Bathroom tile backer questions?


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Old 09-15-06, 06:51 AM
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Bathroom tile backer questions?

Just planning a bathroom remodel, and was looking for some advice... first off the bathroom is 85" X 60" so it's pretty small, I am going to gut it totally and it will be getting new everything.

First off when the bathroom is totally gutted, I plan on installing a 1/2" concrete backerboard subfloor, which will continue under the tub area, to raise the new tub up as well, is this the normal method?

Second, Once the tub is installed I plan on using 1/2" densshield in the tub area, I know with densshield it is bad to install a vapor barrier behind it, but the tub is on an outside wall, so do I just insulate and throw the densshield up? I have also seen it recommended to use roofing tar and roofing felt behind concrete backerboard in a tub/shower area? is this required with densshield??

Third, I am debating continuing the tub surround tile up to the ceiling, if I do, should I continue the densshield up to meet the ceiling, or end the denssheild and transition to M.R. drywall somewhere high behind the tile? I guess what I want to know is if it will be easy to make a tiled denshield wall to painted drywall ceiling transition? am I making sense? Is 1/2 inch MR drywall sufficient for walls and ceiling with 1/2" densshield in the wet tub area?

Thanks for any insight you can provide, it is greatly appreciated!

Matthew
 
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Old 09-15-06, 08:52 AM
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Matthew

When you say 1/2" concrete backerboard as a subfloor, do you really mean underlayment? The subfloor is the first layer attached to the joists. The underlayment is what you will set the tile to. I just want to make sure that you are installing the backerboard over a plywood subfloor and not directly to the floor joists. Cement backerboard is not structural and cannot be installed directly over the floor joists. It has to be installed over the plywood subfloor. That being said, most people use 1/4" backerboard on floors instead of 1/2" backerboard. If you can afford the additional height or are trying to increase the height of the floor to transition to other rooms, the best way to do that is to add another layer of plywood which will indeed maker your floor stronger. You can if you want to run the cement board under the tub but its not necessary to do so.

I don't use Denshield, but have done some reading about it in the past. They specifically tell you not to use a moisture barrier behind the Denshield. You could call Georgia Pacific to see what they recommend for outside walls. Sorry I cant be more help on this.

Run the denshield up the entire wall. Tape the wall to ceiling joint with regular drywall compound. You can tile up to the ceiling and paint the ceiling accordingly. Most dont use the MR drywall anymore as its proven to be no better than regular drywall. Unless is required under code in your area, I wouldnt use the MR drywall on the ceiling. Id use regular drywall.
 
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Old 09-15-06, 10:22 AM
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Thanks Herejohnny, I was actually referring to the 1/2 inch concrete backerboardbeing used as an underlayment. the actual sub floor appears to be 3/4" t+G planking, I planned on using the 1/2" concrete backerboard instead of the 1/4 to add as much stiffness to the floor as I can... The house was built in 1953, and from what I can see it appear to use 7.5"x1.75" (actual measurements) joists at approximately 20" O.C. (measures approx 18" between the joists). The span of these joists is approximately 11 feet?

Right by the entrance door to the bathroom there is about a 10" diameter "spot" that is lower than the rest, it doesn't really feel spongy or anything, I'm not really sure what is the cause, until I start the project and tear up the vinyl floor that is there... I assume it is some kind of patch? as long as everything I find subfloor wise is solid, I planned using some self-levelling compound, then cover the 3/4" t+G subfloor with 1/2" concrete backerboard.... Keep in mind this bathroom is 60" wide and 85 inches long, with the bathtub on the outside wall, will I be safe with probably 12" sq ceramic tile??? the actual total tiled area will be less than 25 ft2.... Plus I plan on installing an electric radiant heating wires under the ceramic..

Thanks for your help!!

Matthew
 
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Old 09-15-06, 11:26 AM
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Matthew

Your joist system should be good for ceramic tile. The 3/4" planking is not a suitable subfloor and you can not just put cement backer board over it. These type floors have too much movement in them. You need to put a minimum 1/2" of plywood over the planking, then a cement board or sheet membrane. Additional plywood will make the floor stronger but the cement board will not. The purpose of the cement board is only to make a good flat surface that the tiles will adhere to - Cement board is in no way structural like the plywood. Any leveling that will need to be done once you remove the vinyl has to be done on top of the cement board, not under it. If there is any 1/4" underlayment under the vinyl this has to go also.

If you are installing the heating mats, your best bet will be to pour 3/4" of slc over them.
 
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Old 09-15-06, 11:34 AM
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Thanks Johnny, so I should use 1/2" plywood over the 3/4" planking? do I use any adhesive or thinset to fasten the plywood or just screws? then use a 1/4" concrete backerboard fastened with thinset and screws?? So do not level the existing subfloor before the addition of the plywood and concrete board? only apply SLC if the finished concrete board has unlevel areas??

many thanks again
Matthew
 
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Old 09-15-06, 12:00 PM
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Matthew

Just screws, no adhesives for the plywood. Avoid screwing into the floor joists. Screw the plywood to the planking only. Make sure the planking is screwed down to the joists good before you install the plywood.

Yes thinset and screws for the cement board. Use unmodified thinset and 1/4" notched trowel. Buy the "square drive" backer board screws. The phillips heads tend to strip out before you can set the heads flush.

Yes - slc on top of the backerboard only in places where it is needed.
 
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Old 10-03-06, 02:04 PM
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Question

hello, i was browsing through, and i am redoing a kitchen, not a bathroom but my question is i have to remove the whole floor to the joists due to h2o damage. i fixed the source and am redy to start. but i was going to screw 3/4" ply to the joists. in this situation is that the way? or should i put something under the plywood and if so waht are the suggestions. thank you,
dave
 
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Old 10-03-06, 04:30 PM
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Dave

Will you be installing ceramic or stone tile or something else?
 
