I am currently working on a Bathroom and shower--shower was just completed today.

Some background

Tile is a polished blanco marble and is a 13/32" thick--(just a hair over 3/8) 12"x12"
1/16 grout spacings.

Room measures 8 ft x17ft
Joists run length wise--supported by a 4ply beam at the 8 ft mark--so both spans are 8ft
Joists are 2x8 SPF #1
Joists are cross bridged--not solid blocking @ 4' off beam
Subfloor is LP Topnotch 450 series 22/32" OSB
Floor is screwed down every 6" in the body and 4" on the seams--ply is glued to joists.

There are a second set of joist running perpendicular to the first set all supported and shimmed at the cross points.

Needless to say this is a very solid floor.

To meet the requirements for Ditra- I am putting down an additional 3/8ply.

My question is this--should I thinset down the second layer of ply to the original?
Other than time and some thinset is there any drawbacks that I am not seeing?

Oh yeah the reason for all of this is that there is a clawfoot tub as well as the shower.

No, do not thinset the plywood down.

Go to the Schluter web site and download the Ditra Installation handbook. There's a section in there that shows how to apply second layer plywood underlayments.

Just remember spec's from Schluter are minimums. I personally, would add 1/2" plywood plus regular Ditra.

Thanks Randall

I know that Schluter said that I didn't have to, but again--was thinking minimum vs better.

Will post pics of finished floor-will have granite accents to match the shower I just posted.

For this application thinsetted plywood is not in the better category.

Looking forward to some photos.

Thanks for replies,

I went with 1/2" plywood, staggered as per Schluter, put down with 1 1/4" screws.

Saturated Surface Dry treated the plywood and put down thinset and Ditra today--back tomorrow to start tiling.

I have the option to place Granite pieces under the feet of the claw foot tub instead of the marble--the tub scares me--any thoughts?

Its alot of weight on four little areas I figure the granite might take it better--also thinking about isolating them with silicone instead of grout--am I just over thinking this?

Anyone done natural stone and claw foot tubs have any suggestions I am all ears.

Can you elaborate on that one?

As long as you have 90 to 100% coverage under the stone where the feet sit, you will likely be ok. Make sure the coverage is full under the foot locations to properly take the point loads.

Double check the marble pcs that are going to sit under the feet for pronounced veining to help eliminate the potential problem for cracking.

Not sure what you mean by isolating them with silicone? Grout joints?

Thanks Randall

I always sponge down porous surfaces before putting down the thinset. Its wet and you can see that it has absorbed some of the water but is dry to the touch. I was told this helps with bonding, prevents "burning"(drying out vs curing) we did this in the concrete industry when patching as well.

I have read it here and there--once in some Schluter literature--so I assumed it was a standard practice.

What I was thinking was to have the pieces under the feet as granite instead of the marble--it being the stronger of the two--and by isolation I mean, setting the pieces seperately so that I can have an clean joint all the way around--including the thinset and using silicone for the grout joints so that any stress is only on the four pieces. But that puts more stress on those four pieces.

Again I am probably over thinking this one--I just don't want to be called back to replace tile under a claw foot tub.

Given a choice between granite and marble, I'd go with the granite, maybe work it into the tile pattern throughout. this is mainly for abrasion resistance. When it comes to cracking, granite is just as bad as marble. Density, hardness is different, but tensile strength is virtually the same.