stone over wood sub-floor

I was contacted by a Contractor to come and take over a stone installation over a wood sub-floor. I was shown the involved flooring areas and given a brief description of the intended tile installation and the sub floor design. About 3000 sq. Ft. 24" x 24" x 3/4" limestone.

Most of the floor was covered with Metal lath (1.75) yd2 on top of cleavage membrane (Aquabar®) on top of crack isolation membrane, (Dal-TS)

The sub-floor panels were comprised of wafer boards ¾” thick, but joints with no gaps for expansion, laid staggered, the joist according to Mr. Grimm are TJI type, 14” high, 16” on center, spanned up to 16’ at some of the areas.
I'm being asked to limit the thickness of the floor finish to the height of the installed baseboard, meaning: to have the tile installed just underneath the existing baseboard. The baseboard is installed to low to accommodate appropriate installation, its height from the sub-floor varies between 1 ⅛” to 2 ¼” (in about 20 ft.) the stone tile thickness is ¾” which leaves ⅜” to 1¼” for mortar bed and bond coat together. Contractor insists on not exceeding the height of the base, claiming that the baseboard was installed very precisely and was brought to a dead level plane, using a laser device, while the sub-floor itself is not level; installing the floor under this conditions, will expose different portions of the base which is unacceptable to the contractor. The contractor would prefer the height differences to be hiding in the mortar bed

Any suggestions about how should I approch It

The tilemen

"BOY" there's a lot of rules already. I'm not sure what can be done to get into compliance with all of that stuff.

What is the load rating of this structure? Does it meet the absolute minimum requirement for a stone installation of L/720?

Who's "tile installation standards" is this contractor following with this approach?

Gil, Is this a new house? If this builder was so precise with his trim work why did he miss so badly on the subflooring? I am sure that this is a good $$$ job but do you know why you were asked to step in, I am assuming for another tile setter, why did that guy leave. Also, by the book you need at least 1 - 1/4" of mud under the tile and that is at the high spot. Is this subfloor built to handle all the weight of doing it correctly? Who is going to warranty this tile work, you or the builder? If you, the install procedures should be on your terms because you are going to bear the burden if a failure occurs. Will this builder develop selective memory in a few months if the installation goes bad and start blaming you for doing a bad job after you properly warned him of the deficiencies in the subflooring? There are a lot of red flags here. Get everything in writing and signed by ther builder, but remember if you accept the job on his terms you are culpable if the customer has problems down the road.

My question is what exactly are the red flags.
You are saing that the minimum is 1 1/4" of mortar
By what book exactly.

I did not see a reference to that measurment in any book and I looked at most of them, MIA Design manual, TCA Hand book, ANSI. CTIOA is actually allowing 3/4" in residential over one layer of 3/4" plywood, the MIA is talking about 2 layer of plywood but do not go into specific thicknes of every one of them nor the minimum thikness of the mortar bed. Is wafer board allowed? What exactly is waferbord is it falling under the definition of partical board, OSB, or mabey nominal board

Fun question.

I really don't see the reason for the cleavage membrane. If the wire lath is stapled to the floor then I really can't see it working as intended. The crack isolation membrane is basically useless with the Aquabar and wire lath on top of it. Wafer board? Under stone? One layer? Huge red flag. Maybe I'm missing something here.

If the subfloor was up to par, I would pull the wire lath and Aquabar and install the tile to the Dal-TS. This would give you an additional 1/4" of height to play with. Then I would wet set each piece of stone, modifying the ingredients of the setting mortar to meet the height requirements.

"Wafer board? Under stone? One layer? Huge red flag"
Think so to, but, do you have any backup, did you see that in any "book"?

About the metal lath: red flag or not, ANSI and TCA are specificaly recomending it in residencial floors
To attach the reinforcing to the sub-floor with fasteners)

Don't need back up. The mere fact it's not listed is testimony to it's discouraged use under tile. The handbook doesn't mention tiling over particle wood either but that doesn't mean it's approved.

Wafer board is not plywood. Wafer board is an early cousin of OSB. In fact OSB was designed to be an improvement over wafer board.

Wire lath is approved for use under 3/4" thick latex modified mortar bed installed over an approved subfloor (which doesn't include wafer board). It's simply an alternative to backerboard.

Cleavage membranes go under mortar beds to allow the mortar bed to move independently from the subfloor. Can't do that with wire lath anchoring the mortar bed to the subfloor. Crack isolation membranes go over the mortar bed- not under it.

It's a real wacky arrangement. I would run from that job so fast I would leave skid marks.