suitability of subfloor for Ditra in renovation
Just a quick question about best practices vs. minimum requirements when trying to match the height of a new tile installation in a 1960's house to adjacent areas of existing parquet maple flooring.
My background: I'm a flooring newbie, I've spent past few days reading through posts on this forum, I've looked at TTMAC specs, researched articles in Fine Homebuilding, visited the Schluter website, and have attended some big box store seminars on floor tiling.
My situation: our entrance hall and kitchen floors were signicantly higher than adjacent rooms (connected by openings, not doorways), because layers of flooring were applied over each other. I have just removed these layers (clay tile over 3/16" luan plywood stapled 6" o.c. and screwed 3" o.c. ((yes, I said 3" o.c.)), over layers of vinyl tile over 1/4" underlayment screwed 6" o.c., over layers of vinyl tile over 3/8" particle-board-like-stuff nailed at 6" o.c.) down to the original subfloor. The original subfloor is 5/8" plywood, nailed to the joists with ring nails, and seems to be in decent condition, apart from the occasional daub of mud left by the original drywallers. I was able to unscrew all fasteners from the subfloor (even those with shanked heads) instead of tearing them out.
The floor structure is 2x10's in good condition bearing on steel beams and foundation walls, but the joists are at varying spacings- apparently a result of coordination with the ductwork. The widest joist span is 13', and the weakest section has two single 2X10's at 16" o.c., while the rest of the floor is doubled 2X10's at spacings varying from 16" down to 8" o.c. The deflecto tells me that the weakest section is still suitable for ceramic tile.
My concern: the new tile will abut existing maple parquet flooring which is about 5/16" thick (applied directly over the plywood subfloor). Having removed about 1-1/2" worth of cumulative flooring at this point, I would prefer to have the new tile installation come as close as possible to the surface of the existing parquet, but clearly need to construct the new floor properly. Both Schluter and TTMAC say that Ditra can be applied directly over 5/8" plywood, which would allow me to bring the new tile flush with the existing parquet.
1) Is applying Ditra over the existing 5/8" plywood subfloor advisable? If so, should i be scrubbing it clean of all traces of compound, screwing it to the joists and removing existing nails?
2) If i should add another layer of plywood before the Ditra, what is the least thickness advisable, and should it be fastened to the subfloor and not the joists, or to both floor and joists?
3) If i have to apply another layer of plywood and thus raise the new tile above the existing parquet, am I better off (from a cost balanced against performance perspective) removing Ditra from the assembly entirely and simply applying a thicker second plywood layer?
Any advice you can offer would be greatly appreciated.