Gypsum board, commonly called drywall, consists of a non-combustible gypsum core sandwiched between two layers of heavy paper. Gypsum is calcium sulfate dihydrate (CaSO4·2H2O), a naturally occurring mineral that contains roughly 21% chemically bound water by weight. When exposed to fire, this water is released as steam, absorbing heat and slowing flame propagation. This inherent fire resistance, combined with low cost and ease of installation, explains why drywall dominates North American interior wall construction, with over 20 billion square feet produced annually across the United States alone.
Standard Drywall: Dimensions and Types
Standard drywall panels measure 4 feet wide by 8, 10, 12, or 16 feet long. The two primary thicknesses serve different purposes. Half-inch (12.7 mm) board is the standard for single-layer residential walls and ceilings on 16-inch stud spacing. Five-eighth-inch (15.9 mm) board provides greater rigidity, better sound control, and is required by many building codes for ceilings with 24-inch joist spacing and for garage-to-living-space separation walls.
The gypsum core incorporates additives including starch, paper fibre, and foaming agents that control density and workability. Standard board weighs approximately 1.6 pounds per square foot for 1/2-inch thickness and 2.2 pounds per square foot for 5/8-inch. The paper facing provides tensile strength and creates a smooth surface for paint or texture.
Type X and Type C Fire-Rated Boards
Type X gypsum board contains glass fibre reinforcement within the core. Under ASTM E119 fire testing, a single layer of 5/8-inch Type X provides a one-hour fire resistance rating when properly installed on wood studs. The glass fibres, typically 10 to 13 mm long and added at roughly 0.5% to 1% by weight, hold the gypsum core together after the calcination process drives off chemically bound water. Type C offers further improvement through the addition of vermiculite, which expands under heat to compensate for gypsum shrinkage. Two layers of 5/8-inch Type X can achieve a two-hour fire rating on many wall assemblies.