Most marble tile is 1 cm thick (3/8" +/-). What you have is fine for the floor, so long as your floor is up to standard for natural stone. It has to be literally twice as stiff as for any other flooring, including ceramic tile.

Once again Bill, you are a source of endless information! Thank you.

I have seen that repeatedly (the additional thickness for stone) and have adhered to it on blind faith -- but what is the reason??

It's not just additional thickness of the subfloor. It's also additional strength of the joisting. The reason is that stone will tend to crack much easier than ceramic with the slightest deflection (bending) in the floor, so for example, whereas 2x10 joisting, 16" on center would be fine for ceramic tile up to an unsupported length of about 13 feet, with marble, they could be no longer than about 10', without additional bracing. As for the thickness, you can use 1/4" cement board over 3/4" plywood to set ceramic over. With stone, you need an additional 3/8" of plywood in a second layer before the cement board, first.

3/4" plus a minimum of 3/8" ply, and then either 1/4" cement board or Ditra. One thing about Ditra that alot of people don't realise-- you have to meet deflections requirements BEFORE the Ditra goes down, not as a result of it.

Should I assume that installing marble is the same on any floor, bathroom or otherwise?

Absolutely. The stone doesn't know what room it's going into! :-)

Since it seems that tile requires more subflooring than wood I'm wondering whether it's just a calculated risk that one takes that the tile will crack?

There are four possibilities I can think of:

1) They're taking their chances using 1/4" cement board directly over the subfloor.

2) They're taking their chances using 1/4" plywood and going directly over THAT (problem is that 1/4" ply tends to delaminate under tile)

3) They're using 3/8" ply and tiling directly over it, and letting the stone ride just a hair high of the hardwood

4) They've added the extra layer under the entire floor, and used 1/4" cement board under the stone to bring it up flush to the wood.

As for the charity, thank you. I appreciate it, as do alot of police dogs across the country (link's below). 100% of every donation goes toward buying bulletproof vests for the dogs. Please don't feel this is necessary, because it's not. If you do wish to donate, though, thank you.

Here is a link that might be useful: Vest 'N PDP

Thank you Bill. Dog vests it is! You never cease to surprise! (Until I can truly give back through experience, it's the least I can do to show my appreciation for your patient support.)

OK, I suppose I could be brave and post to the John Bridge website but GW is my safe zone for the moment. I went over there to use the Deflectometer or whatever it's called, after looking at my drawings. I have 2x12 DF#2 joists in the addition and 2x12 (unspecified circa 1920) in the existing, both 16" on center. (The bathroom floor will straddle new and existing.) So, I have everything I need to enter into the Deflectotellmemeter except the unsupported joist length. How do I figure that out? Is that just the width of the room running the same as the joists run?

If so, it looks like those 2x12s will save me and my marble flooring plans!

Your unsupported length would be from the foundation to the center beam, unless you have any walls up downstairs.

OK, bear with me. I had walls downstairs but they are being replaced with additional posts and beams. The joists will be attached to the beams with Simpson hangers. Is that considered supported or unsupported for purposes of figuring deflection?

Any place where there's support holding up the joists, it's considered support in relation to deflection.

At the risk of obfuscating communicative efficacy of this electronic missive -- here are some thoughts (questions?). :)

With the limited exposure I've had to tile work - this is what I've learned:

Tile, directly on plywood, can peel off and cracks easier

It is best to use concrete backer board - but it is very thick (3/8" ?)

Another alternative that I've found is hardy backer board (gypsom board) -- half as thick as concrete backer board but very rigid and works well with mortar.

I've used concrete backer board for all floors and inside tub areas. I also tiled a ceiling above a tub, but used the gypsom board for the ceiling.

I have used gypsom board for the backsplash behind the sink and stove as well.

- is the gypsom board rigid enough for floor use? Perhaps... but I wouldn't test it with natural stone.

I have also found some specialty thresholds that make a nice transition between a high tile area and a low carpet area.

The side profile of the threshold transition looks something like:

./``````\
/

Although it is sbtle enough that you wouldn't even notice it unless you were looking for it.

At the risk of obfuscating communicative efficacy of this electronic missive

Talk about self provocating! :-)

Tile, directly on plywood, can peel off and cracks easier

Short answer-- I disagree.

Longer answer-- If done properly, it's actually a much stronger installation. However, the person doing the installation must be VERY familiar with the procedures for installing over plywood. I wouldn't even trust most pros. All it takes is one shortcut or missed spec, and it can cause the floor to fail. something so small as the species of the plywood is all it would take. So, for all intents and purposes, you're better off with the CBUs and membranes available today.

It is best to use concrete backer board - but it is very thick (3/8" ?)

There are two thicknesses of cement board (CBU)-- 1/4" and 1/2". So long as you have a 3/4" subfloor (for ceramic, or 1 1/8" in two layers for stone), you can use 1/4" CBU. The 1/2" is for walls.

Another alternative that I've found is hardy backer board (gypsom board) -- half as thick as concrete backer board but very rigid and works well with mortar.

Completely wrong. First, as I just stated above, cement boards also come in 1/4" version. Even with Hardi, it should be 1/2" version used on walls. Secondly, it is NOT a gypsum core product. It's a cement/ fiber board. The only gypsum core product approved as a tile backer is Denshield, and for that very reason, I don't like using it.

is the gypsom board rigid enough for floor use? Perhaps... but I wouldn't test it with natural stone.

Assuming you're again talking about Hardi, it's MORE than strong enough. Keep in mind (and this is why I said setting tile directly over double layer plywood is a stronger installation)-- ALL CBU's, whether they be gypsum core, cement/ fiberboard, or cement board, are there as nothing more than a bonding surface. The subfloor gives you all the strength. Try putting a piece of even 1/2" CBU across a pair of 2x4's set 16" apart, and step on it-- you'll see what I mean. It has practically no structural strength at all. Now do the same thing with a piece of 1/2" plywood. You'll see a HUGE difference. The only strength the CBU needs to have is strength of bond.

I have used gypsom board for the backsplash behind the sink and stove as well.

For any place other than wet areas (those areas where direct constant water spray is imminent), sheetrock is sufficient to tile over, even using the heavier stone tiles.

OKAY!! What else have you learned? :-)

Is 18mm thickness ok for marble flooring?