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Cross plank sub floor for tile?

I am helping my brother remodel his bathroom which is we will say 7'x5.5'. I have the old vinyl floor and sub floor plywood up and we are down to the 1/2"x6" cross plank sub floor which rests on the 2x10 floor joists spanning 11'. The cross plank sub floor is not t&g. I have checked the deflection with an online calculator and it is ok for tile. My main question. I plan to lay 1/2" ext. grade plywood down over the cross plank and then 5/16" durock down on that. How should I screw these two layers down with respect to floor joists, orientation to the subfloor (perpendicular or parallel)? And should either layer be screwed to the floor joists? Is it best to keep the full sections of plywood or durock around the toilet where there is a lot of weight or in the higher traffic areas? I forgot to mention that it is just a sink and toilet bathroom, no shower or tub. Please explain your answer because I like to understand why I should do something rather than just knowing what to do? Thank you so much for your help!

Re: Cross plank sub floor for tile?

i would put the plywood down and make sure there are no seams within about 2 feet of the toilet, that's the area that you want to be most secure. then screw the plywood into the joists, not only the planks about every 6"-8" and you can stick an extra screw down between the joists where the sheets meet. now you can put your durock on top of the plywood and it does not have to be screwed into the joists, the plywood is strong enough to hold it down, just use screws that are at least 1 1/2" long to get a good bite into the plywood. make sure to tape (fiberglass mesh) and skim coat the joints of the durock for a nice level substraight.

Re: Cross plank sub floor for tile?

I would recomend schluter membrane over the plywood rather than durock, you can find it at schluter.com

Re: Cross plank sub floor for tile?

Another suggestion would be to use 5/8 exterior plywood with the rough side up.
Use construction adhesive to adhere down to the sub floor along with 3 inch screws into the joists .... 2 inch screws in the field to clamp the plywood down while the adhesive cures.

Then use the uncoupling membrane like the Schluter Ditra

or like this membrane by Superseal

These would get set in mortor on top of the rough surface of the plywood. The reason for the rough side up is to provide some texture for the mortar to grab.
The membrane will work to help prevent any cracks happening .

This will also save the 1/2 inch the backer board would have added.

Hope this helps.:)

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