Marble bathroom subfloor?

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Old 11-11-10, 10:22 AM
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Marble bathroom subfloor?

Hi, I've read what I can, but am still not sure as to how best to approach my subfloor.

I'm putting a marble floor (12" x 12" tiles) in a 2nd floor bathroom (6'x8'). 90 year old house, so the joists etc. are a bit beefier than current, but well aged! Which I guess can cut both ways....

The bathroom is 6' wide, one side on brick semi dividing wall, over a 15' wide kitchen below, so joist span is 15', but bathroom is concentrated only on first 6.... The "2x8" joists are actually 1 7/8 x 7 7/8, unknown wood, 16" centers. There are 7/8" planks on the joists, varying widths.

The deflection calculator shows that this won't handle marble (or even ceramic). However, I think we will give it a shot anyway - the room has brick on two walls, and I think it is maybe a bit more stable than the calculator's standard assumptions would indicate (hope so anyway). I will brace the joists and beef them up as much as possible.

Question #1: Should I keep the planks and put ply over them, or take the planks out & put 3/4 ply or whatever in their place? Not sure what would be more stable ... these planks do feel nice & stable anyway....

Then, on top of either the planks or the replacement ply, I'm guessing another sheet of ply, 5/8 or maybe 3/4? Then maybe Ditra? I can deal with a bit of extra height if that will make the difference between better chance of a successful installation. If it ends up not working, then so be it, will have to deal with that later.

Thanks in advance for any ideas
Al
 
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Old 11-11-10, 10:43 AM
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The planks can stay but you need to add quite a bit of structure before you'll have the stable base required for marble, so I'd pull them up to keep the height from getting out of hand

I'm still not convinced the joists are going to be enough for this, I'll let the pros chime in there
 
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Old 11-11-10, 04:54 PM
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Mitch is correct. You need to beef up your joists if you want the marble to last.

I'de put a layer of 3/4" ply over the planks as a minimum.
 
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Old 11-11-10, 07:44 PM
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Hi Al,

Adding another 5/8 or 3/4 ply is the most important thing to do since most failures happen because of the deflection between the joists. But that will not stiffen the joists themselves. Since you were thinking of removing the planks, why not do that and sister the joists as far as possible. Even 2x4's, 2x6's or strips of plywood will do it. Then install a 3/4" sub + a second sheet of min. 1/2", prefer thicker.

The industry norm is for joists' defection to be L720 for stone tiles. I'm not sure what the new rating would be doing the above, but surely much better than now. You can also search out mortars that claim will work even at L480 for stone. Laticrete has many (if not all of) mortars that are approved for L480 for stone.

What type of tile underlayment were you gonna install? Cement backer, Ditra?

Jaz
 
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Old 11-11-10, 09:06 PM
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Thanks for the replies guys

I guess if I'm going to beef up the joists, I'll lose the planks, beef them as much as possible, then go with ply, 3/4 then maybe 5/8 on that(?) ... maybe 3/4 again. With that 2nd sheet I should try to avoid hitting joists screwing it down I think?

For underlayment, I was thinking the ditra might be a bit thinner and perform just as well?

Will look for a good mortar as well Jaz, thanks for the tip
 
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Old 11-19-10, 08:26 AM
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UPDATE: The planks are gone, my modified stud walls are up, and I'm pretty much done with the beefing up of the joists - rebuilt one section with 2x8s, did some doubling with 2x8 & 2x6, lots of braces, brackets, etc. Definite improvement

Please let me know if any of the following is totally misguided!

Now I will install 3/4 plywood, of an appropriate grade (once i look that up again). I got some 2" flooring screws for this, and will try to hit all the joists/braces etc every 6" or so(?)

Next, a layer of 1/2 ply (or maybe 5/8?) ... will I want to use a 1 1/4" floor screw & miss joists this time i think i read? on a 6" grid? Nothing between layers of ply like construction adhesive I'm assuming?

Spot level anything that might be required with SLC.

Then, I was thinking of 1/4 cement board set in latex mod thinset under border of room and under vanity, toilet & tub areas. leave a gap in open floor area, where....

radiant heating installed in open gap, then SLC'd to height of surrounding cement board border.

then tile the marble on top of all that?

Thanks for taking the time to read this - it is greatly appreciated!
 
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Old 11-19-10, 08:53 AM
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Have you considered floor height in your plans? You've removed 3/4" plank and whatever the thickness was of your original floor.

Your plans call for adding 3/4" ply, 1/2" ply, 1/4" Hardibacker and marble tile that may be 1/4" to 1/2" or more.
 
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Old 11-19-10, 11:28 AM
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Hi Wayne, yes there wasn't that much height on the old floor, some vinyl tiles & some kind of linoleum over 7/8" planks, so it will be higher. I'd like to keep it as little higher as possible, but will accept extra height if it's needed for the marble. If there's some way to do it without the extra height so much the better, but i haven't seen anything that fits that bill recommended....(?)
 
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Old 11-19-10, 02:54 PM
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Sounds like you doing well. You could get away with use 1/2" for your second layer if you'd like. You don't need want to use construction adhesive between the two plys.

You can eliminate some labor by going with Ditra instead of cement board.
 
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Old 11-20-10, 07:09 AM
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Good to hear, always nice for a bit of reassurance, thanks!!
 
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