The deflection rule for ceramics is L/360, where L = the length of the floor in inches. So for example, if your floor is 12' wide, then 12' x 12" = 144", divide that by 360 gives you 0.4". This rule tells you your floor is allowed to deflect (sag) .4" under normal loading. The rule for stone tiles is L/720, essentially twice as stiff. With the same example as above, the floor would only be allowed to deflect .2" (less than a 1/4" over it's entire span).
http://www.mikeholmesfanforum.com/ a good forum to ask these questions!
