Unless your home was seriously underbuilt, your floors are strong enough so that you can install carpeting or another type of resilient flooring with confidence. But ceramic tile has much more demanding requirements. The weight of the tiles is not usually the problem; deflection is. If a ceramic tile floor flexes, grout and even tiles can crack. If you have any doubts about the strength of the floor, ask a contractor to inspect it before you begin tiling.
Sometimes a bouncy floor can be firmed up by driving screws through the subflooring and into joists. Or, you may have to add another subfloor layer, or even beef up the joists. However, if you build up the floor so much that the new tile surface will be a half inch or more higher than an adjacent floor surface, it will look and feel awkward. That’s why in some situations it is simply not practical to lay a ceramic tile floor.
Anatomy of a floor. Floor joist strength is determined by the size of the joist, the length of the joist’s span between supports, and the amount of space between joists. If your joists have spaces larger than 16 inches between them, for instance, then you will need an extra strong subfloor. Plywood is the best subfloor material, but many older homes have strong subfloors made with planks of lx lumber.
Backerboard - Older installations set tiles on a thick bed of mortar. Backerboard is an excellent modern day substitute. Use as thick a board as possible, installed over a plywood subfloor.
Plywood - Although plywood is somewhat soft and flexible, it also has great strength. When two sheets are laminated together, the result is a very firm surface.
Concrete - Concrete is the best subsurface and works well for straightening out-of-level floors. Do not use curing or acceleration chemicals if you pour a concrete floor. An older concrete surface must be sound; tiling will not strengthen it.
Preparing a wood floor for tile. If you will be tiling directly over plywood, it is best to install two layers of 5/8 inch-minimum plywood, rather than one thick sheet. Use exterior-grade plywood for the top layer. The edges should always fall over a joist, but stagger the sheets so that the joints do not fall directly over each other. Coat the bottom sheet with construction adhesive before setting the top sheet in place. Leave a gap of 1/8 inch or more around all edges of the top sheets, including at the joints. Fasten the plywood with screws or ring-shank nails. If an existing plywood subfloor is strong enough, sand the surface thoroughly, then vacuum. Talk to your tile dealer about the best adhesive to use. If the existing floor is composed of lx or 2x planking in good condition, drive screws into joists wherever it seems at all loose, and perhaps install a layer of plywood over it. If it has cracks and does not feel strong, remove the planking and install plywood.
Preparing a concrete floor for tile. Concrete is a great base for tile, as long as it is structurally sound and flat. Some slabs may actually be too smooth, and should be scruffed up a bit by grinding with an abrasive wheel. Do not install tile over concrete that was treated with a curing or acceleration chemical when it was poured. These additives will prevent adhesive from bonding properly. If you are uncertain about whether or not such additives were used, try to locate the builder of the house or the concrete contractor. They may have a record. You can also test the slab yourself by sprinkling water on it. If the water isn’t absorbed, the concrete was probably treated. Apply a latex bonding agent or add a sub floor.
Install plywood. For the top layer of plywood, leave 1/8 inch gaps between the sheets. Fasten with screws or ring-shank nails in a 6-inch grid in the field, and every 4 inches at the joints and around the perimeter.
Beef up a wood floor. Strengthen a weak subfloor by installing wood or metal cross bridging between joists. Close small gaps between joists and the existing subfloor with shims, or drive screws from above.
Fill in low spots. Clean out low spots in a concrete slab and fill with thinset mortar. Use a trowel or straightedge to level the surface.
Dealing with cracks. Cover small cracks in concrete with an isolation membrane. Cracks that result in uneven surfaces indicate underlying structural problems; do not tile over such a surface.
■ As long as you’re tiling your floor, you may want to update your baseboards as well. New tile may well emphasize your old baseboard’s imperfections.
■ Old vinyl cove base can get pretty ratty-looking. Install new cove base after the tile job is done. Make sure that the new material is as wide as the original stuff or you will have an ugly line on the wall. Apply with cove base adhesive or latex silicone caulk.
■ The base shoe gets banged up in time, too, so go ahead and replace it as well. Use stain or paint on it, then cut it with a miter box and fasten it with finishing nails.
Remove the baseboard. Before tiling a floor, remove the baseboard from the surrounding walls. If the joint between the baseboard and wall is sealed with paint, score it with a utility knife first. Pull vinyl cove base away with a putty knife. If it resists removal, try heating the vinyl with a hair dryer to loosen the adhesive. Use a pry bar to remove wood baseboard. Protect the wall with a thin piece of wood. If your baseboard has a shoe—a small rounded molding at the bottom— remove that piece.
Trim casing and doors. It is usually not a good idea to remove door casings. But cutting tile to fit around casing is difficult, and usually leads to a sloppy-looking job. So trim the bottom of the casing, and fit the tiles beneath it. With the subfloor installed, place a tile up against the casing. Lay a handsaw on the tile as you cut through the casing. To make sure a door will swing freely after the tiles are installed, place tiles on the floor nearby. If not, use the tiles and a pencil to scribe a cut line at the door’s bottom. Allow for a gap of at least 1/4 inch. Remove the door by popping out the hinge pins. Place masking tape along the bottom of the most visible side of the door, and mark a cut line. Cut through the tape using a circular saw with a clamped straightedge as a guide.
Check floors for level. Use a carpenter’s level and a straight board to check the floor for level and to find any spots that are not flat. If the entire floor is out of level with the wall, it can still be tiled. If you plan to extend tile up the wall, however, you should consider leveling the floor or using tapered baseboard to make the transition attractive.
Planning Transitions - A newly tiled floor may be higher than the adjoining floor. Plan your approach to these transitions before you begin any work. The most common technique is to install a transitional piece called a threshold. You can buy metal or wood thresholds that are sloped to ease the transition. Some are designed to be installed after the tile is laid and grouted, and others should be installed at the same time as you lay the tile. Using a table saw, you can make your own threshold out of oak or another hardwood.
Level a floor. Small bumps in the floor must be dealt with before you begin tiling. If the wood subfloor comes up in places, try driving screws through the flooring and into a joist to level it out. You may be able to take out small high spots with a belt sander. To straighten out dips and low areas, or to level out an entire floor, use a self-leveling floor patch. These are made by manufacturers of tile adhesive. Place barriers where necessary to keep the compound where it belongs. Mix the dry ingredients with water, then pour it on the floor. The mixture will level itself out to a certain degree, but use a long flat trowel to help things along. The compound should be cured and ready for tiling within a few hours. Most self-leveling compounds are intended to function at depths no greater than 1 inch. If the work seems intimidating, talk to a contractor about preparing a level subfloor for you.