Published on October 8th, 2015 | by Henri


Bracing floor for the weight of ceramic tiles

Q.  My house is five years old and I am interested in tiling the kitchen and eat-in area with large (12 inch or bigger) ceramic tiles. However, I notice some floor and joist movement when people walk in this area. I am concerned that the large tiles will pop up due to the flex.

The floor itself is plywood (exact thickness unknown) on top of engineered (Silent Floor brand) wooden “I” joists. The span between support walls is 18.5 feet, the joist depth is 14 inches, and the joist spacing is 19 -19.5 inches on center. There is no bridging or blocking. Although the basement below is unfinished, I would prefer not to install an extra beam and lolly columns at the mid point of the span to stiffen the floor.

Do you have any alternative suggestions? Would adding plywood to each side of the “I” joists be effective? This would create a boxed-in joist with three vertical support walls, instead of one. I would think if I staggered the ends of the plywood, the joists would be significantly stronger. I have fairly good access to the joists and could glue and screw the plywood. Would bridging or blocking be effective? I’d be interested in hearing your opinions and options.

A. Here is what my structural engineer friend says with some additional information to help you carry out the repair: “The main problem is a lack of bridging. The depth of the joists is appropriate for the span, but the spacing seems odd (it must be 19.2″, which is one-fifth of 96″) since 48-inch plywood would have to be cut to accommodate it. In any case, for this span and the given spacing of joists, two lines of bridging will greatly increase the stiffness of the floor and prevent tiles from cracking. I would suggest the bridging be either solid plywood diaphragms 5/8 inch thick and solidly blocked in place with screws and glue or steel cross diagonals (Simpson Strong Tie).”

If you opt for the plywood diaphragms, cut them to tightly fit the shape of the I-joists (notches top and bottom), screw and glue 1-inch by 2-inch wood grounds to the web of the I-joist, apply construction adhesive to the space where the plywood will be placed. Drive the plywood diaphragms against the grounds and screw them top and bottom to the I-joist rails and in the center to the web. Screw and glue another set of grounds to the other side of the plywood. This should make the floor rigid enough for tiling.

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