Sorry if this is a repeat, I am new here and didn't find an answer to my specific question. Here is my situation - built 1947, joist and subfloor in good shape. Subfloor is diagonal 3/4 boards nailed to 2 x 10 joists with about 1/4 in. gap between the boards. I would like to use the THINNEST stiffest backer for real slate tiles (ashlar pattern, 12 in and smaller), and I thought maybe if I buy thin (1/4 or 3/16) and double it, screwing, overlapping the seams. everything I read says 1/2 inch, but this seems like overkill if using concrete board. Wouldn't this be stiffer than using 1/2 inch? . Help?
Since you don't mention the length of the joist, it's almost impossible to determine the floors ability to handle the load. Head on over to the John Bridge forum and use his Deflecto meter, and that'll give you an idea of what will and will not work.
I'm sure a pro will chime in...but the backer board or underlayment is NOT typically considered to improve the structural integrity of the sub-floor. Using two sheets of 1/4 is nothing but more work than using a single 1/2". What ever you end up using...make sure to use thin-set between the backer and subfloor.