of the sheets arent shaped (only the sides are). As sheet length increases, so does weight, unwieldiness, and the need for helpers. Standard lengths are 8, 9, 10, 12, and 14 feet. Sixteen-foot lengths are also available. Use the thickness that is right for the job. One-half-inch drywall is the dimension most commonly used. That thickness, which is more than adequate for studs 16 inches on center (OC), is also considered adequate where studs are 24 inches OC. Where ceiling joists are 16 inches OC, use 1/2-inch drywall, whether it runs parallel or perpendicular to joists. Where ceiling joists are 24 inches OC, though, use 1/2-inch drywall only if the sheets are perpendicular to joists.
Drywall of 1/4- and 3/8-inch thicknesses is used effectively in renovation to cover existing finish walls with minor irregularities. Neither is adequate as a single layer for walls or ceiling, however. Two 1/4-inch-thick plies are also used to wrap curving walls.
Drywall of 5/8-inch thickness is favored for quality single-layer walls, especially where studs are 24 inches OC. Use 5/8-inch drywall for ceiling joists 24 inches OC, where sheets run parallel to joists. This thickness is widely used in multiple, fire-resistant combinations.
There are several types of edging in common use. Tapered allows joint tape to be bedded and built up to a flat surface. This is the most common edge used. Tapered round is a variation on the first type. Tapered round edges allow better joints. These edges are more easily damaged, however. Square makes an acceptable exposed edge. Beveled has an edge that, when left untapped, gives a paneled look.
Commonly used tools in drywall application include a tape measure, chalk line, level, utility and drywall knives, straightedge, and a 48-inch T square (drywall square) or framing square. Other basic tools include a keyhole saw, drywall hammer (or convex head hammer), screw gun, drywall trowel, comer trowel, and a foot lift. Some of these tools are shown in figure 5-2.
Figure 5-2.Common tools for drywall installation.
The tape measure, chalk line, and level are used for layout work. The utility and drywall knives, straightedge, and squares are used for scoring and breaking drywall. The keyhole saw is used for cutting irregular shapes and openings, such as outlet box openings. A convex head or drywall, hammer used for drywall nails will "dimple" the material without tearing the paper. The screw gun quickly sinks drywall screws to the adjusted depth and then automatically disengages.
Drywall knives have a variety of uses. The 6-inch knife is used to bed the tape in the first layer of joint compound and for filling nail or screw dimples. The 12-inch finishing knife "feathers out" the second layer of joint compound and is usually adequate for the third or "topping" layer. Knives 16 inches and wider are used for applying the topping coat. Clean and dry drywall knives after use. Use only the drywall knives for the purpose intended-to finish drywall.
The drywall trowel resembles a concrete finishing trowel and is manufactured with a 3/16-inch concave bow. This trowel, also referred to as a "flaring," "feathering," or "bow" trowel, is used when applying the finish layer of joint compound. A comer trowel is almost indispensable for making clean interior comers.
For sanding dried joint compound smooth, use 220 grit sandpaper. Sandpaper should be wrapped around a sanding block or can be used on an orbital sander. When sanding, ensure youre wearing the required personnel protective gear to prevent dust inhalation.
A foot lift helps you raise and lower drywall sheets while you plumb the edges. Be careful when using the foot liftapplying too much pressure to the lift can easily damage the drywall.
Which fasteners you use depends in part upon the material underneath. The framing is usually wood or metal studs, although gypsum is occasionally used as a base. Adhesives are normally used in tandem with screws or nails. This allows the installer to use fewer screws or nails, leaving fewer holes that require filling. For reasons noted shortly, youll find the drywall screw the most versatile fastener for attaching drywall to framing members.
Figure 5-3.Drywall fasteners.
A number of metal accessories have been developed to finish off or protect drywall. corner beads (fig. 5-4) are used on all exposed comers to ensure a clean finish and to protect the drywall from edge damage. corner bead is nailed or screwed every 5 inches through the drywall and into the framing members. Be sure the corner bead stays plumb as you fasten it in place. Casing beads (fig. 5-4), also called stop beads, are used where drywall sheets abut at wall intersections, wall and exposed ceiling intersections, or where otherwise specified. Casing beads are matched to the thickness of the drywall used.
Figure 5-4.Corner and casing beads.