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Old 10-03-06, 07:16 PM
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sorry, i want to use ceramic tile. there were 3 layers of plywood down all were softer than a babys but when i ripped it up. it was real bad, but the joists dont seem to be affected. thank god.
 
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Old 10-04-06, 06:16 AM
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3/4" t&g plywood is the way to go. Since you had 3 layers of plywood already, you probably want to get the floor back to a similar height which is a good thing. If you can afford to height wise, add an additional 1/2" layer of plywood on top of the 3/4" t&g. Then 1/4" cement board and then the tile.

What size are you joists, what is there on center spacing and what is there unsupported span? This will tell you if your joists are adequate to support a tile installation.
 
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Old 10-05-06, 01:00 PM
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Originally Posted by HeresJohnny
Matthew

Just screws, no adhesives for the plywood. Avoid screwing into the floor joists. Screw the plywood to the planking only. Make sure the planking is screwed down to the joists good before you install the plywood.

Yes thinset and screws for the cement board. Use unmodified thinset and 1/4" notched trowel. Buy the "square drive" backer board screws. The phillips heads tend to strip out before you can set the heads flush.

Yes - slc on top of the backerboard only in places where it is needed.

I also have planking but I doubt I have the height to add 1/2" ply. What is the minimal ply thickness I should have for a marble tile? I was thinking of pulling all the planking and replacing it with 3/4" plywood. Would that be enough? Should I make that over 1 inch?
 
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Old 10-05-06, 01:29 PM
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ok HeresJonny,the joists are 2x8s 16"oc and span 12' before they are supported by a steel ibeem. they have cripple studs(if thats how you call them) made of 2x8s at various points in between them, so i think they should be good for tile.
 
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Old 10-05-06, 04:44 PM
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nycemsmedic

2x8's 16 oc 12' span do not meet minimum deflection standards for ceramic tile. You can cut that span down by adding a support wall or you can sister the joists. You cant install ceramic tile on the joist system you currently have.
 
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Old 10-05-06, 04:46 PM
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Badmana

All natural stone needs a minimum 2 layers of plywood for a minimum of 1 1/4". On top of that youll have to install a sheet membrane or cement board before you can install the marble tile. If you dont have the height for this consider installing some other kind of flooring.
 
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Old 10-06-06, 08:59 AM
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Thanks for the reply HeresJohnny.


I checked my planking and it seems intact and in good condition so I want to keep it. It looks like it's 3/4".

So with another 1/2" ply I should be ok with marble right? I want to use membrane to keep it as thin as possible as I'll be tiling over heating wires (I need to lay down mortar thick to cover it).


Thanks!
 
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Old 10-06-06, 10:44 AM
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badmana

You should be ok with going over the 3/4" planking with 1/2" ply and then a sheet membrane.

What size are your joists, what is there on center spacing and what is the unsupported span of the joists? Your joists also need to be strong enough to support a natural stone installation.
 
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Old 10-06-06, 11:22 AM
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Originally Posted by HeresJohnny
badmana

You should be ok with going over the 3/4" planking with 1/2" ply and then a sheet membrane.

What size are your joists, what is there on center spacing and what is the unsupported span of the joists? Your joists also need to be strong enough to support a natural stone installation.
I haven't seen them yet but they are probably what I have in my basement. 16" OC 2x8s. I don't know much else other than my bathroom is in the corner (2 walls are external).

I was only able to sneak a peak at my sub-floor from a hole I punched to check the plumbing of the tub. I'll know more when I tear up the floor.

Is there a 'rule of thumb' for joist strength? Should I put in cross beams between joists? I've been doing that in my basement so I have the material.
 
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Old 10-06-06, 12:00 PM
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If you have 2x8's 16" oc and the unsupported span is more than 8' you will have to do something to beef them up. Youll either have to add a support wall from below to cut down the span or sister the joists. You joists must meet L720 for natural stone.
 
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Old 10-06-06, 12:40 PM
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The wall opposite of the external wall is ~8 feet away. If it's load bearing (and it must be, it's the only wall between the 2 external walls 24 feet apart) I should be ok right?


I can't sister the joists (second story bath) so I either hope it's not more than 8 feet or I switch to regular tile.



I just thought of a possible problem. Since I'm 'raising' the floor height by over an inch (1/2" ply + membrane + 1/2" marble + mortar) my toilet waste pipe is going to probably be too low. What would be the name of the item I need to get to raise it? It's an older cast iron. Could I just fit a piece of PVC into the pipe?


Or will I still be ok when I reinstall my toilet?
 
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Old 10-06-06, 01:19 PM
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If you will be raising the finished floor height 1" higher than the previous finished floor height you should take the time now to raise the flange. I would not recommend a flange extender or thicker wax ring to make up a full 1".

What was over the planking before you tore it up?
 
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Old 10-06-06, 01:41 PM
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It is currently covered with some sort of wood covering. Thinner than 1/4" with glued on floor covering on top (it's one piece but looks like some sort of laminate).


I haven't removed it yet but the edges (around the tub and walls) have curled up so I can see the planking.



The flange is cast iron. Is raising it going to be a pro job? I'll have to check if my plumbing book deals with this.
 
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Old 10-06-06, 02:29 PM
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Just to clarify something HeresJohnny. I've been looking and it seems to me that you need both membrane and cementboard (ply - membrane - cementboard).

Your post implied I could do either membrane or cementboard.

Will I be alright without cementboard?



Thanks for all your help by the way!
 
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Old 10-06-06, 02:34 PM
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Yes one or the other. You dont need both.
 
 

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