How to install cement backer board
For areas like showers, tub surrounds and sink backsplashes, install cement backer board as it is one of the few recommended subfloors. Cement backer board will not warp, rot, mold or deteriorate if exposed to moisture and water.
Installing cement backer board is similar to hanging drywall or laying a plywood subfloor. With careful attention to a few differences, you’ll get a flat, rigid and durable surface.
Cutting and cleanup
The first step to install cement backer board is cutting. Most cement backer board products can be cut like drywall by simply scoring with a utility knife and snapping along the scoring line. Carbide-tipped scoring knives work best, and you won’t dull so many blades. You can also use manual, electric or pneumatic shears.
However, circular, reciprocating or jig saws are not recommended unless you are cutting outside and wear a particulate respirator.
For cutouts and small holes, scoring around the perimeter and breaking from the face with a hammer should provide a neat job. Again, use a jig saw or similar saw only when necessary and with proper ventilation and personal protective equipment.
Note: If you are planning to install cement backer board, respiratory precautions must be taken because most cement backer board contains silica, which is harmful if inhaled. Wear N95 particulate respirators and never dry sweep when cleaning up. Use wet suppression methods or a HEPA-filtered vacuum.
Ensure all framing is structurally sound, spaced no more than 16 inches on-center (OC), and provides proper nailing/attachment at all corners. Backer board can be hung vertically or horizontally, and should be installed ¼ inch above floors, shower pans, tubs, or countertops and caulked with a sealant appropriate for the application.
Fasten with appropriate corrosion-resistant screws or nails, typically no more than 8 to 10 inches OC and ensuring fastener heads are flush or slightly dimpled below the surface (do not overdrive). Keep all fasteners at least ⅜-inch in from the backer board edges and 2 inches in from sheet corners.
Fill joints with the appropriate mortar or thinset for the job, embed a wide, high-strength alkali-resistant glass fiber tape and smooth as you would with drywall.
If necessary, hand-sand seams lightly to achieve a smooth transition between boards. This will generate silica dust, so a minimum N95 particulate respirator must be worn.
Use properly attached ½-inch exterior-grade plywood across cabinets as a backer board subfloor – supported at least every 16 inches OC). Apply a supporting bed of mortar or thinset to the plywood with a ¼-inch square-notched trowel and place the backer board on top, with joints staggered off the plywood subfloor joints. Maintain a ⅛-inch gap between board edges.
Fasten with corrosion-resistant screws or nails, typically no more than 8 to 10 inches OC. Be sure fastener heads are flush or slightly below the surface, and at least ⅜-inch in from the edges and 2 inches from corners. Fill joints with the appropriate mortar or thinset, embed wide glass fiber tape and smooth the finished joint.
Ensure floor framing and subfloor is structurally sound and plywood is a minimum of 5/8-inch exterior grade. If you are not sure the floor is engineered for proper deflection and for the loads of the surface application – for example, when laying natural stone – consult a professional engineer.
Fasten with corrosion-resistant screws or nails spacing no more than 8 to 10 inches OC and with heads flush or just below the surface, and at least ⅜-inch in from the edges and 2 inches from corners. Fill joints with mortar or thinset, embed wide fiber tape and smooth finished joint.
—By Rob Fanjoy