For a tile bathroom floor that isn’t exposed to excessive moisture, there are a few different alternatives to preparing a substrate for tile installation. Assuming that the floor system is suitably strong and that appropriate tiles have been chosen, a ceramic tile can be set on a traditional thick-bed mortar, on backer board, on plywood, or on a concrete slab. Tile can also be set on other surfaces that are in sound condition and properly prepared, including a previous tile installation and uncushioned resilient flooring (providing that there is only one layer). The type of substrate determines the type of adhesive used, though in most cases a latex thinset adhesive offers the best combination of performance and economy.
Substrates
If a floor system is sufficiently strong but has surface irregularities that make tile questionable, a 154-in. mortar bed might be the answer. It can easily accommodate changes in level in the subfloor that would be difficult to do otherwise and is much easier to install on the horizontal surface of a floor than on the vertical surfaces of walls. Mix a 3:1 ratio of mason’s sand and portland cement with a liquid latex mortar additive. Then float it over an asphalt-paper curing membrane and wire-mesh reinforcement to make a strong and smooth setting bed for tile. A latex thinset adhesive is a good choice for setting tile on a mortar bed.
A minimum 1/2-in. plywood subfloor that is flat and smooth can be covered with backer board to make an ideal substrate for floor tile. The backer board should be laminated to the subfloor with a wood-compatible latex thinset or waterproof construction adhesive and screwed to the subfloor on 6-in. centers with galvanized drywall screws. There should be a Vain, gap between adjacent sheets of backer board, which should be filled with thinset and taped with fiberglass mesh tape, as well as a 1/4-in. gap at the walls for differential seasonal movement. Joints should be staggered and fall on joists and should overlap plywood subfloor joints. Again, latex thinset adhesives are a good choice for setting tile on backer board.
Given the choice, I would always pick backer board over plywood to set tile on because it is less expensive and more consistently flat and void free. But exterior-grade CDX or AC plywood can also be used as a substrate for tile floors. Tile requires at least 1 in. of plywood underneath it if the plywood will act as the setting bed, so another layer of plywood should be added to the typical 1/2-in. or 5/8-in. subfloor, with joints staggered so they fall on joists. Fasten plywood down like backer board, using construction adhesive and screws on 6-in. centers. Leave a 1/8-in. to l/4 in. gap between sheets, which will later be filled with adhesive. Either an epoxy thinset adhesive or a latex thinset adhesive works for setting tile on plywood.
Concrete slabs make good setting surfaces for tile, provided the slabs are sound and uncracked. A slab that has been floated with a wood trowel has a rougher surface than a steel-troweled slab and is better for setting tile; the smoother slab may have to be roughed up a bit by grinding it with an abrasive wheel. Unfortunately, most tile adhesives don’t stick well to slabs that have been treated with a form-release agent or curing compound. You can check for this by sprinkling the slab with water. If it isn’t readily absorbed, then you’ll need to choose another flooring material.
It’s a good idea to check the surface of a slab with a straightedge to determine how flat it is. Minor depressions can be filled in with thinset, and minor humps can be ground down with an electric grinder. In some cases, it might be necessary to float an additional layer of deck mud to even out the high and low spots. If there are cracks in the concrete and structural repairs are infeasible, an isolation membrane like NobleSeal T/S will help to insulate the tile from movement in the underlying substrate. Latex thinset is a good choice for an adhesive on most masonry surfaces, including concrete and gypsum-based thin slabs.
Layout and installation
Taking the time to carefully plan the layout and installation of a tile floor is as important as planning the layout of a tile shower. Finding the centerlines of each wall and checking that the room is square are the first steps, and you’ll want to plan the layout so that highly visible areas—door thresholds, areas in front of a tub or shower—look symmetrical and logically laid out. Narrow strips of cut tiles at the edges are distracting and should be avoided, and the layout should try to minimize the appearance of walls that aren’t parallel or square (more common of a situation than you might think). Because narrow tiles visually accentuate this problem, making sure cut tiles that are at least half-sized are used next to out-of-square walls will help. Sometimes letting the grout joint of the last course that is parallel to the offending wall widen or narrow slightly can help to minimize this problem, particularly if the grout joints are fairly wide to begin with.
As with wall-tile layout, it’s very helpful to draw your precise layout on the floor before setting tile. Then you’ll know ahead of time which tiles need to be cut and you’ll be able to plan your work so you’re applying adhesive to only one small area at a time. Tile adhesives skin over fairly quickly after they’ve been troweled on, so working in small areas and having tile precut make the process go more smoothly and enable you to work at a productive rather than frenetic pace.
