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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.


7 Responses to Ceramic Tiles on Wood Subfloors

  1. Our home was built in 1969. There is the original tiled (squares) floor and a linoleum floor over the original. We install a underlayment and linoleum, plus stick-on tile flooring since we lived here. Today we have removed the linoleum and stick-on tiles, leaving the underlayment. Now we want to install Ceramic Tiles on the underlayment. Do you envision install issues?

  2. Your article certainly provides some food for thought. However some explanation on how to measure the 'springiness' of my floor would be greatly received. Maybe further explanation of joist span, size, spacing etc.

    Thank you for taking the time to post this article. I'm still contemplating the use of stone on my raised floor with chipboard covering the joists. I many contradicting arguments from flooring suppliers.

  3. I am looking for recommendations for the most solid and stable 1/2" plywood to use for a bathroom floor which will be tiled. I know it should have exterior grade glue, but don't know which grade is the strongest and most stable. the wood will be glued and screwed on top of Series 360 truss joists that are 16" on center. the bath floor area measures 63" x 65". what would you suggest? Baltic birch? or? (i think I've ruled out advancetech and dryply because of the OSB, and lack of product history) thanks for ideas and benefit of your experience.

    • If the only thing you're putting on top of the truss joists is this plywood, it's NOT ENOUGH.

      Re-read the column above. The MINIMUM wood thickness over the truss joists is 1 and 1/4 inches.

      Use minimum 4-ply plywood - Douglas Fir if you can find it. Just put on two coats of urethane on all sides and edges BEFORE it's installed to protect it from water in the future.

  4. I am putting in a bathroom in the basement and to get the drain pipe at the right slope I cannot bury it, but could get it down to the level of the concrete floor. My question is whether I could just cut a 6" wide trench, lay the drainpipe in, and then cover the entire floor with cement board and then tile it. Is the cement board strong enough to span the 6 inches and support the tile on top? Thanks!

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