||01-06-2013 09:05 AM
This is a pretty common topic on this forum. You need to determine the flex of the floor, which is commonly referred to as the L/D ratio. You can find this from the John Bridge website. To determine the structural flex, you need to know the size of the joists, the spacing, the clear span, and the species. For small tile, minimum floor flex is supposed to be 1/480 if I remember correctly, and for natural stone or large tile (24"x24") you need 1/720.
You also need to determine the flex between the joists, which is determined by the number of layers of subfloor, the type of subfloor, and the thickness of subfloor. One of the tile setters on this forum can describe minimum requirements between the joists, or you can see a full discussion on the Schluter (they make Ditra) website.