Drywall

From Free net encyclopedia

Image:Drywall.jpg Drywall (also called Wallboard, gypsum board, GWB, Plasterboard, SHEETROCK® Gyprock® and ToughRock®) is a low-budget building material consisting of gypsum formed into a flat sheet and sandwiched between two pieces of heavy paper. As of 2005, it is the most common material used globally for the construction of interior walls and ceilings.

Drywall is made primarily from gypsum rock (hydrous calcium sulfate) and is formed by sandwiching a core of wet gypsum between two sheets of heavy paper. When the core sets and is dried, the sandwich becomes rigid and strong enough for use as a building material. Drywall is fire-resistant because in its natural state, gypsum contains water, and when exposed to heat or flame, this water is released as steam, retarding heat transfer. This is the same principle (latent heat of vapourisation) that prevents a boiling pot of water from getting any warmer than about 100 degrees C (212 degrees F; at sea level) until the water is all boiled away, at which point the pot will quickly get much warmer. Therefore a fire of several hundred degrees in one room will not cause an adjacent room to get any warmer than the boiling point until the "trapped" water in the gypsum is boiled away, preventing flashover. A special fire-rated (Type X) drywall is formulated by adding noncombustible fibers to the gypsum, helping to maintain integrity at high temperatures. Fire retardation is the primary advantage of gypsum wall products.

In the United States and Canada, drywall is typically available in four-foot (1220 mm) wide sheets of various lengths, commonly 8 feet (2440 mm), 9 feet, 10 feet, 12 feet, and 14 feet. The newly formed sheets are cut from a virtually endless "belt", the result of a continuous manufacturing process. In some commercial applications, sheets up to 16 feet are used. In other countries, the typical width is 1200 mm, with lengths starting at 2400 mm and available in 300 mm increments up to 3600 mm. Larger sheets make for faster installation, since they reduce the number of joints that must be finished, especially when the sheets are installed horizontally as is often done in American home building. Often, a sizable quantity of any custom length may be ordered, from factories, to exactly fit ceiling-to-floor on a large project, similar to the picture of sheets installed vertically shown in the upper right corner of this page.

Available thicknesses vary by country. The most commonly used drywall is about 1/2 inches thick (12 mm, 12.5 mm or 12.7 mm depending on country) thick, but can range from 6.35 mm (1/4 inch) to 25 mm (1 inch) thick. For soundproofing or fire resistance, two layers of drywall are sometimes used. In North America, drywall in 5/8-inch thickness with a one hour fire resistance rating is also often used.

Image:Screwgun.jpg Drywall 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. In some applications, the drywall may be attached to the wall with adhesives.

After the sheets are secured to the wall studs and/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 (actually "finish plaster") that contains some gypsum and some 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 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. A later developement was the use of plasterboard that replaced the wooden lath and the first of three coats (the scratch coat) of early plaster. Plaster board was usually smaller than later drywall sheets, being only about 2 foot by 4 foot pieces. Either of these methods are time consuming and labor intensive, although skilled plasterers could do the job with great rapidity. 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.

Greenboard is water resistant drywall. Early versions had asphalt added which gave it a brown gypsum core. It is suitable for humid areas, but not areas that actually get wet. Concrete backerboard (concrete reinforced with fiberglass) should be used where actual moisture is expected. Concrete backerboard is typically used as the underlayment for ceramic tile. Enviroboard is a board made from recycled agricultural materials.

Drywall provides an R-value of 0.32 for 3/8" board, 0.45 for 1/2" board, 0.56 for 5/8" and 0.83 for 1" board. In addition to increased R-value, thicker drywall has a higher STC.

Because up to 17% of drywall is wasted during the manufacturing and installation processes, 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. Technology is currently being developed to recycle drywall by removing the paper, pulverizing the gypsum, and removing metallic objects such as nails and screws with electromagnets.

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