[Editor's note: This article originally appeared as a sidebar to the article Tiling Over Plywood Subfloors, JLC Mar 2011]
For ceramic and stone tile installations, two different kinds of floor deflection must be measured: uniform and concentrated. Uniform deflection is the movement of the subflooring or setting bed as measured across the entire floor span when a load is spread evenly over the floor. Concentrated deflection is the movement of the subfloor or setting bed measured between two joists under a concentrated, or point, load.
In simple terms, deflection is the amount of sag or bounce that results from loading a particular floor with home furnishings, office equipment, people, and whatever else usually occupies the space. For ceramic tiles and some stone tiles, allowable deflection is usually limited to L/360; that is, the maximum deflection of the floor under tile cannot exceed 1/360th of the length (L) of the span.
When measuring uniform deflection, the span is the overall length of the floor from one support to the other. When measuring concentrated deflection in a wood floor, the span is the clearance between two joists. So for a 30-foot (360-inch) span, L/360 deflection is 360 inches divided by 360, or 1 inch. Allowable deflection between joists on 16-inch centers would be 14.5 inches divided by 360, or .040 inch, just under 3/64 inch.
To test uniform deflection, I typically load a 4-square- foot area at the center of the span directly over two joists. Using tile, sacks of mortar, or water containers, I place at least 300 pounds on the floor and measure the movement with a dial indicator positioned below one of the joists. (You might also use a laser, but it would not be as accurate as my indicator, which has a .001-inch readout up to 1 inch.)
To measure concentrated deflection, the load must be positioned so that it bears midway between joists. Be aware that floor systems that meet ceramic tile's L/360 requirement for uniform loading may fail the concentrated loading test. Stone tiles may require L/480 to L/1080, depending on stone type and thickness.
