Ceramic floor tiles require a strong, flat, immobile material under them, or they can move and crack when you walk on them. If you’re tiling in a bathroom, it’s important that that underlayment also be moisture-proof. Cement board is a common choice for ceramic tiling, because it lays and cuts like drywall but is as solid and waterproof as a layer of cement once it’s down. HardiBacker is one popular brand of cement board.
Lay your first HardiBacker board in one corner. Run your pencil around the perimeter of the board, marking the subfloor.
Remove the board. Spread thinset mortar within the area that you marked, using your notched trowel.
Set the HardiBacker board back in place. Press it firmly down, stepping on it at various points.
Sink screws into the surface of the board and down into the subfloor every square foot, making sure the screw heads aren’t sticking up anywhere.
Set the next board in place in the same manner, installing it at the end of the first board. Lay the rest of the boards in courses, end to end and then next to each other. Cut the end boards as needed by running a razor knife along the edge of a T-square to score the board, then snap it. (Note: Make sure there are no four-way intersections between the boards as you lay them.)
Lay adhesive mesh drywall tape over each seam between the boards, pressing it firmly into place. Spread a thin coat of thinset mortar over all the lines of tape with a wide putty knife, getting the mortar flat and smooth. Let the mortar set overnight. The surface is now ready for tiling.