There’s a lot of information (and mis-information) online about what constitutes a suitable subfloor for tile, and it’s tough to find a clear answer. The difficulty is that tile can be installed over a number of substrates, and each requires specific preparations to ensure a long-lasting tile floor. To determine the requirements for a tile subfloor, I partnered with Jim and Rich from Diamond Tile located here in Maryland. Jim and Rich are professional tile installers with over 27 years of experience and have worked with a variety of tile substrates.
Deflection, or bounciness, can cause tiles and grout to crack or come loose, and the tolerance depends on what type of tile is being installed. For instance, ceramic tile can withstand more deflection than natural stone.
Many tile contractors can judge a floor’s deflection by walking around. To calculate a specific value, the Tile Council of North America (TCNA) offers a standard formula.
Divide the total span of the floor joists (in inches) by 360 to determine the maximum amount the floor can give in the middle under a live load of 40 lb./sq. ft., plus any long-term deflection due to the weight of the floor. For example, the maximum deflection for a joist span of 15′ is [15 * 12] / 360 = 1/2″. The L/360 formula is useful for most ceramic, porcelain, and hard stone. For soft-stone tile, such as limestone or light marble, the L/720 formula applies (cutting the maximum allowable deflection in half).
John Bridge Forum also offers this helpful deflection calculator.
According to TCNA, there are several factors that affect deflection including condition of the subfloor and joists, joist size, type and spacing, and how the subfloor was fastened.
Adding another layer of plywood can reduce deflection and curvature of the sheathing between joists. In the past Jim and Rich have glued and screwed plywood, taking care to stagger the joints and gap the panels 1/8″ on each edge. Schluter recommends using ring shank nails or screws, turning panels perpendicular to joists and not aligning joints with joists (pg. 23 of DITRA Handbook). In extreme cases Jim and Rich have advised additional joists be put in place.
Tile over Wood Subfloor
Plywood and OSB are the two most common materials used for floor decking, and typically they are 5/8″ or 3/4″ thick. In this scenario, Jim and Rich install 1/2″ cementious backer unit (CBU) resulting in at least an 1-1/8″ thick subfloor. If they install a second layer of wood underlayment (3/8″ plywood or thicker), the guys would use 1/4″ CBU on top.
Older homes may have a structural plank subfloor, and these can be problematic because they move so much with changes in humidity. In this case, the guys usually install 1/2″ plywood overtop and 1/4″ CBU or DITRA.
Installers sometimes use fir plywood in lieu of CBU. While acceptable, Jim and Rich prefer concrete board because plywood can contain voids, and in rare cases the plywood may delaminate. Also, mortar creates a stronger bond with a concrete board.
Tile over Concrete Subfloor
It’s also common to see tile installed over a concrete subfloor, and the big concern is movement from cracks, or expansion / contraction. Jim and Rich often flash the floor to create a flat surface, and they always recommend installing an uncoupling membrane like DITRA overtop concrete.
Tile over Tile or Resilient Flooring
Tile can be installed directly over existing tile given that it is free of cracks and there are no loose tiles. Jim and Rich advise using a bond-promoting primer to enable the mortar to better adhere to the existing floor, and to set the floor with a specialty polymer modified mortar. If the floor is uneven, it can be flattened just like a wood or concrete subfloor.
Jim and Rich don’t advise tiling over resilient flooring. It’s too risky not knowing the condition and materials of the subfloor, and so they always remove resilient flooring.