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Marble Tile over diagonal T&G subfloor

merlvern | Posted in General Discussion on December 17, 2013 03:46am

howdy all,

i’m doing marble tile in a very small bathroom (25 square feet).

my 1950 subfloor is 1×8 (actual) diagonal T&G hardwood over 16″ oc 2×10 joists spanning @8′.

my intention was to 1/4″ wonderboard over the T&G with thinset and screws, then lay the tile on that.

both here and in “johnbridge”, there seemed to be some disagreement about using the existing T&G as a subfloor or rip it out and replace with plywood as the subfloor (which i’d rather not do). or use the T&G under 1/2 plywood, then the 1/4″ wonderboard (which messes me up on height). i know the wonderboard lends no structural value, so going to a thicker 1/2″ wonderboard doesn’t help either.

can i just use the 1/4″ wonderboard over the T&G subfloor and 1/2″ plywood?

 

thoughts anyone?

thanks in advance for the help.

 

John

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Replies

  1. calvin | Dec 17, 2013 05:48pm | #1

    John

    The span of your joists is a critical item using natural stone-and at 8' I'd guess you passed that test.

    The diagonal subfloor is a weak issue if you consider that on the diagonal the span of these probably just a full 3/4 (perhaps 7/8") increases and might cause some deflection with concentrated weight between the joists.

    This is what I've done with no problem when faced with the same situations-screw the old subfloor to the joists with 1-3/4" subfloor screws (not drywall screws-but deck screws would work).  Over that-1/2" Aruaco (a quality brand of plywood here) glued and screwed to the subfloor. **Do Not Use 3 ply 1/2" sheeting-the Aruaco (sp) is 5 ply**.   A good BC exterior plywood is a good substitute but I use what I know works and is readily available ( not a cheap product-but what sq. footage do you have in that bath?).   PL Premium as the glue, when applied properly will stick tenaciously!  No little squiggle lines like you see on the cheap paneling you can pull off in a couple years.   I screw every 2" along the seams-6" squares in the field-using a standup screwgun w/collated subfloor screws, but you're welcome to do it the hard way.  Done right, the poor bastard that tries to tear it out will call you every name in the book.

    You could add the 1/4" wonderboard-but we usually lay the tile to the plywood with a good full flex thinset (with marble you will be looking for a special thinset anyway-find the one that also is recommended for adhering tile to ply).  Mix and lay according to the directions and don't scrimp on the trowel notch size-too little thinset and it'll get the moisture sucked out of it too quickly and bond poorly.

    1. merlvern | Dec 18, 2013 10:34am | #5

      thanks, i appreciate the help!

  2. amazingman | Dec 17, 2013 06:08pm | #2

    tile job

    I agree with the half  plywood .But x the backer and use kerdi matt it will allow for some movement in substrates

  3. User avater
    Mongo | Dec 17, 2013 10:32pm | #3

    You can't use a cement board directly over plank subfloor.

    You already have 3/4" plank subfloor. On top of that I'd add 1/2" plywood, screwed. Over that, Ditra. Which is what the previous poster was calling "Kerdi matt". The Ditra is about 1/8" thick. Ditra will help protect the marble from any floor movement, and with this being a bathroom, Ditra is waterproof, so the underlayment and subfloor will never see water. 

    If that added elevation (1/2" ply plus 1/8' Ditra) works, great. If you need to go higher, go with a thicker plywood.

    If you do use wonderboard, do not forget that you have to bed the wonderboard in thinset. ie, you'll have your plank subfloor, then your plywood underlayment, then thinset, then cement board. The thinset is not for bonding the cement board to the plwood. The screws do the fastening. The thinset fills any void between the plywood and the cement board, eliminating any micro-vertical movement of the cement board. The thinset is not an option.

    Good luck!

    1. merlvern | Dec 18, 2013 10:34am | #4

      thank you

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