Expert: Dan Griffin - 7/30/2010

Question
I am doing my first tile project. I removed the old material down to the subfloor. I have read that my subfloor need to be a minimum of 1 1/4" thick for stone. The old T&G plywood subfloor is 5/8 and I plan to add 3/4" ACX. The problem I am having is the old subfloor has just a few low spots. I was thinking of using Patching compound ( Henry or Laticrete) to make the floor flat. After the patch was dry I would put the 3/4" down then the rest of the underlayment. Can I sandwich the compound like that in the middle?. And with no readable markings on the old subfloor how can I tell if it is exposure 1, exterior grade and the patching compound will not damage the subfloor? thank you.

Answer
John, using the floorstone between the subfloor layers is fine.  I would either glue the top layer to the bottom layer or install a sheet of rosin or tar paper to prevent flor squeaks,  Make sure you use plenty of fasteners between the two.  If this is an interior application (my assumption) you will not have any problems with leveling compound.  Make sure it one that is recommended for wood floors and if they suggest using an acrylic modifier, I would use it.  You may need an additional application of leveler on the top layer  - check with a straight edge.  The tile adhesive will take up some variations, but you want the tile bed as flat as possible to insure full bond and support.

The big issue is deflection in the floor.  If the joists bounce or move, the tile cannot deal with the deflection.  The larger the tile, the more this is true.  Check with the supplier's tech sheets.  Many tile installations demand a maximum deflection of L/480 or even L/600 even thouygh code for joists is L/360.