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Discussion Starter · #1 ·
I'm doing a bathroom job, and need to install marble floors. Tore out the old, saw significant water damage to the sub floor. Tore it all out, going to put new 23/32" ply for subfloor, quarter inch hardibacker cement board, then flexbond and the marble. Problem is with this subfloor, the bathroom is a 5 x 4 powder room, and the sub that was in there was part of sub from the hallway also. I'm not very experienced installing new sub, but I do know that I need this floor to be very sturdy so as to not have cracks or strain in the marble down the line. Now what I have here is like 4 joists to work on, and on the side with the doorway, I don't have a joist. I can't attach to it either, there is a duct in the way, would be difficult to move. As it is right now, if I installed the sub, the first 10 inches or so would have no support from a joist. Can I bridge a little side off the closest joist? Ideal would be to hook up to the joist under the hallway floor, but vent is in the way.

And if I were to do that (move vent, hook up 2x4 or something to the under hallway joist), do I just nail those boards in, or notch out the 2x10 joists spaces for the 2x4. I want to do this right, its for a close friend.
 

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I couldn't follow exactly what your situation is, but what is called for when installing new sub-flooring and dealing with existing joists is sistering and cross bracing. Go to 2x6s, you can even use deck hangers if you really want to be anal, the ones made by Simpson strong tie would work. But if you do it right all you need is wood. When in doubt use more wood.
 

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I'm having trouble picturing your problem. I would be nailing in 2x4 between the visible joist and the joist hidden under the hallway floor from the floor below (if possible/accessable), avoiding the vent. Again, I may not be picturing this correctly.

Whatever you do to try to get added support, I would encourage the use of 1/2" cement board for added stability. 1/4" Hardibacker, although usable, could be too flexible for marble especially considering this other issue you have to deal with.

- Bob
 

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Discussion Starter · #5 ·
Tileworks said:
I'm having trouble picturing your problem. I would be nailing in 2x4 between the visible joist and the joist hidden under the hallway floor from the floor below (if possible/accessable), avoiding the vent. Again, I may not be picturing this correctly.

Whatever you do to try to get added support, I would encourage the use of 1/2" cement board for added stability. 1/4" Hardibacker, although usable, could be too flexible for marble especially considering this other issue you have to deal with.

- Bob

Fixed the issue, was a pain in the ass, had to move that vent. Attached over to the other joist, mounted it from the floor below. Still sticking with quarter in hardi, as I said I'm redoing with all brand new sub and its dead level. Shoudl hold up fine, just a powder room.

Thanks for the help guys. I like these forums, just signed up the other day.

-Chris
 

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Wellp, - - good luck with it, anyway, - - unfortunately, 'dead-level' has nothing to do with rigidity, - - personally, my absolute minimum is 1 1/4" 'built-up' subfloor for marble or ceramics, - - but hey, maybe it will hold for you. Hope so.
 

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Tom R said:
Wellp, - - good luck with it, anyway, - - unfortunately, 'dead-level' has nothing to do with rigidity, - - personally, my absolute minimum is 1 1/4" 'built-up' subfloor for marble or ceramics, - - but hey, maybe it will hold for you. Hope so.
For any natural stone, I'm with you. 3/4" ply plus 1/2" Durock at the least. 1/4" Hardibacker is all right for ceramic or porcelain though.

I have a horror story for you (sort of)...

Did a job where the existing tile needed ripped out to be redone with travertine stone (16x16) to match an addition. Ripped out the tile, which was porcelain set over hardibacker, which had been set with mastic and staples. The subfloor was particle board.

GC wouldn't replace the particle board with plywood, so we followed suit and went with 1/4 hardibacker set with mastic and roofing nails, and set the travertine on top.

It breaks about every rule written about tile setting, but a year has elapsed and haven't heard of any problems yet.

- Bob
 

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That is a (potential) horror story. I seriously doubt it will make the 5-year mark, - - which really is still a short life.

Hope you had the GC 'sign-off' on it. Trouble is, - - if and when it fails, - - it still makes YOU look bad.
 

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Seems to me, if the subfloor is that bad, wouldn't the bathroom just need to be gutted, ripped out, maybe even to the point of taking a wall out and replacing it in order to fix the the floor. I mean, we are talking floor here. So, if the the floor is bad, all else comes out, then the floor fixe, then all else comes in. No jimmyrigging allowed...right? ;)
 
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