QUESTION

Stone Installation subfloor - For a travertine install, I have 5/8 subfloor existing (square edge)and plan on using 3/4 u/l. AC fir grade is very expensive, cdx fir square edge in the middle and 3/4 tongue and groove is still $30. a sheet. If I use a crack suppression membrane like "Detra", which u/l should I use for the project?

ANSWER

Regarding your question about a travertine installation first let me qualify by saying:

1.  Marble Institute of American standards say your floor can have no more deflection than L/720.  You need to make sure your structural engineer verifies your floor meets that requirement.

2.  It sounds like you intend to avoid using a mortar bed installation.  You must realize that any other method will not allow you to fully compensate for the irregularities of the floor and achieve a completely flat or sloped surface that will lead to other tradeoffs in the finish surface.

3.  You could apply an anti-fracture membrane but that gives you no additional structural integrity nor will it compensate for too much deflection.  The Schluter Ditra Mat is more than just an anti-fracture membrane; it also provides a cementitious underlayment.  If you use that product I suggest you contact Schluter and ask what type of exterior grade plywood they recommend you use over your existing subfloor.  You might be able to use a thinner sheet of plywood depending on spacing of your joists.

Good Luck!