How to Apply Cement Board Over Brick Fireplace

Brush off any dirt or debris from the brick surface of the fireplace with a stiff bristle brush. Wash the brick with a pH-neutral cleaner and a sponge after brushing to remove ingrained dirt. Rinse with water and pat the bricks dry with rags.

Mask off the edges of surfaces adjacent to the brick with a strip of masking tape to avoid spreading thinset mortar onto them. Cover the floor surrounding the fireplace with dropcloths.

Use a tape measure to measure the sides of the fireplace, then transfer the measurements to a piece of cement board using a pencil and straightedge. Score the surface of the board with a utility knife along the penciled lines, using the straightedge to keep the scored line straight. Apply pressure to the rear of the board along each of the lines to snap the board at the lines.

Apply a layer of thinset mortar to the sides of the brick fireplace with the flat of a notched trowel. Be sure to mix the mortar with a substitute of acrylic additive for the water of the mix. Mix the thinset in a large bucket with an electric drill and grout-mixing paddle. Tilt the trowel at an angle and run the notches through the thinset to raise grooves.

Press the cement board pieces onto the spread thinset. Use a pencil to mark the location of each mortar line between the bricks of the fireplace along the edges onto the cement boards.

Nail the side boards into place with concrete nails and a hammer to hold them in place, driving a nail through the board and into the marked mortar areas between the bricks beneath. Space the nails 8 inches apart along the board lengths.

Measure the top of the brick fireplace hearth and cut a cement board to fit. Follow the same application process as that used with the side pieces; however, walk across the board once you place it onto the hearth to press it firmly into place before nailing it to the brick surface.

Measure and place the front pieces to the fireplace using three separate boards, one for each leg of the fireplace, then a third across the tops of the leg boards across the front of the fireplace to complete the installation.