Cement board is often called a cementious backer unit (CBU) or backer board. Unlike wood-based materials such as drywall, greenboard, or plywood, CBUs have no organic matter that will cause mold, rot, shrinkage, or decomposition. Also, because there is no cellular material in cement boards, it does not draw in moisture like cellulose materials do.
Cement backer board is the recommended substrate below tile.
If you have a good, smooth cement board surface, your tile work will go easier and the results will be smoother. You will have less chance of lippage (tile edges of various heights) and a greater chance that your tile will last for a longer time without cracking. CBUs are high value, low cost items that can only make your tile work look better.
What Goes Below Your Cement Board?
Because cement board can be used either for floors or walls, the surface below the cement board will vary.
- Flooring: Your backer board will usually rest on top of plywood subfloor (minimum 5/8" exterior-grade plywood or even OSB), with the tile atop the backer board. If the bottom-most surface is concrete, there is no reason to install backer board on the concrete. 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.
- Walls: For highly wet areas like showers and bathtub surrounds, the CBU should be installed directly on studs with an intervening waterproof membrane. Walls that you expect to stay dry need no membrane. If you are installing your CBU in a high moisture area, you should not install onto drywall or greenboard. First remove and demolish the board, then install directly on the studs (with the intervening membrane).
1. Purchase the Right Size CBU
Cement board, regardless of brand, tends to come in 3 foot x 5 foot sheets, each about 30 pounds for 1/4 inch board and 36-44 pounds for 1/2 inch board.
Sheets measuring 4 foot x 8 foot are available but tend to be unmanageable by most DIYers. Use gloves when handling as edges are sharp. In transporting CBUs, be cautious as it is easy to break or crack the boards. If only for this reason, bring a partner along when you buy the CBUs.
- Floors: 1/4 inch minimum thickness is needed.
- Walls: Use 1/2 inch or 5/8 inch thick cement board, not 1/4 inch. The extra thickness is needed to span the studs and provide a solid base for the tile.
- Countertops: 1/4 inch, 1/2 inch, or 5/8 inch thick.
2. Choose a Brand That Is Best for Your Project
All available at major home improvement stores, popular brands of cement board include:
- HardieBacker: Imprinted with a 1 inch by 1 inch grid for easier tile-setting, James Hardie Industries' HardieBacker is 90% Portland cement and sand, with a patented MoldBlock additive.
- Durock: USG's Durock tends to run lighter than equally sized HardieBacker.
- Denshield: From Georgia-Pacific, DenShield is a two-in-one product, combining cement board and waterproof membrane. Although DenSheild works in dry applications, it makes more economical sense to confine it to wet applications due to the membrane.
- Wonderboard: Wonderboard is often a cheaper cement board choice, over other brands per 3' foot x 5 foot sheet.
Since major home improvement drivers Home Depot and Lowe's carry HardieBacker and Durock, respectively, most do-it-yourself homeowners end up using either of those two brands.
3. Lay Out CBUs In a 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.
4. 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.
5. 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.
6. 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.
7. Tape Seams
Cover seams with fiberglass seam tape. Fill in and smooth with mortar. Let dry.
What You Need:
- Cement Backer Board (CBU)
- Thinset Mortar
- Notched Trowel
- Fiberglass Seam Tape
- Cordless Drill
- Cement Board Screws
- Safety Glasses
- Utility Knife (optional)
- Contractor's Paper
- Contractor's Pencil