When laying out a drywall job, keep in mind that each joint will require taping and sanding. You therefore should arrange the sheets so that there will be a minimum of joint work. Choose drywall boards of the maximum practical length.
Drywall can be hung with its length either parallel or perpendicular to joists or studs. Although both arrangements work sheets running perpendicular afford better attachment. In double-ply installation, run base sheets parallel and top sheets perpendicular. For walls, the height of the ceiling is an important factor. When ceilings are 8 feet 1 inch high or less, run wall sheets horizontally. Where they are higher, run wall sheets vertically, as shown in figure 5-5.
Figure 5-5.Single-layer application of drywall.
The sides of drywall taper, but the ends dont, so there are some layout constraints. End joints must be staggered where they occur. Such joints are difficult to feather out correctly. Where drywall is hung vertically, avoid side joints within 6 inches of the outside edges of doors or windows. In the case of windows, the bevel on the side of the drywall interferes with the finish trim, and the bevel may be visible. To avoid this difficulty, lay out vertical joints so they meet over a cripple (shortened) stud toward the middle of a door or window opening.
When installing drywall horizontally and an impact-resistant joint is required, you should use nailing blocks (fig. 5-5).
There are several things you can do to make working with drywall easier.
Figure 5-6.Cutting gypsum drywall
When attaching drywall, hold it firmly against the framing to avoid nail-pops and other weak spots. Nails or screws must fasten securely in a framing member. If a nail misses the framing, pull it out, dimple the hole, and fill it in with compound; then try again. If you drive a nail in so deep that the drywall is crushed, drive in another reinforcing nail within 2 inches of the first.
When attaching drywall sheets, nail (or screw) from the center of the sheet outward. Where you double-nail sheets, single nail the entire sheet first and then add the second (double) nails, again beginning in the middle of the sheet and working outward.
Figure 5-7.Spacing for single and double nailing of gypsum drywall.
Figure 5-8.Dimpling of gypsum drywall.
Make sure the first sheet on a wall is plumb and its leading edge is centered over a stud. Then, all you have to do is align successive sheets with the first sheet. The foot lift shown earlier in figure 5-2 is useful for raising or lowering a sheet while you level its edge. After you ve sunk two or three screws or nails, the sheet will stay in place. A gap of 1/2 inch or so along the bottom of a sheet is not critical; it is easily covered by finish flooring, baseboards, and soon. If you favor a clean, modem line without trim, manufactured metal or vinyl edges (casing beads) are available for finishing the edges.
During renovation, you may find that hanging sheets horizontally makes sense. Because studs in older buildings often are not on regular centers, the joints of vertical sheets frequently do not align with the studs. Again, using the foot lift, level the top edge of the bottom sheet. Where studs are irregular, its even more important that you note positions and chalk line stud centers onto the drywall face before hanging the sheet.
Applying drywall in older buildings yields a lot of waste because framing is not always standardized. Use the cutoffs in such out-of-the-way places as closets. Dont piece together small sections in areas where youll notice seams. Never assume that ceilings are square with walls. Always measure from at least two points, and cut accordingly.
Drywall is quite good for creating or covering curved walls. For the best results, use two layers or 1/4-inch drywall, hung horizontally. The framing members of the curve should be placed at intervals of no more than 16 inches OC; 12 inches is better. For an 8-foot sheet applied horizontally, an arc depth of 2 to 3 feet should be no problem, but do check the manufacturers specifications. Sharper curves may require backcutting (scoring slots into the back so that the sheet can be bent easily) or wetting (wet-sponging the front and back of the sheet to soften the gypsum). Results are not always predictable, though. When applying the second layer of 1/4-inch drywall, stagger the vertical butt joints.
The finishing of gypsum board drywall is generally a three-coat application. Attention to drying times between coats prevents rework that has a cost involved as well as extra time.
Where sheets of drywall join, the joints are covered with joint tape and compound (fig. 5-9). The procedure is straightforward.
Figure 5-9.Finishing drywall joints.
