Drywall is the term used for a common method of constructing interior walls and ceilings using panels made of gypsum plaster pressed between two thick sheets of paper, then kiln dried. Many such panels are made with fiberglass instead of paper to prevent mold growth which is common with paper which has been exposed to water due to plumbing leaks or floods. Drywall construction is used globally for the finish construction of interior walls and ceilings. Drywall construction became prevalent as a speedier alternative to using plaster based interior finish techniques, which involved forcefully spreading a substrate of coarse plaster, known as the base (made up of the scratch coat and (optional) brown coat), onto the wall's lath-work before finally applying the smoother finish coat, each layer added in succession and all by hand (see the lath and plaster method).
Drywall, by contrast to plaster, requires hand finishing only at the fasteners and joints. The drywall process requires less labor and drying time, lending its name to the panels used in the assembly
As opposed to a week-long plaster application, an entire house can be dry-walled 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 mud-men, or float crew, who finish the joints and cover the nail-heads with drywall compound.
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, glue, or more commonly in recent years, the now-ubiquitous drywall screws.
A drywall fastener also referred to as drywall clips or stops are gaining popularity in both residential and commercial construction. Drywall fasteners are used for supporting interior drywall corners and replacing the non-structural wood or metal blocking that traditionally was used to install drywall. Their function serves to save on material and labor expenses; to minimize call backs due to truss uplift; to increase energy efficiency; and to make plumbing and electrical installation simpler.
Drywall screws heads have a curved taper allowing them to self-pilot and install rapidly without punching through the paper cover. These screws are set slightly into the drywall. When drywall is hung on wood framing, screws having an acute point and widely spaced threads are used. When drywall is hung on light-gauge steel framing, screws having an acute point and finely spaced threads are used. If the steel framing is heavier than 20-gauge, self-tapping screws with finely spaced threads must be used. 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). This compound is also applied to any screw holes or defects. 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.
Water damage and mold
Drywall is easily damaged by exposure to water. Whilst it can be waterproofed through covalent waterproofing, if waterproofing is absent or if the waterproofing layer is punctured, water will cause drywall to swell and eventually disintegrate requiring replacement. Drywall is a porous, lightweight substance that supports the growth of mold. It is for this reason that greenboard and cement board is used for rooms expected to have high humidity.
When used as a component in fire barriers, drywall is a passive fire protection item. In its natural state, gypsum contains the water of crystallization bound in the form of hydrates. When exposed to heat or fire, this water is vaporized, retarding heat transfer. Therefore, a fire in one room that is separated from an adjacent room by a fire-resistance rated drywall assembly will not cause this adjacent room to get any warmer than the boiling point (100°C) until the water in the gypsum is gone. This makes drywall an ablative material because as the hydrates sublime, a crumbly dust is left behind, which, along with the paper, is sacrificial. Generally, the more layers of Type X drywall one adds, the more one increases the fire-resistance of the assembly, be it horizontal or vertical. Evidence of this can be found both in publicly available design catalogues, including, but not limited to DIN4102 Part 4 and the Canadian Building Code on the topic, as well as common certification listings, including but not limited to certification listings provided by Underwriters Laboratories and Underwriters Laboratories of Canada (ULC). "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 and ceilings that are required to have a fire-resistance rating.