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Ceramic Tiles on Wood Subfloors

DEAR TIM: I am thinking of installing a ceramic tile floor in my kitchen. The floor consists of 2 x 10 southern yellow pine floor joists 16 inches on center. The subfloor is 3/4 inch plywood. Is this floor strong enough to prevent cracks in the ceramic tile. If not, what do I need to do before installing the tile? Can I install the tile directly on the plywood? N.G.

DEAR N.G.: The single layer of plywood is insufficient. You will need to add additional underlayment prior to installing the ceramic tile. However, you need to check something else before you proceed. The floor joist span may be too great.

Ceramic tile is a very rigid material. Most ceramic tiles have very little tensile strength. In other words, if you try to bend or stretch a tile it will crack. Some wood floor systems, when designed to minimum standards, have too much bounce. This bounce will crack the tiles.

Floor joists are basically wood I beams. For a given floor joist, as you increase the distance between supports, the joist will deflect a greater amount. Floor joists beneath ceramic tile floors must not deflect more than 1/360 of their span. Assuming your joists are #1 grade and the span is no greater than 16 feet 9 inches, you can proceed.

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You have two choices of underlayment: 3/8 inch exterior grade plywood or 1/2 inch cementitious board. Both will perform exceptionally as long as you install them carefully. Your existing subfloor should be securely attached to the floor joists. Drywall screws work well. Use ones that have coarse threads.

If you choose to use exterior plywood, the spacing of the sheets is critical. The 3/8 inch plywood edges must be offset from the existing plywood edges at least 2 inches in each direction. You must also be sure to maintain a 1/8 inch spacing around each sheet of plywood.The plywood expands with changes in humidity. If spaced too tightly the plywood will buckle and crack the tile. Attach the plywood with nails or screws that will penetrate the floor joists at least 3/4 inch. Be sure to screw the edges.

Cementitious board makes a great underlayment, however, it requires some additional labor. It is necessary to install the cementitious board in a layer of fresh thinset or dry set mortar. This material acts as a filler for any low spots in the plywood. Without this layer of thinset, the board may flex under foot traffic causing the ceramic tile to crack. The cementitious board is attached to the existing plywood and floor joists with galvanized nails and screws.

If you choose exterior plywood as your underlayment, you must use either an organic adhesive or an epoxy mortar as your bonding agent between the tile and the wood. A latex modified Portland cement mortar will create a long lasting adhesive should you decide to use the cementitious board underlayment. Remember, without a stiff floor your ceramic tile job is doomed to failure.

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Author's Notes:

April, 2002

Tile installed over wood floor systems MUST have a minimum of 1-1/4 inches of solid material beneath it. This means you can have a 3/4 inch wood subfloor covered by 1/2 inch plywood, cement board, or approved gypsum fiber underlayment.

If you want an even stiffer floor, be sure the wood floor system is designed to a 1/480 deflection maximum standard in lieu of the 1/360 standard mentioned above.


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