Hi Harry and All
I need to get some opinions on Cement board. The newest installation method is to set the board in non-modified thin set, so that it's not really fastened to the subfloor, which is fine, but now that means that it's not giving any structural integrity to the floor, so in theory, you would have to have 1" and a 1/4 subfloor, BEFORE the cement board, if you want to have enough strength, which would leave you with more than 2" after the tile is installed. If this makes sense, then we might as well use Ditra on every job(it will go on 5/8th subfloor) or at least start doing Mud floors on every job...even that will be less than 2". Anybody following my line of thinking here? :rolleyes:

Hi BRI

We would need to have many pitchers of beer to go over this topic. Too bad we aren't all a little closer to each other.

Part of what I see as the inconsistency in the logic is the minimum underlayment (which the cbu mfrs include the cbu as part of the 1 1/8" or 1 1/4" total )AND then saying that it is not structural.

So you would be better off to have two layers of plywood for strength, with a membrane to protect against the problems with a direct bond to plywood. I'd rather put down a second layer of plywood than a layer of cement board any day.

Almost everyone I talk to agrees that it seems to stiffen the floor, and I swear it seems stiffer or stronger when 1/2 " is down rather than 1/4".

If multiple plys of wood glued together form a stronger board, then why wouldn't a ply of cement board (glued with latex modified thinset, and fastened with nails/screws)also stiffen the floor?

Well, let's see if someone else jumps in.

Rob

I said almost exactly the same thing over at John's site to Croy regarding his limestone over plywood post.
I've only recently started to see why cbu may in fact not add strength to the floor, for the reasons Brian suggested.

We all know that cbu is rather flimsy by itself, but after being installed properly to the subfloor seems strong and sturdy. Now over time when the plywood underneath expands or shifts in any way ... the cbu loses some of it original bond to the plywood and I think that's where the real problem lies.

The perfect installation short of a mud bed would in my mind be 1/2" plywood glued and screwed into the existing subfloor giving a thickness of about 1-1/4" then as Rob suggests ... go over with either ditra or cbu for a total of about 1-1/2". Now you have your strength and also the independent system holding the tiles.

I'm not sure Rob if the 2 plys of cement would act in the same way as plywood because of the lack of a grain.

Hmmmm.... maybe if they invented a cbu product with elongated slivers embedded into the construction to imitate the grain of plywood.

Hey Harry, new product idea: OCB...Oriented Cement Board. Sounds almost as bad as OSB, doesn't it?

What happened to the top secret forum?

Rob

The contoversy was too much to handle ... and I agree that it might have seemed a little rude having a "Private Forum"

My guess is that the cbu, mfg's. don't want to take on the added responsibility of being a load bearing portion of the house.

You can use cbu's 1/2" with 3/4 ply, to make the 1 1/8" requirement for substrate. Just not recomended to make up for the deficiencies (sp?)of the structure.

If you don't need the water proofing of some of the other products, why not use the material the tile has stuck to, for the ages?

Cement- this product has worked well with tile, for the test of time, find me a better more accepted product to stick your tile to.

Chip