When finishing an inside corner (fig. 5-10), cut your tape the length of the corner angle you are going to finish. Apply the joint compound with a 4-inch knife evenly about 2 inches on each side of the angle. Use sufficient compound to embed the tape. Fold the tape along the center crease (view A) and firmly press it into the corner. Use enough pressure to squeeze some compound under the edges. Feather the compound 2 inches from the edge of the tape (view B). When the first coat is dry, apply a second coat. A corner trowel (view C) is almost indispensable for taping comers. Feather the edges of the compound 1 1/2 inches beyond the first coat. Apply a third coat if necessary, let it dry, and sand it to a smooth surface. Use as little compound as possible at the apex of the angle to prevent hairline cracking. When molding is installed between the wall and ceiling intersection, it is not necessary to tape the joint (view D).
Figure 5-10.Finishing an inside corner.
When finishing an outside corner (fig. 5-1 1), be sure the corner bead is attached firmly. Using a 4-inch finishing knife, spread the joint compound 3 to 4 inches wide from the nose of the bead, covering the metal edges. When the compound is completely dry, sand lightly and apply a second coat, feathering edges 2 to 3 inches beyond the first coat. A third coat maybe needed, depending on your coverage. Feather the edges of each coat 2 or 3 inches beyond each preceding coat. Corner beads are no problem if you apply compound with care and scrape the excess clean. Nail holes and screw holes usually can be covered in two passes, though shrinkage sometimes necessitates three. A tool that works well for sanding hard-to-reach places is a sanding block on an extension pole; the block has a swivel-head joint.
Figure 5-11.Finishing an outside corner.
To give yourself the greatest number of decorating options in the future, paint the finished drywall surface with a coat of flat oil-base primer. Whether you intend to wallpaper or paint with latex, oil-base primer adheres best to the facing of the paper and seals it.
Renovation and Repair
For the best results, drywall should be flat against the surface to which it is being attached. How flat the nailing surface must be depends upon the desired finish effect. Smooth painted surfaces with spotlights on them require as nearly flawless a finish as you can attain. Similarly, delicate wall coverings-particularly those with close, regular patternsaccentuate pocks and lumps underneath. Textured surfaces are much more forgiving. In general, if adjacent nailing elements (studs, and so forth) vary by more than 1/4 inch, buildup low spots. Essentially, there are three ways to create a flat nailing surface:
Frame out a new wall-a radical solution. If the studs of partition walls are buckled and warped, its often easier to rip the walls out and replace them. Where the irregular surface is a load-bearing wall, it maybe easier to build a new wall within the old.
Cover imperfections with a layer of 3/8-inch drywall. This thickness is flexible yet strong. Drywall of 1/4-inch thickness may suffice. Single-ply cover-up is a common renovation strategy where existing walls are ungainly but basically flat. Locate studs beforehand and use screws long enough to penetrate studs and joists at least 5/8 inch.
Build up the surface by "furring out." In the "furring- out" procedure, furring-strips 1 by 2 inches are used. Some drywall manufacturers, however, consider that size too light for attachment, favoring instead a nominal size of 2 by 2 inches. Whatever size strips you use, make sure they (and the shims underneath) are anchored solidly to the wall behind.
By stretching strings taut between diagonal comers, you can get a quick idea of any irregularities in a wall. If studs are exposed, further assess the situation with a level held against a straight 2 by 4. Hold the straightedge plumb in front of each stud and mark low spots every 12 inches or so. Using a builders crayon, write the depth of each low spot, relative to the straightedge, on the stud. If studs arent exposed, locate each stud by test drilling and inserting a bent coat hanger into the hole. Chalk line the center of each stud on the existing surface. Here too, mark the depth of low spots.
The objective of this process is a flat plane of furring strips over existing studs. Tack the strips in place and add shims (wood shingles are best) at each low spot marked (see fig. 5-12). To make sure a furring strip doesnt skew, use two shims, with their thin ends reversed, at each point. Tack the shims in place and plumb the furring strips again. When you are satisfied, drive the nails or screws all the way in.
Figure 5-12.Furring strips hacked with shims.
When attaching the finish sheets, use screws or nails long enough to penetrate through furring strips and into the studs behind. Strips directly over studs ensure the strongest attachment. Where finish materials are not sheetsfor example, single-board vertical paneling furring should run perpendicular to the studs.
Regardless of type, finish material must be backed firmly at all nailing pints, corners, and seams. Where you cover existing finish surfaces or otherwise alter the thickness of walls, its usually necessary to build up existing trim. Figure 5-13 shows how this might be done for a window casing. Electrical boxes must also be extended with box extensions or plaster rings.
Figure 5-13.Building up an intertor window casing.
