Ideally, your backer board will reside on top of plywood subfloor, with the tile atop the backer board. But if height is not a problem and you wish to save time, you can lay your backer board on top of existing finish floor as long as it is in good shape.
- Purchase Backer Board: Buy your cement backer board. HardieBacker, Durock, and Wonderboard are the three main types you will find at your local home improvement store (I prefer Durock). Bring gloves and a friend. The edges are sharp, the boards are heavy.
- Dry Run: Lay out boards but do not affix to the subfloor yet. Mark partial boards by drawing cut lines on the surface of the cement board with a carpenter's pencil. Where the board must go around irregular obstructions, such as plumbing pipes, toilets, etc., draw a template on contractor's paper, cut out the template with scissors, then transfer that pattern to the backer board with pencil.
- Cut Partial Boards and For Obstructions: If you have any obstructions to work around, cut the cement board with a SkilSaw outfitted with a carbide blade. Safety glasses are not optional--cutting cement board is a messy process. An alternative, cleaner way is to score the mesh side with a utility knife and snap off.
- Butter Surface with Thinset: Apply thinset mortar on the surface where you will be affixing the cement board. Using basic tiling techniques, "comb" the thin-set mortar with your notched trowel.
- Install Boards: Lay down the cement boards, keeping them 1/8" to 1/4" apart. With your cordless drill, screw the cement boards down with the cement board screws along the edges, every 8". Screw heads should be slightly below the cement board surface. You do not want screw heads protruding, or you will have problems when you lay down the tile.
(Buy Direct - Gilmour Cement Screws)
- Tape Seams: Cover seams with fiberglass seam tape. Fill in and smooth with mortar. Let dry.