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Old 10-15-2012, 10:30 AM   #1
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Bathroom Floor (Mud Bed) Removal and New Floor


Hi Everyone. I am renovating my 2nd story guest bathroom in a 1911 house. The original installation was mosaic tile set in a roughly 4" thick mud bed. The tile and concrete had become cracked and deteriorated over time so I have removed the tile and concrete down to the joists (16" oc and they have been hatcheted). Between the joists, running perpendicular, there are 1x6" planks sitting on cleats 4" below the top of the joists that supported the concrete bed. I plan on installing a plywood subfloor, ditra and ceramic tile as the new floor. My questions on how to proceed are as follows:

1. Should I remove the 1x6" planks and cleats and sister the joists up to the desired height? Or can I build up off of the 1x6" planks? My concern is that the mud bed previously ran under (about 6") and supported the skirt of my cast iron tub (the tub is temporarily supported on 2x4s sitting on the 1x6 planks). It would be easier to place supports off of the planks for the new floor because I can't get under the tub to the last joist, however the planks were obviously not intended to be used this way. They are sound and happen to be there though so why not?

2. Concerning the new floor, I've read many different opinions on the proper subfloor and underlayment for ceramic tile. I see a total subfloor thickness of 1'-1/8" a lot. Is this the thickness of the plywood layers alone or does it include the cement board layer as well? I thought 3/4" plywood was fine for structural purposes so I'm confused as to why another 3/8" layer is necessary. So, if I am using Ditra instead of cement board can I simply have a 3/4" plywood subfloor, modified thinset, Ditra, unmodified thinset, then tile?

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Last edited by fletch7; 10-15-2012 at 01:11 PM.
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Old 10-15-2012, 06:58 PM   #2
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Bathroom Floor (Mud Bed) Removal and New Floor


Not sure I understand, Are you saying the joist had the top 4" removed?
Just does not make sense to me, how big are the joist?
Then again, it did hold 4" of concrete so should be stout.
I like the idea of sistering them for a few reasons. which would mean removing the 1x6.
Leaving the 1x6 could cause weird squeaks, simply cant do anything good for you.
So pull em out and get them out of the way, I would try to be careful, could be some nice tight grain 100 year old wood you can use on another project.

I would want to use 2x8 to sister the joist, give you enough meat to screw them solid, I also use liquid nails on them to help eliminate squeaks.
Now you want to determine the finished floor hight to the hallway, or whatever room you enter while exiting the bathroom, to get a flush or as close to flush as you can get.

No tall threshold to trip on.
All you need for sub floor is 3/4", go to the big box store, it will look like osb and will be tongue and groove. Is specifically made for sub floor and has the engineered strength.
I would use hardibacker on top of it, you have a choice if 1/4" or 1/2", the 1/4" is good if you have flat surface, since yours will all be new, 1/4" would be fine.
Then the thickness of your tile adding for the thinset under it, is the finished floor hight of your new floor.
This is why the sistering idea is good, you now know how high to set that joist, you can raise or lower it to where you need it. Then you use a level to set the other end.
I really would be surprised if your 100 year old house floors were still level.
By sistering you get level and correct floor hight., and is worth it to me.

I hear a lot of talk about ditra, I did a quick google search for it the other day to see what it was.
Looks like a membrane that I use when building shower pans, do not understand why it would be used on floors, I should read more on it.
I do know that simple hardibacker will work for what you are doing. But is your choice.
I would not mind hearing more about why some choose ditra and what conditions they decide to use it.

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Old 10-15-2012, 07:37 PM   #3
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Bathroom Floor (Mud Bed) Removal and New Floor


@ fletch7, reply back with joist size. Yes you got your plan of attack planed out.... The deck mud & planks need to go, your house is way old so sister your joists to level. 3/4" ply is good but you will have extra hight to add 1/2" for beefier floor structure.
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Old 10-15-2012, 07:54 PM   #4
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Bathroom Floor (Mud Bed) Removal and New Floor


This is one time a picture would be best to get an exact recommendation on the rebuild---

Generally---with the lowered floor joists from an old mud bed---sistering is the way to go---

You are saving the 100 year old tub---so sistering will add to the challenge---(that's where a picture would be helpful)

3/4" BC exposure one ply is good for tile---natural stone will benefit from another layer of BC plywood--3/8 or 1/2 inch---

Your Schluter membrane is a very good plan----Jaz is very experienced with that and I am not----so if he finds fault with my suggestions--listen to him--Mike---
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Old 10-16-2012, 11:15 AM   #5
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Bathroom Floor (Mud Bed) Removal and New Floor


Thanks guys, I appreciate all your responses. I didn't get a picture last night but I've drawn up a quick cross-section that should get the idea across.

@JetSwet, the joists are 2x10 unfinished at 16" oc but as I said they have been hatcheted at the top to accommodate the mud. Due to their size and quality the floor really hasn't deflected much but sisters are still needed due to the hatcheted tops.

@oh'mike, to save the tub I think I would need to add joists perpendicular between the last two joists on the right of the drawing. All while working my way along the apron removing the temp supports, cleats and planks and adding new supports. This sounds difficult and if you had a different idea I'd like to hear it. While I would like to save at least one thing original to the room, it is a 100+ year old tub and would need refinished anyway at about the cost of a new tub. Replacing it is an option.

@funfool, I was thinking Ditra because it was recommended to me by a friend who has used it many times. It accomplishes the same thing as backer board but is apparently much easier to fit and install and also acts as a moisture barrier by itself.

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Old 10-17-2012, 12:13 AM   #6
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Leave the tub supported as is---sister all the joists----fill in the tub joist bay with a carefully ripped 2x4---to the height of the new joists--then sheet over the joists and 2x4 ripper---

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