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Gypsum Wallboard - An Economical and Versatile Wall Surface
Hansen, A. T.
Ser
NA7110
N21h8
no.
25 c. 2BLDG
NATIONAL RESEARCH COUNCIL
CANADA
DIVISION OF BUILDING RESEARCH
HOUSING
NOTE NO.
25
GYPSUM WALLBOARD
-
AN ECONOMICAL AND
VERSATILE WALL SURFACE
A.
T.
HANSEN
Reprinted from
Canadian Builder, Vol. XV, No. 6, June 1965 pp. 48-49,70
This Note may be reprinted without amendment provided acknowledgment is given to Canadian Builder
OTTAWA
JULY 1965
Gypsum
wallboard
-
an economical
and versatile
wall surface
The past ten years have seen remarkable advancements of gypsum wallboard as an interior finish. In 1955 it was used in less than 15 per cent of Canadian dwelling units, but today it is estimated that it is used in about 50 per cent of all units.
Gypsum drywall provides a fast, relatively clean and economical finish almost indistinguishable from lath and plaster if properly applied. It does not require the introduction of large quantities of mixing water into the building. This is especially ad- vantageous in the winter months, when the proper drying of wet fin- ishes is difficult, and excess humidity in the building can lead to undesira- ble results.
Drywall enables the house to be ready for occupancy earlier than would be the case with wet finishes. Generally speaking, the costs are about the same. Gypsum wallboard may be slightly cheaper than lath and plaster, but it must be painted to provide an acceptable appearance and this is not necessary for plaster. Although gypsum wallboard over- comes the problem of surface crack- ing, it is subject to nail popping This occurs when concealed nail heads are
pushed out from the plane of the wall, studs should not be used; they will grooved nails are better than plain sometimes cracking the concealing not provide the even surface essential shank nails. Because of their greater filler. A principal cause of nail for proper wallboard application. This holding power, shorter nails can be popping is wet lumber framing. When is more critical with drywall than with used (1% in. long for %-in. boards the lumber dries it shrinks away from plaster which has the advantage of and 1% in. long for %-in. boards), the nails, and seasonal cycles in the being able to tolerate larger varia- lessening differential shrinkage be- moisture content of the framing may tions in the plane of the wall studs tween the wood and the nails and further lift the nail heads. The long- because it can accommodate subsur- reducing the degree of popping. er the nail or the wetter the wood, face irregularities. Even better results are claimed if the greater the problem will be. Nail the "double nailing" technique is popping can also be caused by poor Mechanical Fastening used. In this case the nails are workmanship, where the board is not The standard method of applica- spaced in pairs about 2 in. apart drawn in tight to the framing during tion consists of applying wallboard every 12 in. along the supporting application. directly to the framing members with members. The first nail in each pair
It is important that the framing annular grooved nails spaced approx- is intended to draw the board in close members provide an even plane for imately 7 in. apart on the ceilings to the member; the second ensures the wallboard attachment. Bowed and 8 in. apart on the walls. Annular that the board will be in tight con-
tact with the member.
Wallboard nailing should be from the centre of the sheet towards the edges. Otherwise the sheet may be forced into compression, which can result in waviness in the finish. All nails should be driven so that the heads are slightly below the plane of the board but d o not break through the paper. If this does occur another nail should be driven close to the first nail.
Recently, screw application of dry- wall has become increasingly popular. Screws made especially for this pur- pose are inserted with an electric screw driver to a predetermined depth beneath the plane of the board. This ensures tight contact with the supports and reduces the possibility that the fastener head may hreak through the paper face.
The screws are usually spaced 12 in. O.C. along the supports, but this
spacing is increased to 16 in. O.C.
along the wall studs if the studs are
not more than 16 in. O.C. The screws
usually penetrate only % in. into the
wood members (less than for nails), further reducing the possibility of differential shrinkage between the wood and steel. Some appliers prefer to use nails near the edges to be taped (since the filled joints are nor- mally free of nail popping), and use screws only at the intermediate loca- tions.
