Cement board is made up of portland-cement / fine-aggregate slurry with polymer coated glass fiber mesh. The board typically comes in thicknesses of 1/4″ (non-wall or ceiling), 1/2″, and 5/8″ with widths of 32″ (1/2″ thick), 36″, 48″ and lengths of 5′ and 8′. It is primarily used as a backer board for hard tile and Exterior Insulation Finishing Systems (EIFS) as it’s water-durable, mold resistant, high-moisture suitable, and interior/exterior nature. Under reasonable circumstances it will not warp, rot, disintegrate, combust, or delaminate. The edges come rounded with the panel edges square.
Cement board is slightly more flexible and less rigid than drywall. It is seen as an advantage over paper-faced water resistant gypsum products due to it’s mold-proof nature, which leads to a backing material which does not break down as quickly as quickly from moisture penetration because of it’s superior drying properties. Cement board weighs almost twice as much as equal thickness gypsum board. Many installers will predrill fasteners due to the hardness of the material
It is also popular for use behind stone and thin brick in exterior applications. Joints/seams between panels are typically sealed with 2″ fiber reinforced mesh (or less commonly joint tape) followed by thinset adhesive.
It is good practice to install a plastic vapor barrier in between the cement board and the wall framing as an additional waterproofing measure if the intended area is anticipated to be high-moisture. There is a smooth side and a rough side with the rough side commonly facing outward if thinset mortar-adhesive will be used. The smooth side is commonly faced outwards if a latex mastic will be utilized. The boards are most commonly installed horizontally with the joints abutting eachother without being forced together and the end joints are staggered for each row. Wafer head cement board screws should be used as they have a chemical coating which resists corrosion from the high alkali content of the cement, drywall screws or nails should not be used.
Fasteners should be spaced 8″ on center for walls and 6″ on center for ceilings. placed at least 3/8″ from edges . Fasteners should be +/- 1-1/4″ long for steel framed walls and +/- 2-1/4″ for wood framed walls.
Cement board can be cut with a knife and straight edge but a dust-collecting power saw makes the cutting process much more efficient. Many cement board manufacturers have skim coats, base coats, and waterproofing products that can be used over the surface for a smoother, more water resistant assembly.
If being installed on a floor surface, cement board should be laminated to the floor or framing with a suitable latex modified thinset mortar.
Cement board is typically designed for a max load of 60 pounds per square foot with max stud/joist spacing of 16″ for walls and 24″ for floors with a suitable subfloor material underneath. Cement board typically should not be applied to metal framing lighter than 20gauge. Lightweight ‘setting’ type compounds or ready-mixed joint compounds should not be used directly over cement board.