Tile floors are installed on top of cement backer boards.
Cement backer boards are designed and manufactured for use with ceramic tile and natural stone installations, acting as an underlayment layer between the wooden substrate below the board and the tile installation on top. If you are remodeling an old tile floor and are trying to decide whether you need to replace the old cement boards, there are a few things you can look for to help save time and money on the remodel.
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The first thing to look for when you are checking to see if you need to replace the cement backer board is whether the pieces were in some way glued down to the floor. If the underlayment is simply floating on top of the substrate with nothing more than nails or screws holding it in place, you need to remove it and either replace it with new or install it with some type of mastic or thinset mortar to help bond the backer board in place.
Cement boards are generally unharmed by minimal water damage, such as underneath a toilet where water eventually seeps out over the years. However, excessive water damage such as a broken pipe that leaks for weeks or months can eventually break down the concrete board's particles, leaving behind a crumbly mess that was once solid board. If you have this type of water damage, you need to remove the board as well as any rotted substrate material and replace them with new.
Walk across the backer board surface to determine if there are any loose sections on the floor; you will feel them under your feet as you walk, or you will hear them and see the floor move under your weight. This is a sign that the board was not glued down in the first place or that the adhesive bond has broken over the years. In either case, you need to firmly adhere the loose section and nail or screw it down.
If the boards appear to be in good shape and firmly adhered to the floor, you can re-use the old backer boards for your new tile installation. However, you still need to prepare the boards for installation, taking them back down to the bare cement board. Removing the old glue or thinset mortar can be a challenging task that requires a hand scraper as well as a drum sander for large, open areas.