Cement board is a building material made of a mixture of glass fibers and cement, usually formed into 4 by 8 foot sheets, 1/4 or 1/2 inch thick. It is typically used as a backing for tiled walls and floors, or on the exterior of buildings. If used on building exteriors, cement board may either be stuccoed, or simply painted as the final exterior surface.
While cement board is rigid and very strong and can be cut to desired sizes, because it is cement-based the cutting tools and saws need to be carbide-tipped. Further, regular wood drills will not work; metal drills will be needed to bore holes. Unlike sheet rock or paper-backed gypsum board, when attached to studs, floors or exterior walls cement board forms a strong, impact-resistant surface, strengthening the structure it is attached to.
Common, or regular, cement board is water resistant, though not actually waterproof. It is often used in bathrooms as a backing for sink, tub and shower tiles. This board is constructed of a gypsum-based core with a reinforced cement covering. When installed in wet areas the edges must be sealed to maintain water resistance. Though it may absorb some moisture, if properly sealed, it will not deteriorate when placed in moist areas, nor will it mold.
Portland Cement makes a board that is totally waterproof for use in areas of constant contact with water. This product is made of a cement-based core with a glass fiber mat reinforcing the cement on both sides making it totally waterproof.
Waterproof Portland board can be used in places where the board is actually and constantly in contact with water. Boat docks and boat houses can be sheathed with waterproof board, and it is an ideal material to use for shower pans. Since the waterproof version is much heavier than common board, it is usually formed in smaller board sizes, typically 30” by 48” or 36” by 60.”
While cement board has many advantages over sheet rock, one disadvantage is that it is much heavier. Attaching it to walls is at least a two person job. Also, nail or screw holes may need to be drilled before fasteners can be used. It is also more expensive than sheet rock, and in construction where cost is a factor, water resistant gypsum board or sheet rock, though not as ideal, may be used as tile backing rather than cement board.