Masonry surfaces must be smooth, clean, and dry. Where the walls are below grade, apply a vapor barrier of polyethylene (use mastic to attach it) and install the furring strips. Use a power-actuated nail gun to attach strips to the masonry. Follow all safety procedures. If you hand nail, drive case-hardened nails into the mortar joints. Wear goggles; these nails can fragment.
Most drywall blemishes are caused by structural shifting or water damage. Correct any underlying problems before attacking the symptoms.
Popped-up nails are easily fixed by pulling them out or by dimpling them with a hammer. Test the entire wall for springiness and add roils or screws where needed. Within 2 inches of a popped-up nail, drive in another nail. Spackle both when the spots are dry, then sand and prime.
To repair cracks in drywall, cut back the edges of the crack slightly to remove any crumbly gypsum and to provide a good depression for a new filling of joint compound. Feather the edges of the compound. When dry, sand and prime them.
When a piece of drywall tape lifts, gently pull until the piece rips free from the part thats still well stuck. Sand the area affected and apply anew bed of compound for a replacement piece of tape. The self-sticking tape mentioned earlier works well here. Feather all edges.
If a sharp object has dented the drywall, merely sand around the cavity and fill it with spackling compound. A larger hole (bigger than your fist) should have a backing. One repair method is shown in figure 5-14. First, cut the edges of the hole clean with a utility knife (view A). The piece of backing should be somewhat larger than the hole itself. Drill a small hole into the middle of the backing piece and thread a piece of wire into the hole. This wire allows you to hold the piece of backing in place. Spread mastic around the edges of the backing. When the adhesive is tacky, fit the backing diagonally into the hole (view B) and, holding onto the wire, pull the piece against the back side of the hole. When the mastic is dry, push the wire back into the wall cavity. The backing stays in place. Now, fill the hole with plaster or joint compound (view C) and finish (view D). (Note: This is just one of several options available for repairing large surface damage to gypsum board.)
Figure 5-14.Repairing a large hole In drywall.
Compound sags in holes that are too big. If it happens, mastic a replacement piece of drywall to the backing piece. To avoid a bulge around the filled-in hole, feather the compound approximately 16 inches, or more. If the original drywall is 1/2 inch thick use 3/8-inch plasterboard as a replacement on the backing piece.
Holes larger than 8 inches should be cut back to the centers of the nearest studs. Although you should have no problem nailing a replacement piece to the studs, the top and the bottom of the new piece must be backed. The best way to install backing is to screw drywall gussets (supports) to the back of the existing drywall. Then, put the replacement piece in the hole and screw it to the gussets.
Most of the plywood used for interior walls has a factory-applied finish that is tough and durable. Manufacturers can furnish prefinished matching trim and molding that is also easy to apply. Color-coordinated putty sticks are used to conceal nail holes.
Joints between plywood sheets can be treated in a number of ways. Some panels are fabricated with machine-shaped edges that permit almost perfect joint concealment. Usually, it is easier to accentuate the joints with grooves or use battens and strips. Some of the many different styles of battens are shown in figure 5-15.
Figure 5-15.Battens used for paneling joints.
Before installation, the panels should become adjusted (conditioned) to the temperature and humidity of the room. Carefully remove prefinished plywood from cartons and stack it horizontally. Place 1-inch spacer strips between each pair of face-to-face panels. Do this at least 48 hours before application.
Plan the layout carefully to reduce the amount of cutting and the number of joints. It is important to align panels with openings whenever possible. If finished panels are to have a grain, stand the panels around the walls and shift them until you have the most pleasing effect in color and grain patterns. To avoid mix-ups, number the panels in sequence after their position has been established.
When cutting plywood panels with a portable saw, mark the layout on the back side. Support the panel carefully and check for clearance below. Cut with the saw blade upward against the panel face. This minimizes splintering. This procedure is even more important when working with prefinished panels.
Plywood can be attached directly to the wall studs with nails or special adhesives. Use 3/8-inch plywood for this type of installation. When studs are poorly aligned or when the installation is made over an existing surface in poor condition, it is usually advisable to use furring. Nail 1- by 3- or 1- by 4-inch furring strips horizontally across the studs. Start at the floor line and continue up the wall. Spacing depends on the panel thickness. Thin panels need more support. Install vertical strips every 4 feet to support panel edges. Level uneven areas by shimmying behind the furring strips. Prefinished plywood panels can be installed with special adhesive. The adhesive is applied and the panels are simply pressed into place; no sustained pressure is required.
