QUESTIONStone 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?
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.