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07-04-2008, 08:59 AM
I'm doing a bathroom floor with 12" x 12" granite tiles. I've done a lot of ceramic tile, but this is my first job with using granite. My floor is 2 x 10 16" oc with 3/4" t&g plywood. Will 1/4" hardie backer board be enough? or should I use 1/2" hardie backer board. An other question, would adding 1/4" underlayment plywood and then 1/4" hardie backer board be stronger then adding 1/2" hardie backer board? Also I think I'm going to use Custom marlbe and grante thinset. Any advice will be appreciated. Also the backer board will be glued and screwed.

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07-04-2008, 09:04 AM
cement boards ad no structural support to a floor, they only act as a proper tiling surface.
your support has to be made up in the plywood.

check your floors Deflection ( with this link to see if your floor passes.

Crestone Tile
07-04-2008, 09:05 AM

What is the span of your joists?

You will need a second layer of ply to accommodate a proper stone installation.

Crestone Tile
07-04-2008, 09:06 AM
Apparently, Brian has his PDA phone in a quick draw holster. :)

07-04-2008, 09:08 AM
Use the deflecto tool at the top of the page to see if your joists are good enough for natural stone. In addition to what you posted above, you will need to know how far your joists run unsupported.

Regardless of what the deflecto tells you you will need a second layer of plywood over your 3/4" plywood, and then your 1/4" backer.

Customs Marble and Granite is good for this project.

the backer board will be glued and screwed

The backer should be set in thinset and screwed or nailed. Not glued. Follow the manufacturers instructions.

07-04-2008, 09:28 AM
Apparently, Brian has his PDA phone in a quick draw holster.or it could be your just sloooooow ! :yeah:

07-04-2008, 10:50 AM
I will check the floor joist span and run it through deflecto to see if works out.

07-04-2008, 01:47 PM
Does increasing the thickness of the subfloor increase the deflection? I'm asking because I did'nt see any metion of it on the deflection calculator.

07-04-2008, 02:03 PM
No, adding extra plywood to make up for inadequate joists wont work. The extra plywood will help with your deflection between the joists, but not along the joists.

07-04-2008, 05:44 PM
The whole floor can move up and down which is the joist deflection. The subfloor also deflects between the joists. Both of those things need to be strong enough to not deflect. Say you had 1/4" ply on top of great, strong joists...standing on the joist it would be solid as a rock, but as soon as you stepped away from the joist, the ply would bow and might even crack. The ply layer must be strong enough as well as the joists, and while there is a little interaction, not enough for one to compensate for the other. You need at least a layer of 3/8" ply, and preferably more, on top of what you have now (assuming your joists are good to go for stone).

07-05-2008, 06:38 AM
So you are saying to put at least 3/8" plywood on top of exsisting 3/4" plywood and then the 1/4" hardie backer board. And if my joists pass the deflecto test for stone, I'm good to go?


07-05-2008, 07:55 AM
That's correct, Dave.