A cement board is a combination of cement and glass fibers formed into 4 ft x 8 ft sheets, 1/4 to 1/2 in. thick, that are typically used as a tile backing board. Cement board can be nailed or screwed to wood or steel studs to create a substrate for vertical tile and attached horizontally to plywood for tile floors, kitchen counters, and backsplashes. It can be used on the exterior of buildings as a base for exterior plaster (stucco) systems and sometimes as the finish system itself.
Cement board offers an extremely stable, strong bond for most tile mortars and any materials that use cement-based materials to create a finish bond. Cement board also adds impact resistance and strength to the wall surface as compared to water-resistant gypsum boards. Cement board is also fabricated in thin sheets with polymer-modified cements to allow bending for curved surfaces.
As a tile backing board, cement board has better long-term performance than paper-faced gypsum core products because it will not mold, mildew, or physically break down in the continued presence of moisture or leaks. Cement board is not actually waterproof, but it is highly resistant to absorbing moisture and has excellent drying properties. In areas continually exposed to water spray (i.e., showers), a waterproofing barrier is usually recommended. The barrier can be applied behind the boards or to the face of the boards behind the finish system.
Cement board is composed of aggregated Portland cement with a glass-fiber mesh on the surfaces. This 5/16 in. (7.9 mm) thick cement board is designed as an underlayment for tile floors. These are 3 x 5 ft (91 x 152 cm) sheets manufactured by the United States Gypsum Company under the DUROCK brand name. In Europe, the board is manufactured by Etex group and has the Hydropanel brand name.
One major disadvantage of cement board is the weight per square foot. It is approximately twice that of gypsum board, making handling by one person difficult. Cement board is also a very hard material, so the cutting must be done with carbide-tipped tools and saw blades, and pre-drilling of fasteners is often recommended. Finally, cement board is initially more expensive than water-resistant gypsum board, although it may provide better long-term value.
Cement board is hung with corrosion-resistant screws or ring-shank nails. Cement board has very little movement under thermal stress, but the boards are usually installed with a slight gap at joints in shower pans, in bathtubs, and between boards. These joints are then filled with silicone sealant or the manufacturer's taping compounds before the finish is applied. The filled joints are taped like conventional gypsum board, but with fiberglass tapes that provide additional water resistance. Combined with a water-impermeable finish, cement board is a stable, durable backing board.
The category of construction material know as cement board includes both water-resistant and waterproof board. Each has its own best use.
Typically water-resistant cement board is composed of a treated gypsum core with a non-organic-fiber reinforced covering, either on one or both faces. This type of board requires fastidious sealing of all cut edges and penetrations to maintain the manufacturer's warranty for wet area installations. Gyspum core "cement" board panels are ideal for moist but not truly wet installations of tile or stone walls.
There is a class of cement board strictly constructed of a Portland-cement-based core with glass-fiber matt reinforcing at both faces. This type of board is truly waterproof. These panels can be immersed in water without any degradation (excluding freeze thaw cycles). These panels do not require the sealing of edges and penetrations to maintain their structural integrity. These Portland-cement-based products are smaller in size than the gypsum-core-based products. Typically they range in size from 30 in. x 48 in. to 36 in. x 60 in. They are, as one would expect, considerably heavier than the gypsum-core panels.
Portland-cement-based panels are ideal for truly wet locations like shower surrounds and for locations where a Portland-cement-based thin-set material is used for bonding tile and stone surfaces to a substrate. They are also ideal for floor tile and stone installations over a structural subfloor.
