Subfloor, underlayment and eventual floor material question

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Old 02-11-12, 10:58 PM
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Subfloor, underlayment and eventual floor material question

Remodeling an old bathroom. Currently have everything down to
studs and joists. The room is 10' x 7', with 2x10 joists spaced 16"oc

There is just over 3/4" between joists and the hallway floor, so I'm wondering what my options for floor material are. Can I do tile? If I did, would I want to go with 1/2" plywood and 1/4" concrete board?Or would 3/4" plywood and no backer board be better? Or does tile absolutely require a thicker than 3/4" subfloor/underayment combination?

Tile isn't a must have, just thought it would look nice if the floor could support it.

Thanks much.
 
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Old 02-12-12, 10:19 AM
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Bad day. You can't use 1/2" plywood as subflooring which will eventually be tiled. Too much movement. You can't use 3/4" plywood without cbu prior to tile. Unless you choose to have a transition to the other floor from the bathroom, your choices are limited.....say linoleum.
 
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Old 02-12-12, 12:27 PM
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Thanks for the response. Just a couple of follow ups:

1. Would some kind of blocking allow me to use 1/2"? Maybe 2 or more blocks between each joist?

2. Is 1/2" out entirely, even if I put down 1/2" then 1/2" then 1/4" concrete backer? (I ask because I have a bunch of 1/2" on hand.)

3. In your opinion, what is the maximum height difference at a transition that would look good and be functional? I don't mind exceeding the height of the hallway, I just can't visualize how much height difference would be acceptable. 1/2"? 3/4"?

Again, thanks for the help.
 
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Old 02-13-12, 04:28 PM
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Your subfloor plywood needs to be something thicker than 1/2". Absolute minimum would be 5/8" and 3/4" would be better. If you go with 3/4" plywood you can use 1/4" cement board or a sheet membrane like Ditra over the plywood. This assumes that your tile will be ceramic/porcelain, and not natural stone. Natural stone is a different ball game.

2 sheets of 1/2" plywood will not be good either. You need for the subfloor layer to be minimum 5/8". Additionally, the subfloor needs to be t&g plywood.

What are you transitioning to in the adjoining room. I assume that the 3/4" you are talking about is just the subfloor in the adjoining room and not the finished floor height.
 
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Old 02-13-12, 06:46 PM
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I believe a single 3/4" would be close to or stronger than two layers of 1/2". 1/2" ply should never be used as a subfloor, even if it were t&g.

Tiled bathroom floor over wood framing will almost always the thicker than the other adjacent floors. Years ago it was common the a bathroom to be 1 1/2 or more higher than the hall's subfloor. Add carpeting or hardwood and you're much closer.

Jaz
 
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Old 02-13-12, 06:48 PM
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FWIW, the existing 2 x 10 floor joists would happily support a tile floor. Assuming they are spanning in the long direction (10') and composed of typical SPF of reasonable quality; even better if they span in the 7' direction. Bridging between all of the joists with 2 x 8s at 12" centers (using joist hangers) just might make using 1/2" plywood under 1/2" cement board stiff enough to keep deflections to a minimum, and make ceramic tile workable. Gluing and screwing both the plywood and backer board would also help in developing enough composite action to further minimize net deflections.
 
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Old 02-13-12, 06:59 PM
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Gluing and screwing both the plywood and backer board would also help in developing enough composite action to further minimize net deflections.
What do you mean by the above? Of course you should glue the subfloor to the joists, but what are you saying about the backer board?

Jaz
 
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Old 02-14-12, 12:26 AM
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Composite action is an engineering term used to describe how the section modulus of load carrying members (in this case, floor joists), can be increased by tightly adding a top flange (the glued and screwed plywood and backer board). The result is both the plywood and backer board act in unison with the joists, and with each other. With respect to stiffening up the plywood, think of it as adding additional plies, strong in compression, to the plywood underneath (assuming a tight-enough bond, of course).

Typical highway bridges designed today make use of the same principle, with concrete decks above the girders tightly tied to their top flanges with shear connectors, resulting in stiffer and stronger composite members. Moreso than things would be if acting individually, and allowed to float "free". I can remember 40 years ago, early in my bridge inspection career, when many structures were non-composite (concrete decks not firmly attached to girder flanges, other than by gravity). Standing at an abutment end of a long-span steel girder bridge, you could actually observe the concrete displace longitudinally (more than 1/4" at times) with respect to the top steel flange it rested on, under passage of multiple heavy trucks out at mid-span.
 
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Old 02-14-12, 12:29 AM
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If 1/2 inch backer board is being suggested in order to add strength to the floor, forget it. Backer board, either 1/2 or 1/4, does not add strength to the floor. That is accomplished with the wood subfloor. The backer board is only a medium between the tile and wood to compensate for the dramatically different expansion and contraction rates between wood and tile. Typically, 1/4 is for floors and 1/2 is for walls. If, on the other hand, 1/2 is being suggested for height compensation, you're better off following the previous advise and using 3/4 subfloor and 1/4 backer board. As to gluing the backer board, the instructions on the sticker affixed to the new sheets of backer board will instruct you to thin set the backer board to the subfloor. The purpose of the thin set is not to glue it in place. It is to bed the board so there are no voids between the layers. Voids allow deflection and tile will not respond well to deflection.
 
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Old 02-14-12, 06:59 AM
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What Smokey and Jaz said.

I'll add that 1/2" plywood and 1/2" cement board will not be adequate for a ceramic tile floor, but rather, is likely to result in failure. You need thicker plywood, and the cement board will add no strength at all. Your theory may work well for bridges but it doesnt fly for tile, nope.
 
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