Begin installing panels at a corner. Scribe and trim the edges of the first panel so it is plumb. Fasten it in place before fitting the next panel. Allow approximately 1/4-inch clearance at the top and bottom. After all panels are in place, use molding to cover the space along the ceiling. Use baseboards to conceal the space at the floor line. If the molding strips, baseboards, and strips used to conceal panel joints are not prefinished, they should be spray painted or stained a color close to the tones in the paneling before installation.
On some jobs, 1/4-inch plywood is installed over a base of 1/2-inch gypsum wallboard. This backing is recommended for several reasons. It tends to bring the studs into alignment. It provides a rigid finished surface. And, it improves the fire-resistant qualities of the wall. (The plywood is bonded to the gypsum board with a compatible adhesive.)
Through special processing, hardboard (also called fiberboard) can be fabricated with a very low moisture absorption rate. This type is often scored to form a tile pattern. Panels for wall application are usually 1/4 inch thick.
Since hardboard is made from wood fibers, the panels expand and contract slightly with changes in humidity. They should be installed when they are at their maximum size. The panels tend to buckle between the studs or attachment points if installed when moisture content is low. Manufacturers of prefinished hardboard panels recommend that they be unwrapped and placed separately around the room for at least 48 hours before application.
Procedures and attachment methods for hardboard are similar to those for plywood. Special adhesives are available as well as metal or plastic molding in matching colors. You should probably drill nail holes for the harder types.
Plastic laminates are sheets of synthetic material that are hard, smooth, and highly resistant to scratching and wear. Although basically designed for table and countertops, they are also used for wainscoting and wall paneling in buildings.
Since plastic laminate material is thin (1/32 to 1/16 inch), it must be bonded to other supporting panels. Contact bond cement is commonly used for this purpose. Manufacturers have recently developed prefabricated panels with the plastic laminate already bonded to a base or backer material. This base consists of a 1/32-inch plastic laminate mounted on 3/8-inch particleboard. Edges are tongue and grooved so that units can be blind-nailed into place. Various matching corner and trim moldings are available.
SOLID LUMBER PANELING
Solid wood paneling makes a durable and attractive interior wall surface and may be appropriately used in nearly any type of room. Several species of hardwood and softwood are available. Sometimes, grades with numerous knots are used to obtain a special appearance. Defects, such as the deep fissures in pecky cypress, can also provide a dramatic effect.
The softwood species most commonly used include pine, spruce, hemlock and western red cedar. Boards range in widths from 4 to 12 inches (nominal size) and are dressed to 3/4 inch. Board and batten or shiplap joints are sometimes used, but tongue-and-groove (T&G) joints combined with shaped edges and surfaces are more popular.
When solid wood paneling is applied horizontally, furring strips are not required-the boards are nailed directly to-the studs. Inside corners are formed by butting the paneling units flush with the other walls. If random widths are used, boards on adjacent walls must match and be accurately aligned.
Vertical installations require furring strips at the top and bottom of the wall and at various intermediate spaces. Sometimes, 2- by 4-inch blocking is installed between the studs to serve as a nailing base (see fig. 5-16). Even when heavy T&G boards are used, these nailing members should not be spaced more than 24 inches apart.
Figure 5-16.Vertical wood paneling.
Narrow widths (4 to 6 inches) of T&G paneling are blind-nailed (see insert in fig. 5-16). The nailheads do not appear on finished surfaces, and you eliminate the need for countersinking and filling nail holes. This nailing method also provides a smooth, blemish-free surface. This is especially important when clear finishes are used. Drive 6d finishing nails at a 45° angle into the base of the tongue and on into the bearing point. Carefully plumb the first piece installed and check for the plumbness at regular intervals. For lumber paneling (not tongue and grooved), use 6d casing or finishing nails. Use two roils at each nailing member for panels 6 inches or less in width and three nails for wider panels.
Exterior wall constructions, where the interior surface consists of solid wood paneling, should include a tight application of building paper located close to the backside of the boards. This prevents the infiltration of wind and dust through the joints. In cold climates, insulation and vapor barriers are important. Base, corner and ceiling trim can be used for decorative purposes or to conceal irregularities in joints.