Q: The Schluter manual recommends that tile underlayment be installed so that the distance between its edges and the nearest joist is one-quarter of the joist spacing. What is the logic behind this advice?
A: Bryant Bouchard, an installation specialist at Schluter Systems, responds: This recommendation seems counter-intuitive, but it’s quite ingenious once you take time to understand it.
Typically, the subfloor (which in many cases is 3/4-inch T&G plywood or OSB) bends the most at the center of the space between the joists when a load is applied. With 16-inch-o.c. joists, the center is approximately 8 inches from the joists on either side. At the same time that a load is bending the panel down at the center, the areas of the subfloor that are under the second-most amount of stress from the applied load are directly over the joists. Those areas want to lift under the load. So the area in the subfloor that is changing the least—or is under the least amount of stress—is halfway between the center of the panel and the joist, or at the quarter point of the joist spacing.
When installing underlayment for tile, you try to have the least amount of movement to make the floor assembly as stable as possible beneath the tile. The weakest spots in the underlayment are the seams, so when you’re putting down the underlayment before applying Schluter’s Ditra Mat, we recommend you place the weakest points—the seams—of that second layer of plywood over the most stable area of the subfloor—at one-quarter of the joist spacing. So with 16-inch-o.c. joists, the quarter point would be 4 inches from the joists; with 19.2-inch spacing, it would be 5 inches; and with 24-inch-o.c. joists, the quarter point would be 6 inches from the joists. (Be aware that with wider joist spacing, a thicker subfloor might be needed to meet the deflection requirements.)
And while we’re on the subject of underlayment, always remember to leave the proper gap between sheets of underlayment to allow for expansion. We recommend 1/8-inch gaps between sheets and a 1/4-inch gap around the perimeter of the floor.
