Backerboard usually has to be cut to size before it can be installed. You may also have to drill holes in the board so that it will slide into place. If you have ever installed drywall, you will find the score-and-snap method to be very familiar. There are several types of backerboard on the market, and new materials are introduced from time to time. Be sure to follow the manufacturer’s instructions if they vary from the process described here. You can cut backerboard with power tools, but it will be messier, not any faster, and may damage your blade or motor.
Tools: Drywall square or straightedge, utility knife or scoring tool, and rubbing stone.
1. Score the board. For cement-based board, measure carefully and mark cut off lines on both sides of the backerboard. Align a straight edge with the line on one side. Pull the scoring tool along the straight edge; make as many passes as are necessary to break through the mesh on the surface. Place the straight edge on the other side and repeat the process. The mesh must be completely severed on each side. (A faster but somewhat riskier method: Proceed as you would for drywall, cutting one side, snapping the cut edge over, then cutting the other side.)
2. Snap. Place the backerboard on a flat surface. Press down with your hand on one side of the cut line. With the other hand, lift up just enough to snap the board along the scored edges. Some types of backerboard may break easier if you elevate the board on one side of scoring line, then press down.
3. Smooth edges. Be careful when handling the cut board. Some types of backerboard may leave a sharp edge along the cut line. The best way to smooth the edge is to use a rubbing stone.
A Hole Saw - Cement-based backerboard is often used under tiled surfaces in wet areas. That means you may have to fit it over plumbing protrusions in the wall, countertop, or floor. Most holes can be drilled quickly and effectively using a power drill equipped with a carbide-tip hole saw. The hole saw should be available where you buy your tile or at any large home center. Another method is to mark and score a circle on both sides of the board, then tap through with a hammer.
If it has been cut correctly, backerboard is fairly easy to install. Each type is installed with screws or nails, then the seams are joined with fiberglass tape and mortar. If you are planning to tile in a wet area, remember to take proper waterproofing steps. Cement-based backerboard itself is not damaged by moisture, but it is not waterproof. Water can permeate the board and the underlying framing, causing serious damage. For best results, install a waterproofing membrane behind the backerboard. Use the type of fasteners recommended by the manufacturer. Roofing nails work, but corrosion-resistant screws offer superior holding power. Edges of backerboard must by supported by studs or joists, or glued with construction adhesive to a sound wall surface.
1. Attach to walls. Attach backerboard directly to bare studs or over an existing layer of drywall. In either case, make sure the drywall screws or nails are long enough to penetrate the framing at least 3/4 inch.
2. Attach to floors. Coat the clean sub floor with adhesive applied with a notched trowel. Set the boards so that all joints fall over floor joists. Press the board into the adhesive before fastening with screws.
Expansion Gaps Between Boards - One of the most important steps you can take to ensure a long-lasting tile installation is to plan for some movement on and below the finished surface. Expansion gaps, filled with a flexible material, allow for normal movement without jeopardizing the integrity of the tile and grout. Each manufacturer has specific recommendations for expansion gaps around board edges. As a general rule, you should leave a 1/8-inch gap between boards and a 1/4 inch gap around bathtubs and shower pans.
3. Finish the joints. With all boards fastened, apply adhesive-backed fiberglass-mesh tape to the joints. Holding a trowel nearly flat, spread adhesive over the tape, pressing it into the mesh. Feather the edges of the adhesive for a smooth finish. Make sure there are no high spots; shallow low spots are not a problem.