Adhesive Applications
Nail popping problems can be entirely eliminated if the board is applied with adhesives. There are two principal methods. The first is the double layer method whereby the first layer of board is nailed as described for the "standard" method of applica- tion. A second layer is then applied at right angles to the first layer. It is attached with the same drywall ce- ment as is used in filling the joints. Cement is applied by means of a scored spreading tool somewhat sim- ilar to that used in applying floor tile adhesive. The second layer may be temporarily shored in place while the adhesive sets or, more commonly, it may be held in place with double headed nails that are removed after the cement sets. The temporary nail- ing is usually spaced about 16 in. apart
on the walls and 12 in. on the ceiling. w
The second method is called "strip- laminating." In this case the framing members are covered with 6-in. by %-in. strips of gypsum board attached to the framing with nailing equivalent to that used in the single layer appli- cation. The wallboard is cemented to these strips and temporarily shored or nailed until the cement has set. The strips are usually applied either across the framing members (at 16-in. or 24- in. O.C. spacing) or parallel to them
(Fig. 1 ) This method is usually somewhat cheaper than the double layer method.
Gypsum wallboard should be at least 3/8 in. thick on supports up to
16 in. O.C. apart and 1/2 in. thick on
supports up to 24 in. O.C. apart for
the single layer applications. Two ?/a-
in. layers are adequate for supports up to 24 in. O.C. More and more appliers, however, are using M-in. thick gyp- sum board in the single layer appli- cations, even where %-in. thick board is permitted. The additional cost per house is quite low (about $30 to $40) and a superior wall finish is produced.
Direction of Application
Wallboard should be installed so that the taped joints are horizontal, using the longest lengths possible in each room to reduce the necessary amount of joint treatment (horizontal joints are less conspicuous than verti- cal joints). Ceiling panels should also be applied so that there is a minimum amount of joint finishing.
The ends of gypsum board panels are not usually recessed to receive the tape and joint filler, as are the sides of the panels, and are somewhat harder to conceal. Wherever possible, therefore, end butt joints should be avoided. As sheets are usually avail- able in lengths up to 12 ft and even 14 ft, the rooms can be spanned in many cases with continuous sheets.
It is usually recommended that the
ceiling panels be applied first, fol- lowed by the horizontally applied
..-
finish layer of wallboard to the top half of the wall, so that it is in contact with the ceiling panels.
Wallboard Over Masonry
Wallboard may also be laminated directly to interior masonry surfaces, using the same drywall cement as is used in treating the joints. Other proprietary adhesives have also been developed for this purpose. Direct application should not be attempted on exterior walls or walls exposed to soil moisture. Damp conditions will lead to the early failure of the wall- board.
Spqcial proprietary . adhesive sys-
tems have been developed for attach- ing gypsum board to foamed polysty- rene insulation board, and these sys- tems may be used on walls exposed to the weather.
Comer Reinforcing
External corners are reinforced
with metal corner beads specially manufactured for the purpose. These provide not only protection against damage, but also a screed for the finishing operations, ensuring straight, even corners. Internal corners are reinforced with perforated paper strips of the same type as those used in reinforcing the joints between sheets.
Joint Treatment
Joints, corners and nail or screw heads are usually treated with three separate layers of gypsum wallboard cement. Each layer is allowed to dry about 24 hr before the next is applied because of the relatively high shrink- age of the cement.
After the first layer has been ap- plied to joints and internal angles, perforated paper reinforcing tape is embedded in it. The tape is then smoothed out and excess cement re- moved with the spreader tool. The wider the spreader the better, for this
will contribute to the ease with which an acceptable joint treatment can be made.
The second layer of cement is applied to the joints so that it com- pletely beds the tape, and is feathered out at the edges.
The third layer of cement is also feathered out at the edges. Figure 2 shows the typical finishing treatment for recessed and butt joints. After drying, the rough or uneven areas are lightly sanded to provide a smooth even surface. About 48 hr after the application of the third coat, the sur- face is ready for painting. An interior latex primer sealer is recommended as the prime coat.
There is a variety of equipment available for mechanical filling and taping. Many users, however, still prefer the manual method; a good craftsman can provide a finish at least as good and about, as fast as that applied with mechanical equipment.
