Cementitious-fiber backer board, installed over a wood subfloor, is the industry standard for professional tile setters. After ensuring that the subfloor is sound, the installer applies a layer of thinset mortar to the subfloor, beds the cement-fiber board in the thinset and then secures it to the subfloor with screws. The thinset forms a strong masonry bond between the underlayment and the subfloor, creating an ideal surface for laying ceramic tile.
When installed correctly, ceramic floors are hard-wearing, require very little maintenance and can last for decades. If they do fail, the culprit is usually an inadequate substrate or faulty installation. Because ceramic tile is rigid and unyielding, the base beneath it must be structurally sound, level and free from settling or movement.
Cement Backer Board
One of the great things about ceramic tile is that you can install it right over concrete, which makes it a sensible choice for basements. As long as the concrete floor is clean, level and free from anything more than a couple of hairline cracks, a layer of thinset is all you need to set the tiles. The caveat here is that if you tile over cracks or expansion joints, the tiles could crack or pop if the floor settles.
Tile Over Existing Flooring
You can install new ceramic tile over existing ceramic tile as long as the existing tile is in good shape. If any tiles are cracking or popping, though, it’s best to remove all the tiles and then install cement backer board before laying new tiles. Always remove existing hardwood, and install backer board before laying new tile.
Vinyl tiles and linoleum must also come off before installing backer board and ceramic tile, but if the flooring is older than 1980 or so, there is a chance that the flooring, or the adhesive used to install it, contains asbestos. Only an asbestos-remediation professional should remove flooring if you suspect the presence of asbestos.
Other Underlayment Types
Manufacturers of underlayment membranes -- the new kids on the block -- such as Ditra, claim their flexible, corrugated membranes reduce the risk of stress cracking when installed with adhesive or thinset between the tile and the substrate. Cork underlayment, which adds a noise control factor, might also reduce minimal stress cracking, but don’t install it over floor expansion joints. Luan, an underlayment used with vinyl or linoleum, is not suitable for use with ceramic tile.
What Lies Beneath
Before installing underlayment on a wood subfloor, inspect the framing. If your home has floor trusses instead of a stick-framed floor, a professional tile installer might not guarantee the floor against future cracking. Floor trusses tend to move, or deflect, which increases the risk of grout-line cracking and tile pop. Building codes use the formula L/360 (length of the truss divided by 360) to determine the maximum allowable deflection. In a 20-foot truss, maximum allowable deflection would be a sag in the center of no more than 2/3-inch. Even after the installation of the subfloor and concrete backer board, which add rigidity to the floor, the leftover deflection is often too great for successful ceramic tile installation.
Substandard stick-frame floors, often found in older homes, can also experience some bounce, often signified by china rattling on shelves when someone walks across the floor. It’s a good idea to ask a reputable contractor or engineer if your home’s floor system is adequate for ceramic tile.
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