Drywall board (gypsum board, wallboard) was developed in the 1930s and is, basically, a gypsum product in sheet form with heavy paper facings to give it shape and abrasion resistance. It became popular during the building boom of and after World War II, replacing traditional lath and plaster because of its comparatively low cost even though it is less durable if not installed properly. It is also replacing traditional masonry fire division walls in new construction and renovation because of its comparatively low cost.

Gypsum is hydrous calcium sulfate (CaSO4-2H2O) and is a naturally-occurring mineral. This means that it is calcium sulfate (plaster of paris) combined at the molecular level with water of crystallization, about 20 percent of the gypsum by weight, and about 50 percent by volume. After gypsum is mined, it is crushed and ground to the fineness of flour; heated to drive off its water, making plaster of paris; recombined with precise amounts of water and other ingredients; and pressed between two sheets of heavy paper to create drywall board. Traditional plaster is a similar material except that the plaster of paris and other dry ingredients are mixed with water and other ingredients at the construction site and are applied by hand in layers over wood or expanded metal lath.

Gypsum can also be manufactured from industrial byproducts such as flue gas from power plants. The flue gas desulfurization (FGD) process results in a solid that can be purified and refined into gypsum with the same chemical formula as the naturally-occurring mineral. This “synthetic gypsum” has been manufactured since the 1980s and accounts for a significant amount of the drywall board used in North America each year. Some other industrial processes also have byproducts that can be refined into gypsum.

Drywall board is naturally fire-resistive because of the large amount of water contained within the gypsum molecules. A large amount of heat is required to drive off this water. As the water is driven from the gypsum during a structural fire, it slows the spread of the fire as the gypsum is breaking down chemically and structurally and its volume and dimensions are reduced, causing it to shrink, crack, and ultimately fail. (When overhauling burned-out rooms, areas of drywall or plaster that were heavily heat-damaged are pulled much more easily than areas of the same materials with little or no heat damage.)

Type X (fire code) drywall board has additives to the gypsum that make it more fire-resistive than the standard board including glass fibers to control shrinkage and cracking. Type X drywall board is commonly used in wall and ceiling assemblies with one- and two-hour ratings (such as Underwriters Laboratories Fire-Rated Assemblies U495 and V416). Special drywall assemblies have received higher ratings, but these are less common.

Type C drywall board, available since the 1980s, has the same gypsum core as Type X but with more glass fibers and with a form of vermiculite added to the gypsum. When the gypsum is exposed to heat, it shrinks because of the loss of the water molecules, but it is held together by the glass fibers. At the same time that the gypsum is losing its water and volume because of the heat, the vermiculite is expanding at about the same rate that the gypsum is shrinking. The result is drywall board that holds together longer under fire than Type X, with no loss of volume or dimension. One manufacturer’s data shows that equivalent thicknesses of Type X drywall board last 10 times longer in a fire than standard gypsum drywall, and that Type C drywall board lasts twice as long as Type X.

Although the cost of 5/8-inch (15.88 mm) Type C drywall board is about 10 percent higher than that of 5/8-inch Type X, the one- or two-hour interior partition fire rating can be achieved in some assemblies (like UL Design U412) using ½-inch (12.7 mm) Type C board. This makes the cost competitive and reduces the labor for installation because of the reduced weight, both of which can make this material attractive to builders.

Gypsum drywall board is usually sold with two sheets held face-to-face by paper end bindings (photos 1, 2), which are marked with the manufacturer, the size, and the thickness of sheet and the type of gypsum core. These paper bindings are removed to separate the sheets.

Drywall board is usually imprinted on the exposed back of each sheet with similar information (photo 3). After installation, this imprint is visible only by opening the wall in the right place on the opposite side. Some manufacturers also imprint the face of each sheet at the tapered edge (photo 4); this imprint will be covered by one or more applications of tape and drywall joint compound.

Some manufacturers use different colors of face paper for each type of drywall board, but these colors are not universal among manufacturers, and the colored paper will be concealed by wall covering or paint before the building is complete.

Building officials inspect frequently during construction so that they can ensure that fire-rated partitions use the proper type and thickness of gypsum drywall board as well as to note other structural elements and connections. After the building is completed, it is too late to verify that the fire-rated partitions are correctly assembled of the proper materials.

Firefighters can and should visit buildings under construction and undergoing renovation to see firsthand the materials and construction methods that combine to form the finished product. These buildings will be our workplaces for the next several generations, and the more we know about them, the better decisions we can make to work safely in them.

NFPA Codes and Standards are published by the National Fire Protection Association (www.nfpa.org), and can be purchased on CD, as an online resource, or on paper. Your local fire prevention bureau may allow you to use its copy for reference. Technical college and fire training school libraries often have these codes and standards available online or on paper for use by instructors and students.

The Underwriters Laboratories Fire Protection Directory is available online at http://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/fireressrch.html or at www.ul.com.

The Factory Mutual Approval Guide is available online at www.fmglobal.com or www.approvalguide.com  There is no charge for its use after free registration and log-in with password.

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Gregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 35-year veteran of the fire service. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College; has more than 35 years of experience in facilities management and building construction; and has presented classes at FDIC.