Floor under Flange

[aberger]

I am in the process of remodeling a bathroom and wanted to double check my thinking on something. The flange and waste pipe are in good shape (all lead). Right now there is about 1 1/4" between the subfloor and the underside of the flange. I've already determined that the subfloor should have another sheet of plywood over it before I put down cement board for tile. I'm thinking a 1/4" sheet of plywood, 1/2" sheet of cement board and then 1/4" thick tiles. This equals total of 1" thickness. I figure the thin set mortar that goes between the new subfloor and the cement board and then the thin set mortar that goes between the cement board and tile should add another 1/4", which should then equal up to the 1 1/4" height I need to get the tile right under the flange. I plan on using a 1/4" notch trowel for the mortar. In theory, does my logic sound OK? I guess I'm worried that the mortar will actually increase the height by more than a 1/4".

 

[IanSpaid]

I'm not much of a toilet expert, but it sounds logical to me. But, you might want to get an expert opinion

 

[Sponsor]

 

[Jadnashua]

If the 1/4" plywood is for strength, forget it, you need thicker. Also, if this is luan, forget it big time - never put this down under tile - it delaminates if you even think about moisture - the thinset is enough to do it. If you need it for thickness to match up other things, then use 1/2" cbu instead. If you really need additional strength in the subflooring, you could use 1/2" plywood and a membrane line Ditra from www.schluter.com .

 

[Pitterpat]

Good advice Jim. I was going to tell him the same thing about the 1/4', I think we had the discussion a couple of weeks ago on John Bridge.

Pat

 

[Lithnights]

If the 1/4" plywood is for strength, forget it, you need thicker. Also, if this is luan, forget it big time - never put this down under tile - it delaminates if you even think about moisture - the thinset is enough to do it. If you need it for thickness to match up other things, then use 1/2" cbu instead. If you really need additional strength in the subflooring, you could use 1/2" plywood and a membrane line Ditra from www.schluter.com .

I'm in a similar situation that aberger was in. I have 1 1/4" from subfloor to bottom of flange.

1/2" for cementboard 1/4" for tile. Thus, I need another 1/2" (1/4" if the 2 different layers of thinset takes up 1/4" as asberger assumed.. does it?)

Would 1/2" plywood do the trick here?

Thanks,

 

[Jadnashua]

The cbu is NOT for strength, it is for isolation on the floor, so 1/4" cbu is fine for a floor. If you want to save some height, you can use an isolation membrane instead of the cbu. It is crucial that the floor be strong enough on its own to support tile before the cbu or membrane is installed. One example is a product made by www.schluter.com called Ditra. This won't work if you are going to use a tile smaller than 2" though. When installing the cbu, make sure to follow the manufacturer's instructions - use thinset underneath and fasteners at the prescribed intervals. Use the cbu screws or galvanized roofing nails. The nails are quicker, some find the screws easier, but they can be a pain if you don't have a hefty drill. Watch for "tenting" where the screw pushes the backer up when it doesn't immediated cut into the subfloor, leaving a gap between the board and the floor. This can be fatal to the tile, and makes a nasty hump in the floor, too. The thinset layer will likely be thinner than the 1/8", but that is not a bad estimate.

 

[Lithnights]

The cbu is NOT for strength, it is for isolation on the floor, so 1/4" cbu is fine for a floor. If you want to save some height, you can use an isolation membrane instead of the cbu. It is crucial that the floor be strong enough on its own to support tile before the cbu or membrane is installed. One example is a product made by www.schluter.com called Ditra. This won't work if you are going to use a tile smaller than 2" though. When installing the cbu, make sure to follow the manufacturer's instructions - use thinset underneath and fasteners at the prescribed intervals. Use the cbu screws or galvanized roofing nails. The nails are quicker, some find the screws easier, but they can be a pain if you don't have a hefty drill. Watch for "tenting" where the screw pushes the backer up when it doesn't immediated cut into the subfloor, leaving a gap between the board and the floor. This can be fatal to the tile, and makes a nasty hump in the floor, too. The thinset layer will likely be thinner than the 1/8", but that is not a bad estimate.

I guess I'm still a bit confused. So if you had 1 inch from subfloor to bottom of flange, what combination of plywood and cbu would you use under the tile??? Tile height is a shade over 1/4" (5/16").

I would think either..
1/2" plywood, then 1/4" cbu on top of that then tile OR
1/4" plywood, then 1/2" cbu on top of that then tile

Not sure if I need Ditra in my situation.

Thanks,

 

[OldHouse]

correct me if I'm wrong...

...but why would you tile under the toilet flange. I think that's what you are intending by your post. Maybe I just mis-interpretted it.
Quinn

 

[Jadnashua]

There is no 1/4" structural plywood generally available, so you need, preferably, the sandwich of thicker plywood, then either the 1/4" cbu, or Ditra. And, to address the other comments, ideally, the flange is installed on TOP of the finished floor, so yes, that means tiling under it. To make things easier, you can notch the tile so you don't have to drill through it, but it is better to have it under there.

 

[OldHouse]

Thanks..

... for the clarification. I think I have almost always seen the folk in this area screw them fast to the backer-board without the tile under it. Always seem to learn things here. Thanks again.