Designing Kitchens and Bathrooms - Putting in new tub question

I have one of those americast plastic tubs that cannot be set in mortar. (I am tiling this bathroom) My question is, should I set the tub on the plywood subfloor or go ahead and put down the cement board and just set the tub on top of that?

I have one of those americast plastic tubs that cannot be set in mortar.

I not following this statement. Why not. It'll need the support.:thinker:

I not following this statement. Why not. It'll need the support.:thinker:

Under the tub is a support system that keep the tub base from actually resting on the ground. It says mortar is not recommended, to use shims where needed? Can you set a tub on the cement board?

No reason you cant set a tub on cement board. That being said most times its just set on the plywood, then cement board is put down up to the tub.

No reason you cant set a tub on cement board. That being said most times its just set on the plywood, then cement board is put down up to the tub.

well here's another possible problem. there were two 3/4" sub floors put in that bathroom for stregnth reason im guessing. The top one was bad so I ripped it out. Will the cement board add the stregnth needed to support the tub? As i have it now, after the cement board and tile, I will be flush with the hard wood in the adjoining room. Therefore i dont really want to replace the subfloor I ripped out or I will be stepping up into my bathroom. I guess I COULD just put the cement board up to the tuba dn add more ply wood under the tub. Is there any logic in that?>

If your floor needs strength then yes plywood is better. Cement board does not give your floor strength like plywood does. Cement board's main function is to provide a good bonding surface for thinset and tile. It adds little to no strength to the floor. Cement board under the tub wont hurt anything but plywood will indeed make the floor a little more stiffer.

You had two layers of plywood down and you removed one. Is that the case? Heres an option for you. Put down another layer of 1/2" plywood on the entire floor. Then put the tub in place. Then 1/4" cement board on the area to be tiled. Are you planning on installing ceramic tile or a natural stone product? Please say ceramic, please.:)

If your floor needs strength then yes plywood is better. Cement board does not give your floor strength like plywood does. Cement board's main function is to provide a good bonding surface for thinset and tile. It adds little to no strength to the floor. Cement board under the tub wont hurt anything but plywood will indeed make the floor a little more stiffer.

You had two layers of plywood down and you removed one. Is that the case? Heres an option for you. Put down another layer of 1/2" plywood on the entire floor. Then put the tub in place. Then 1/4" cement board on the area to be tiled. Are you planning on installing ceramic tile or a natural stone product? Please say ceramic, please.:)

yes im using ceramic. If I use the 1/2" plywood, then the 1/4" cement board, then the tile, I may be slightly above the hardwood in the connecting room. Will this cause a problem or will a simple threshhold make it look okay?

The height will not be an issue. Once you know exactly where you stand with the transition of the two floors we'll come up with a good solution for you. Its not a problem at all.:)

Are you planning on installing ceramic tile or a natural stone product? Please say ceramic, please.:)

Why so strong against natural stone? Yes it needs to be sealed, but otherwise?

Why so strong against natural stone? Stone requires a floor that is double the strength that's needed for ceramic. Code will usually have the floor assembly at a deflection of L/360 at 40/10 load ratings when joists are 16 oc both along the joist and for the subfloor span between. Stone requires a floor that's 2x as strong both between the joists (which is easy to accomplish with the addition of additional plywood) as well as along the joists. Gaining access to the joists unless over a basement involves tearing out more than most people bargain for. Unless stone is specified as the floor covering during the design phase, it's a lot more work to get it to par. Stone requires L/720 deflection across the whole floor system.

Stone requires a floor that is double the strength that's needed for ceramic.

Does that include limestone such as Travertine Marble ?

Yes, it includes all stone. :)

Joe

Not just the maintenance issues, but as Tilebri described. To many diyers come on these boards thinking that all you gotta do is slap some thinset on the floor and stick the stone down. Theres always structural work to do before natural stone can go down and most folks dont know that til its to late. Most existing floors are ok for ceramic tile but most arent ok for stone.:eek:

Good advise on the stone. I have tried looking up the spec in the IRC and can't find any reference to stone finished floor. The tables all reference a live load of 30psf and dead of 10 psf for joists and sheathing, but nowhere does it mention requirements for a stone finish floor. Can you point me in the right direction or give me source to refer to. Many thanks.

IRC provides minimum requirements for a safe structure. The requirements for certain finishes may exceed the minimum for code. Tile and stone are an example of those finishes. The TCNA dropped the L/360 requirement, instead changing it to verbiage that dictates not only does the structure need to conform to IRC but also to have the additional load of the tile or stone considered in the planning or the structure.

Thanks for all the help. I found some excellent documents regarding setting stone from the Marble Institute of America. In addition to structural information they also provide information on acceptable setting techniques and products; acceptable grouting techniques and products. Very detailed and valuable information. Again, thanks.