Subfloor Deflection Under Tile
Tile does not flex and a timber floor does, which is why walk in shower installation lakewood ca nexusphiladelphia measures deflection before any large format tile is ordered, since a Lakewood shower deck that moves an eighth of an inch under load will crack the grout first and the tile shortly afterwards.
The Standard
Ceramic tile wants a floor no more flexible than the span divided by three hundred and sixty, and natural stone twice that. Those numbers come from the joist size, the span and the spacing rather than from anything visible. A twelve foot span on two by eights at sixteen inch centres is already marginal for stone.
Measuring It
Walk the floor and watch a level or a glass of water on it, then check the joists from below for size, spacing and any notching a previous plumber left. A single notched joist under a shower is a common find and it removes a surprising amount of the strength that the calculation assumed was there.
Stiffening a Deck
Sistering a full length member alongside each joist is the most effective and the most work. Adding blocking between joists helps with twist rather than with deflection. A second layer of plywood glued and screwed over the existing subfloor adds real stiffness and adds height, which then has to be resolved at the door and the threshold.
Uncoupling Membranes
A membrane between the deck and the tile allows small in plane movement without transferring it to the tile above. It is not a substitute for a floor that is too flexible, and it will not save tile over a bouncing deck. Used on a sound floor it is excellent insurance, particularly over a seam or a change of substrate.
Flatness Is Separate
A stiff floor can still be out of plane, and large format tile shows every hollow because it cannot follow the surface. Straightedge it in both directions and correct with a self levelling layer where needed. Deflection and flatness are two different faults with two different fixes, and passing one says nothing at all about the other. Check both before the tile is ordered, because the correction for either one adds height to the floor and that height has to be resolved at the threshold rather than discovered there.