[frazwood]

Hello, I just want to butt in for a quick second to not only provide you with some additional info, but also because I firmly believe that for each person who posts a question, there is probably 7 to 10 who read to research but never post a question of their own.

Your floor, as it is described by you, meets the minimum standard for a ceramic installation but not stone. Stone needs a floor that is twice as rigid as that which is required for ceramic and unless the floor is designed at the time of construction for a stone surface, it is not structurally rigid enough for a stone installation. It is not because of the weight of the stone but because of the natural fissures and fault lines within the stone that create weak spots along which it will easily crack. You would need to add an aditional layer of at least 1/2" bc ply prior to installing your cement board. This second layer is never to be glued, only screwed. It is also to run 90 degrees to the joists, even if the subfloor panels run 90 degrees to the joists. The strength axis of the ply is along the face grain.

Your joist structure would also need to be twice as rigid for stone. That's not as easy to accomodate on an upper level of the home

Whatever was listed on a hardwood floro site for nail holding power may apply for hardwood, but you cannot install a cement board over a dimensional lumber subfloor. To clarify for those who may read this thread doing research, you would need to install a layer of 1/2" bc plywood over the t-g plank subfloor first then you could install your cement board or membrane.

It is good that you did not follow the advice to take out the original subfloor and replace with AC plywood. If you had, you would have needed to overlay the new subfloor with an additional layer of bc or better 3/8" plywood as the minimum of properly installed glued and nailed 3/4" subfloor requires that it be t-g plywood. Replacing it would have significantly reduced the rigidity of your floor.

Good luck with your project and come back with any other questions. If you have not used Hardi before, it is a very thirsty cement board. Make sure you wet it with a sponge before combing any thinset onto it. Tape your joints as you set the tile so you don't create any ridges to try to ride over. Use a good quality unmodified underneath. If you have a Lowes near you, Megabond with just water, or from Home Depot, Multiset with just water are great choices for the bedding layer. Over top, any modifed thinset, versabond from HD or "Multipurpose" from Lowes will be fine.

Thanks for butting in, if that's what you want to call it. I'll take any and all advice possible -- I'm perfectly capable of making a final decision after collecting the information. :p

As of this moment, we're going to put down 3/4" AC plywood on top of the subfloor and then the 1/4" cement board on top of that. There is some debate as to whether to use 1/2" plywood, but we're probably not going to do that because: (1) my wife actually likes the higher floor because she thinks it'll make the cutting of the transition tile easier (FYI: she does the tile work; I do everything underneath), (2) we like the idea of a stronger, better floor, and (3) we got the 3/4" AC plywood at a ridiculously good price (i.e. better than 1/2" bc plywood).

We actually have a decent amount of experience laying tile -- enough to know that it's absolutely critical to have strong, non-flexing floor underneath. In our last house, though, the subfloor was conceptually simpler for us because it had 1" x 6" boards installed diagonally across floor joists 16" on center(OC) (and we could obviously see it).

Also, that's a very good point about natural stone having weakness/fissures in it. I don't think that we're going to be using natural stone tile, but thanks for the heads up (my wife might want to try natural stone... so that's good to know).

[mtconc]

Bljack.. if I understand correctly you are implying that a stone floor would require a
Minimum substrate of 2 1/2" from the joist up?

[Bljack]

bljack..if i understand correctly you are implying that a stone floor would require a
minimum substrate of 2 1/2" from the joist up?

No, a layer of 3/4 t-g Properly installed and a layer of 1/2" or 5/8" properly fastened would be sufficient for the subfloor and underlayment layers. It's the joist structure that would need the strengthening, either by supporting it from below with posts and a beam to effectively reduce the span of the joists or by sistering them to effectively double the strength.

For ceramic, the minimum deflection criteria is L/360, or for every inch of unsupported span, when a load (300lbs for a ceramic assembly) is placed at mid span, the joists cannot deflect more than one 360th of an inch for every inch of span. This would mean that for a 10 foot span, the joist cannot deflect in the middle more than 1/3". That might seem like a lot, but you wouldn't feel it. More often, the killer is deflection between the joists. That too, must be at L/360, or in numeric terms, the subfloor panels cannot sag between the joists more than 4-one-hundredths-of-an-inch. (.04"). Where on earth the "You must have 1.25" of subfloor" comes from I don't know. If you know what you are doing, how to select the right grade and species of plywood and the proper setting materials and exactly how to dot your I's and cross your T's, that would be the case for a direct to plywood installation.

More plywood is always better since cement boards are not a structural part of the floor, but the minimum is 3/4" tongue and groove plywood on joists 16" oc, attached to the joists with a single bead of subfloor panel adhesive where panels cross the joist and a double bead where two panels join on a joist. The panels must be fastened a minimum of every 6" where they meet on a joist and every 12" where they cross a joist. Providing the joist system is designed to achieve L/360 or better using 40/10 loading, your subfloor between the joists will be between .04 and .025, well within the limits. Follow?

Note: If plywood is to be added to the subfloor, the minimum thickness of plywood allowed in a ceramic floor assembly is 3/8" plywood and the minimum quality is cc plugged and sanded , rated as "exterior" or "exposure 1." CDX, Sheathing rated plywood, pressure treated and all 1/4" plywood products are specifically excluded from ceramic and stone floor assemblies. I'll cover the fastening below.


Frazwood

INSTALLATION OF PLYWOOD UNDERLAYMENT PANELS

Install the panels so the long edge crosses the joist. Do not use glue. Using beads of construction adhesive creates voids between the panels as it does not compress into a full spread layer between the two (as does the thinset used under cement boards) and in time the underlayment panel fatigues (creep) and results in failure. The exception would be full lamination of the panels with a trowelable adhesive but even that is not desirable as you lose the effect of the underlayment helping to isolate the floor above from some of the seasonal movement of the dimensional lumber joists.

The underlayment panels should be placed so that the short edge of the panel ends 2-4 inches past the joist, not on the joist. Offset the long edge from the subfloor panels by a foot or more. Stagger the panels so that 4 corners do not come together. 1/8" gap between panels and 1/4" gap around the room. Fasten the panels with coated screws every 6" around the perimeter and every 8" within the field, fastening in straight rows so any bowing in the plywood is removed. Do not fasten the underlayment into the joists, fasten it only to the subfloor panels. Excessive bowing in the underlayment panels can be removed by setting a circular saw at the depth of the bottom veneer and making a series of cuts along the length of the ply, in dashes, to release some of the pressure of the odd number of veneers. Since it is along the strength axis, it does not reduce the strength of the panel.

Happy tiling :D

[mtconc]

You are correct in the maximun deflection. The 1.25" comes from ansi spec A108.
It is good to see so much info from a pro like yourself. People like you make us all better..
I never stop learning myself.. thanks again

[Bljack]

ANSI 108.01.3.4 19/32" sub with 1.25" mud bed, 19/32" sub with 7/16 cement board, 19/32" sub w/ 15/32" underlay with organic adhesive bond coat.

ANSI 108.11.4.4.1 (covering interior cementitious backer units) 19/32" or thicker subfloor (with deferring to manufacturer minimum subfloor requirements, often 3/4" t-g ply for 1/4" cbu)

:)