Type X versus Type C

Gypsum board with glass fibers in the core, made to hold together in a fire long enough for the assembly to earn its rating. Five-eighths Type X is the standard one-hour board. It is on the rack at every yard, and it is most of what goes out.

More glass fiber, plus additives — usually vermiculite — that expand as the board heats. When gypsum calcines in a fire it shrinks; Type C's core expands to compensate, so the board stays in the assembly longer. That is why some listings require it and Type X cannot substitute. At Outwest, the 1/2" fire-rated board is Type C; 5/8" Type C is supplied when the assembly calls for it.

Horizontal assemblies, where the board is holding itself up against gravity as it degrades. Shaft walls. Some two-hour walls. Certain column and beam wraps. If a listing says Type C, it is because the test was run with Type C and the assembly failed the time with Type X.

The wall type on the drawings references a UL design number — U305, U419, L502. Look the number up. The listing names the board type by name, and often by manufacturer product. Send us the number and we will match it. Guessing is how a wall gets opened after inspection.

At 1/2", fire-rated board is Type C. At 5/8", Type X is common, but some assemblies require Type C. When in doubt, read the UL number, not the box.