Scrap the marine grade plywood, and use exterior grade. No need for marine grade at 3 times the price.

Also, what's with the 7/8" sleepers? You're not on concrete.

There's no need to put durock under the terrazzo base, either, as far as I know, nor is it necessary to thinset the base down. I would think that just the weight of it should be anough to keep it in place.

Concerning the use of the 3/4" stone-- that's usually set in mud instead of thinset. For thinset applications you might look for the 3/8- 1/2" stone.

Also, concerning that this is a stone install, have you checked your joisting to make sure it'll handle stone? The floor must be literally twice as stiff for stone as it does for ceramic or any other type of flooring. You can check it at the link below:

Here is a link that might be useful: John Bridge's Deflectolator

Thanks for the quick reply Bill!

Easy enough to use exterior grade plywood, I was just trying to be cautious (and the total floor space is something like 45 sq. feet (including the shower area so not much wood is needed).

Not on concrete... As I mentioned in the post, the bathroom is resting on the basement ceiling (3/4" tongue and groove planking which rests on 16" 2x10 joists which rests on foundation wall at one end and a 7" wood beam on the other). The sleepers are for air circulation (and needed to match the floor height to the original floor in the hallway).

The durock is probably not required under the shower base, but I want to keep the shower floor at the same height as the floor, I guess I could use 3/8" sleepers and 3/4" ply with 1/8" thinset. I think the purpose of the thinset is to allow minor adjustments to the ensure the shower base is truly level... Its use was specified in the Florestone installation document.

Since the only stone we found was the 3/4" Cotswold Limestone, I think that is what we are going with... How thick is mud? Am I going to need to adjust my sleeper thickness? Any preference for the mud (brand)?

Per the Deflectolator the joists are suitable for stone... If necessary I can always double up the joists.

What about my questions? Hydroban? Vapor barrier? Hydroban with the heated floor? Sealer? Greenboard?

Any advice would be appreciated.

Thanks,

Derek

Are you by chance an engineer? :-)

1) Does everything look okay?

Asked and answered.

2) Should I use the Laticrete Hydroban on all surfaces (floor, ceiling and walls) throughout the bathroom (as opposed to just the shower area)?

Throughout the bathroom would give you no benefit at all. Actually, it's not needed at all, although you could use it in the shower instead of a vapor barrier. You could also use it on the floor, but again, unless you flood the floor on a constant basis, it's really not necessary.

3) Should I put up a plastic vapor barrier behind the walls and ceiling throughout the bathroom (as opposed to just the shower area)?

No. Whichever you use-- the plastic or the Hydroban-- use it in just the shower.

4) If I should use the laticrete on the floor, I assume it should be on top of the Durock... Could it be effected by the heat of the radiant heating mat?

I don't think so, but you might want to call Laticrete on that. (203) 393-0010

5) Any recommendation for a particular mortar?

I'm assuming you're going with Laticrete products, so I'll give you their product names.

Walls and ceiling-- 255 lightweight thinset.
Under the cement board-- #17 unmodified thinset
Between the tile and cement board-- either 253 or 254 modified thinset

Grout?

Again Laticrete has some good conventional grouts. For joints under 1/8", une unsanded grout. For 1/8" and over, use sanded grout.

Sealer for the limestone? Sealer for the grout?

Before I give you a recommendation, take a sponge and put a small puddle of water in the center of a tile. Let me know if it just sits there, or does the tile suck it right in?

6) Regular Drywall or greenboard?

In every place but the shower, either one is fine in most places. However, some do require greenboard by code. You might want to check to be sure.

7) Anything else I should know or keep in mind?

Not that I can think of off hand!!

Thanks again for the reply Bill!

In a previous life I was an engineer... It shows?

Pretty much retired now... The house, and family are my job now.

Just for clarification... The datasheet on Hydroban (and all the Laticrete products I looked at) specified that it was not to be used as a vapor barrier... Perhaps it works like Gortex allowing vapor to pass but blocking liquids.

We are getting a sample of the tile shipped to us, I will let you know the results of the water test when I receive it.

For the mortar under the (3/4") stone would you recommend Laticrete 226? 209 (and should I use the 3701 admix)?

Did you mean 317 under the cement board (as opposed to #17)?

Thanks again!

Derek

Bill,

As I indicated in my first post, I have a total of 3.25" from the basement ceiling to the top of the bathroom floor... If I do not install any sleepers, but keep the plywood, I would have

3.25"
- .75" Plywood
- .75" Stone
----------
1.75"

This dimension assumes the electric floor mat can be placed in the mud bed without additional space being required.

One concern is the weight of a thick mud floor... How much weight would I be placing on the (2x10) joists below?

Actually this is sounding a bit overwhelming... I am starting to think that I might just prep everything and have the local flooring company do the stone install... And to think, I thought you just slapped some thinset on the floor and set the stone in place. LOL

Thanks,

Derek

Even with the thinner stone, there's no such thing as just "slapping it down"... not and being happy with it afterward.

As for weight, I could be mistaken, but I believe that mud adds about another 6 pounds per square foot, based on a 1 1/2- 2" mud bed.