As a dense material, drywall is a better barrier against fire (and a more efficient sound absorber) than lighter materials such as plywood, but one type of drywall is particularly effective in containing fire. Fire-resistant drywall panels have a gypsum core with special additives and glass fibers that aid in containing fire. On the surface, these panels look the same as regular drywall, except for a stamp that indicates that they are fire resistant. Fire-resistant drywall is a little harder to cut than regular drywall, because the gypsum core is tougher.

"Fire resistance" signifies the ability of a constructed assembly (the wall or ceiling that is covered with drywall) to contain a fire. The fire-resistance rating for each thickness of drywall is indicated with intervals of time: 45 minutes for 1/2-in. fire-resistant drywall, 60 minutes for 5/8 in. and 120 minutes for 3/4 in. The panels can be layered to increase the fire rating.

Many building codes specify fire-resistant drywall for attached garages, for furnace or utility rooms, and for ceilings and walls separating dwelling units in apartment and condominium complexes. The two most commonly used fire-resistant panels are 1/2 in. and 5/8 in. Half-inch panels are convenient when you need to cover only part of a wall or ceiling with fire-resistant drywall and the rest with regular 1/2-in. drywall (the most common standard thickness). An example would be on a garage ceiling, where typically only the first 5 ft. of the ceiling adjacent to the house wall has to be fire resistant. Using matching thicknesses allows you to make a smooth transition at the joint. If you use different thicknesses (say, 5/8-in. fire resistant and 1/2-in. regular drywall panels), you'll create a seam that's difficult to hide during the taping process.

Five-eighth-inch panels are the most commonly used fire-resistant panels. They provide the one-hour fire rating that many building codes require. Since most garage-ceiling joists are 24-in. o.c, 5/8-in. panels work best because they are approved for 24-in. spacing. Although manufacturers approve 1/2-in. panels for joists 24 in. o.c, these panels are more, likely to sag, especially in a garage that is exposed to temperature and humidity extremes. Because of the extra thickness and stronger core, 5/8-in. fire resistant is a little more resistant than 1/2 in. to denting and other types of abuse. Since garages can come in for a lot of abuse from car doors, bikes and tools, you'd like to cover the entire surface with 5/8 in.

Three-quarter-inch fire-resistant drywall has a two-hour fire rating, or a four-hour rating if double-layered. It is used where a high fire rating is a must, as between apartments or to separate offices from a garage or factory. The extra thickness and fire rating allow for fewer layers, cutting down on the cost of materials and labor.