Gypsum wallboard is the flat panel material that forms most interior walls and ceilings in homes and commercial buildings. It consists of a noncombustible gypsum core sandwiched between two sheets of paper, and it goes by several common names: drywall, sheetrock, plasterboard, or simply “board.” The gypsum core is made of calcium sulfate dihydrate, a naturally occurring mineral that contains about 21% water by weight locked inside its crystal structure. That built-in water is what makes gypsum wallboard uniquely useful as a building material.
What’s Inside a Gypsum Panel
The core of every gypsum panel is essentially rock. Calcium sulfate dihydrate, the mineral’s chemical name, means each molecule of calcium sulfate is bonded to two molecules of water. This isn’t moisture that can be squeezed out; it’s part of the crystal itself. Surrounding that core are two paper facers that bond chemically and mechanically to the gypsum during manufacturing. The front face (the side you see after installation) is smooth and ready for paint or texture. The back face is rougher and designed to grip framing members.
Specialized versions of gypsum board add ingredients to the core. Fire-rated panels include glass fibers mixed into the gypsum to reinforce it at high temperatures. Moisture-resistant boards use a thicker paper coating with a wax layer to repel water. The base recipe, though, is always the same mineral core wrapped in paper.
How It’s Made
Manufacturing starts with raw gypsum rock, which is crushed and heated to drive off most of its crystalline water. This process, called calcination, converts the mineral into a powder similar to plaster of Paris. That powder is then mixed with water and additives to form a slurry, which is poured onto a continuous sheet of paper moving along a conveyor belt. A second sheet of paper is laid on top, creating the familiar sandwich.
As the board travels down the production line, the calcium sulfate recrystallizes, reabsorbing water and reverting to its original rock-hard state. The paper bonds tightly to the core during this process. Once set, the board is cut to length and sent through large dryers to remove any remaining free moisture, producing a rigid, ready-to-install panel.
Standard Sizes and Weights
Gypsum wallboard comes in several thicknesses, each suited to different jobs. The most common panels are 4 feet wide and 8 feet long, though 10- and 12-foot lengths are available for taller walls.
- 1/4 inch: Weighs about 1.2 pounds per square foot. Used mainly for curving over arched surfaces or as a laminating layer over existing walls.
- 3/8 inch: Weighs about 1.4 pounds per square foot. Sometimes used in repairs or for covering old plaster.
- 1/2 inch: Weighs about 2.0 pounds per square foot. The standard choice for most residential walls and ceilings with framing spaced 16 inches apart.
- 5/8 inch: Weighs about 2.5 pounds per square foot. Required for fire-rated assemblies and preferred for ceilings because of its added stiffness, which resists sagging.
A standard 4-by-8-foot sheet of 1/2-inch board weighs roughly 64 pounds. The 5/8-inch version comes in at about 80 pounds per sheet.
Why Gypsum Resists Fire
The single most important property of gypsum wallboard is its fire resistance, and it comes down to that crystalline water. When fire heats a gypsum panel, the energy goes into converting the trapped water into steam rather than raising the temperature on the other side of the wall. As long as water remains in the crystal structure, the unexposed side of the panel stays relatively cool. This buys critical time for occupants to evacuate and for firefighters to respond.
The process is the same calcination that happens during manufacturing, just uncontrolled. A standard 1/2-inch panel provides meaningful fire protection, but building codes often require thicker or specially formulated boards for walls and ceilings that separate living spaces, garages, or stairwells.
Fire-Rated Types: Type X and Type C
Type X gypsum board is the most widely specified fire-rated panel. It contains glass fibers mixed into the core that reinforce the gypsum as it loses its crystalline water during a fire. Without those fibers, a calcined panel would crumble and fall apart. With them, the board holds together much longer. In testing, a Type X panel resisted fire exposure for about 57 minutes before collapsing under load.
Type C board takes the same concept further, using a significantly higher percentage of glass fibers by weight. The difference is dramatic: in the same test conditions, a Type C panel lasted over two hours without failure, more than twice the endurance of Type X. Type C is typically specified for assemblies requiring two-hour fire ratings, such as walls between units in apartment buildings or corridors in commercial spaces.
Moisture and Mold-Resistant Boards
Standard gypsum wallboard is vulnerable to water. If the paper facers stay wet, they become a food source for mold. Two specialty products address this problem.
Green board (officially called water-resistant gypsum board) uses a thicker paper facing treated with wax to resist moisture absorption. It’s been a go-to choice for bathrooms and laundry rooms for decades, though it isn’t waterproof and shouldn’t be used in direct contact with standing water, like inside a shower enclosure.
Purple board (a brand name that’s become a generic term) combines moisture resistance with mold resistance. It provides the same fire rating as standard drywall while also standing up to humidity and resisting mold growth on its surface. It can be used anywhere standard drywall goes, making it a popular upgrade in kitchens, basements, and bathrooms.
Sound Control Performance
Gypsum wallboard plays a significant role in how much sound passes between rooms. Sound performance is measured using the Sound Transmission Class (STC) rating, where every 10-point increase cuts the perceived volume of transmitted sound in half.
A basic residential wall, one layer of 1/2-inch gypsum on each side of wood studs, scores an STC of about 30. That’s enough to muffle normal speech but not enough to block loud conversation or music. Adding a second layer of gypsum to each side and staggering the joints pushes the rating into the 45 to 49 range, a noticeable improvement. For serious sound isolation, a double-wall system with steel studs, insulation in the cavity, and multiple layers of board on each side can reach STC 55 to 59, which blocks most everyday noise between rooms.
The key insight is that gypsum board alone isn’t enough for good soundproofing. The entire wall assembly matters: the framing type, insulation, number of board layers, and whether the two sides of the wall are physically connected or isolated from each other.
Other Specialty Types
Beyond fire-rated and moisture-resistant panels, gypsum board comes in several other specialized forms. Gypsum sheathing board is designed for exterior walls, providing a weather-resistant layer behind siding. Gypsum ceiling board is engineered for overhead installation with extra sag resistance. Gypsum shaftliner board lines elevator shafts and stairwells. Veneer plaster base has an absorbent paper face designed to accept a thin coat of finish plaster rather than joint compound and paint.
All of these products fall under the same industry standard, ASTM C1396, which sets requirements for core hardness, flexural strength, nail pull resistance, water absorption, and other physical properties. If a panel meets this standard, it performs predictably regardless of the manufacturer.
Why It Replaced Plaster
The earliest version of gypsum board was patented in 1894 by Augustine Sackett, consisting of alternating layers of wool felt and plaster. The modern version, with paper facers bonded to a gypsum core, evolved over the following decades but didn’t become the dominant wall material until the mid-20th century. The reason was speed: traditional plaster walls require skilled tradespeople to apply multiple coats over wood lath, with drying time between each coat. Gypsum wallboard can be screwed to framing and finished in a fraction of the time, making it dramatically cheaper to install. Today, virtually every new home and commercial interior in North America uses gypsum wallboard as its primary wall and ceiling surface.