Things You'll Need
Cement backer board
Fiberglass mesh tape
Concrete is usually a good subfloor on which to install ceramic tile. However, a homeowner may need to raise the floor slightly before installing tile and lack the experience or time to install a mortar bed. Placing cement boards over the concrete subfloor is an excellent way to create a solid and smooth subfloor. While cement backer boards are normally nailed to wood studs or a wood subfloor, you can use thin-set mortar to bond the cement boards directly to the concrete.
Measure the floor and determine if and where you can use whole sheets of the cement board. They typically come in 3-foot by 5-foot sheets, but can be found in larger sizes. You should lay out the area starting with whole sheets then measure and make cuts to fit smaller pieces around them.
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Mix thin-set mortar in a clean bucket and according to manufacturer’s instructions. Use a notched trowel to spread the thin-set on the concrete where your first sheet of cement board will be placed. Use the flat side of the trowel to spread the mortar firmly against the concrete to achieve a good bond, and then rake back across it with the notched side to leave ridges in the mortar. Lay the cement board sheet in the mortar and use the weight of the board and a twisting motion to force the sheet into the mortar and flat on the concrete.
Measure and cut the cement board to fit in around the whole sheets. Leave 1/8-inch between sheets and cuts. Mark the cut on the cement board using the straight edge and pencil. With the straight edge in place over your mark, cut through the surface of the board with the knife. You are only cutting through the fiberglass mesh on the surface. Pick up the edge of the sheet and put the straight edge under it next to the cut. Use slight pressure to break the sheet at the cut. Use the knife to cut through the fiberglass mesh on the other side to complete the cut. Use the thin-set mortar to affix these smaller sheets of cement board to the concrete subfloor. Once all cement board is in place, allow the floor to dry for 24 hours.
Cut fiberglass mesh tape to fit over the joints between the sheets. Mix more thin-set and spread it into the joints using the flat side of the trowel. Place the tape into the mortar over the joints and use the flat side of the trowel again to push the tape flat against the boards and scrape excess mortar off the surface. You want to leave a smooth flat surface over the joints. Allow this to dry before beginning your ceramic tile installation.
If the taped joints are a little rough after the mortar dries, use a brick or a rubbing stone to sand the rough spots.
Work in one direction across the floor as you lay the sheets. You want to avoid getting on boards already set until the mortar is dry.