Sound substrate?

Planning a smaller kitchen job. Have ¾” OSB butt joint with ¼” ply on top (and vinyl). Joist are 16 on center and span 12”. According to what I read, should suffice for L/360 although it surprises me feeling how “soft” it feels. Plan on removing the vinyl and probably the ¼”. Plan on using Ditra. Should probably glue and screw ½” ply on top or would that be an overkill? If I do, how should I offset seams and should I place the seams off the joist like when putting in CBU? Should I fill in the Ditra and then lay the tile as I see some do? Should I put the el. Heat under the Ditra as I see some recommend?

Thanks,
Emil, Grand Forks ND.

The joists-- are they 2x8's or 2x10's?

Oh yes, forgot, they are 2x10, which from what I can find out matches code.

Thanks,
Emil

I'll assume your 2x10 joists span 12 feet NOT 12" as you wrote.

I say 3/4" subfloor that is not t&g is not suitable as a single layer subfloor, you need another structural sheet. Therefore please install 1/2" underlayment grade plywood in accordance with good installation practice. No glue. Your subfloor feels soft because it is not t&g.

You need to remove that 1/4" underlayment, take it up with the vinyl still attached.

Once the new 1/2" ply is installed, you can install the Ditra. (Good choice). Filling Ditra and setting the tiles the next day is silly and a waste of time. May even cause a possible future failure.

The tile warming mats are NOT "heat" as in the heating system for the room. They will not heat the room. Yes the mats go under Ditra.

Jaz

Thank you Jaz.

Yes, I ment 12' span. I havn't looked, but I assume Ditra has some good tips on proper install of the ply, seam location, fasterner etc. I am supriced with "no glue" since anything I work with if I want to improve stiffness, I glue "laminates" which this would in essense be. But I take your word for it.

And yes, the "warming mats" are just for cozyness for our bare feet in the mornings, already have forced heat. Although I have used floor "warming" in a basement wc as a sole heat source (only one small outside wall) and it works very well.

Thanks,
Emil

A good article about installing underlayment for a tile installation was published last year by Professor Emeritus, now Wood Construction & Engineering Consultant Frank Woeste, Ph. D. , P. E. , & Peter A. Nielsen, Technical Director, Schluter Systems. http://www.tile-assn.com/tileletter/pdfs/Underlayment-Nielsen-Woeste-0604.pdf

If you could laminate the new underlayment to the subfloor, that would increase the overall stiffness for sure. What you described is not lamination. Laminating means glued over 100% of the surfaces, not just beads. The construction adhesive is thick and therefore creates voids where there is not glue. These voids cause vertical movement which is not good.

Having said that, it is also best to isolate the new underlayment from the subfloor sheet and especially the framing. This is the main reason you fasten the underlayment into the subfloor only.

Jaz