Fix-It Chick: Patch a cement board

Hardie Board, WonderBoard and Durock are all classified as cement boards. These sheet goods are made with a mixture of Portland cement and sand instead of the powdery gypsum that’s used in drywall. The cement basis of the product dictates the use of thin-set mortar to properly repair cracks or holes.

Step 1: For holes larger than 1/2-inch in diameter, use a carton knife or carbide-tipped tool to cut a V-shaped groove behind the edges of hole. This will allow the patching compound to lock in behind the cement board.

Step 2: Use a clean, damp sponge to wipe down the area to be patched.

Use thin-set mortar to repair cracks or holes in cement boards.

Step 3: Mix up a small amount of thin-set mortar following the manufacturer’s instructions. Thin-set mortar typically has a working time of less than 15 minutes, so smaller batches are the way to go.

Step 4: Use a putty knife or trowel to fill the hole or gap with a layer of thin-set mortar.

Step 5: For holes larger than 1/2-inch, press a length of 2-inch-wide polymer coated, alkali-resistant fiberglass tape into the layer of mortar. Overlap the edges with a half inch or so of tape. Trim away the excess before proceeding.

Step 6: Spread more thin-set mortar to level the patch.

Step 7: Once the patch is dry, sand the mortar smooth and finish as desired. Always wear a NIOSH-approved (e.g. N95) respirator when sanding, especially when sanding cement-based products.

Step 8: For holes more than a few inches wide, use a carton knife or carbide-tip tool to cut back the cement board and expose a stud on either side of the damaged area. Cement board contains silica. Avoid cutting the board with power tools when indoors. Clean up any dust and debris with a damp sponge or towel.

Step 9: Use a carton knife to score and break a new piece of cement board to fit into the newly created void.

Step 10: Attach the new board to the studs with corrosion resistant screws. Pre-drill the screw holes for easier installation. Be sure to wear a respirator and protective eyewear when drilling.

Step 11: Fill the voids with a layer of thin-set mortar topped with fiberglass tape. Sand the mortar smooth and finish as desired.