» Drywall

Drywall is typically available in 4 ft (approx. 1200 mm) wide sheets of various lengths. Newly formed sheets are cut from a belt, the result of a continuous manufacturing process. In some commercial applications, sheets up to 16 ft are used. Larger sheets make for faster installation, since they reduce the number of joints that must be finished. Often, a sizable quantity of any custom length may be ordered, from factories, to exactly fit ceiling-to-floor on a large project.

The most commonly used drywall is about ½ in. thick, but can range from ¼ in. (6.35 mm) to 1 in. (25 mm). For soundproofing or fire resistance, two layers of drywall are sometimes used laid at right angles to each other. In North America, drywall in ⅝ in. thickness with a one hour fire resistance rating is also often used where fire resistance is desired.

Drywall provides an thermal resistance R-value of 0.32 for ⅜" board, 0.45 for ½", 0.56 for ⅝" and 0.83 for 1" board. In addition to increased R-value, thicker drywall has a higher Sound transmission class.

Construction techniques

Electric screwgun used to drive drywall screwsDrywall is cut to size, using a large T-square, by scoring the paper on the front side (usually white) with a utility knife, breaking the sheet along the cut, scoring the paper backing, and finally breaking the sheet in the opposite direction. Small features such as holes for outlets and light switches are usually cut using a keyhole saw or a small high-speed bit in a rotary tool. Drywall is then fixed to the wall structure with nails, or more commonly in recent years, specially designed screws. (Drywall screws have an acute point, widely spaced threads, and a curved top, allowing them to self-pilot and install rapidly without punching through the paper cover.) In some applications, the drywall may be attached to the wall with adhesives.

After the sheets are secured to the wall studs or ceiling joists, the seams between drywall sheets are concealed using joint tape and several layers of joint compound (sometimes called "mud"). The compound is allowed to air dry then typically sanded smooth before painting. Alternatively, for a better finish, the entire wall may be given a skim coat, a thin layer (about 1 mm or 1/16 inch) of finishing compound, to minimize the visual differences between the paper and mudded areas after painting.

Another similar skim coating is always done in a process called veneer plastering, although it is done slightly thicker (about 2 mm or 1/8 inch). Veneering uses a slightly different specialized setting compound ("finish plaster") that contains gypsum and lime putty. For this application blueboard is used which has special treated paper to accelerate the setting of the gypsum plaster component. This setting has far less shrinkage than the air-dry compounds normally used in drywall, so it only requires one coat. Blueboard also has square edges rather than the tapered-edge drywall boards. The tapered drywall boards are used to countersink the tape in taped jointing whereas the tape in veneer plastering is buried beneath a level surface. One coat veneer plaster over dry board is an intermediate style step between full multi-coat "wet" plaster and the limited joint-treatment-only given "dry" wall.

The name drywall derives from drywall's replacement of the lath-and-plaster wall-building method, in which plaster was spread over small wooden formers while still wet. As opposed to a week-long plaster application, an entire house can be drywalled in one or two days by two experienced drywallers, and drywall is easy enough to use that it can be installed by many amateur home carpenters. In large-scale commercial construction, the work of installing and finishing drywall is often split between the drywall mechanics, or hangers, who install the wallboard, and the tapers and mudman, or float crew, who finish the joints and cover the nailheads with drywall compound.

Because up to 17% of drywall is wasted during the manufacturing and installation processes,[citation needed] disposal has become a problem. Some landfill sites have banned the dumping of drywall. Therefore, used drywall and gypsum are often dumped into the ocean where it is not known to cause harm to sea life. The EPA regulates this ocean dumping by permit. Most manufacturers with an environmental concern take back the boards from construction sites, they are then burned at high temperature (thus burning away the paper and bringing back the gypsum to its initial plaster state). Since recycled paper is used during manufacturing, the environmental impact is minimal.

Fire resistance

When used as a component in fire-barriers, drywall is a passive fire protection item, subject to stringent bounding. It exhibits fire-resistance because it is endothermic. In its natural state, gypsum contains water of crystalisation, bound in the form of hydrates. When exposed to heat or fire, this water is released as steam, retarding heat transfer. Therefore a fire in one room, which is separated from an adjacent roon by a rated drywall assembly, will not cause this adjacent room to get any warmer than the boiling point until the "trapped" water in the gypsum is boiled away. This makes drywall an ablative material. "Type X" drywall is formulated by adding glass fibers to the gypsum, to increase the resistance to fires, especially once the hydrates are spent, which leaves the gypsum in powder form. Type X is typically the material chosen to construct walls that are required to have a fire-resistance rating.

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» Citation

The above was extracted from Wikipedia on 11/15/06.

Drywall. (2006, November 8). In Wikipedia, The Free Encyclopedia. Retrieved 23:18, November 15, 2006, from http://en.wikipedia.org/w/index.php?title=Drywall&oldid=86409977