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Unread 09-07-2020, 08:41 AM   #1
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proper plywood


I've been educating myself through various threads in between working on a master bath remodel. I recall reading specifics about what type of plywood to use as subfloor but now doing a search there are so many responses I am not finding what I need.

I plan on doing 2 layers of 3/4" (23/32) plywood. I recall reading something about marine glue? and maybe BC sanded? Also is OSB ok or no?

My deflection is L/735 if that makes a difference and plan on using natural stone. I would love confirmation/explanation from the pros. Plan on laying it down soon (hopefully today).


Last edited by ETOH; 09-07-2020 at 08:44 AM. Reason: update info
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Unread 09-07-2020, 09:41 AM   #2
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Exterior glue is what you're after. No face less than C, so B/C ply would be fine.

Strength-wise, either OSB or ply. The concern with OSB is surface coating on some that would interfere with bonding of, say, thinset mortar.

You want to offset joints in second layer regardless.

You've got some beefy structure to get to that deflection. It's atypical of conventional framing unless built or upgraded to that spec. We just don't want to get someone started down a wrong road by using room size as the gauge.

Having seen cracks in natural stone floors where this spec was disregarded, I can tell you it's not there for no reason.

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Unread 09-07-2020, 09:55 AM   #3
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Welcome back, Erik.

What Peter said.

Is there a reason you want to use nominal 3/4" plywood for your second layer when nominal 1/2" would suffice and be much easier to fasten properly?

Keep in mind also that some thinset mortar manufacturers specifically recommend against use of their product to bond to OSB.

My opinion; worth price charged.

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Unread 09-07-2020, 10:15 AM   #4
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Thanks for the replies!

So atypical to most, I need to be thicker to match my height of the existing Adjacent room. I’ve got 3/4” diagonal planks that I am almost done ripping out and it had 3/4” hardwood on top of that, then Backer board then tile. Again that is what used to be there. Plan was 3/4”, 3/4”, ditra, tile on the new installation.

I think I did the deflection calc correctly but would not say that with 100% certainty. I will be blocking between joists to stiffen things up even more, as I like to over engineer things.

I have the orange and blue big box stores nearby. Do I need tongue and groove for the bottom layer? Both layers? CX, I could do 1/2” for the second layer, but any negative for going 3/4” again?
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Unread 09-07-2020, 10:49 AM   #5
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Blocking can help with a point load as it helps share the load between those two joists, but has no deflection benefit for a distributed load. It can change the resonant frequency of the floor, but is most beneficial prior to the installation of the subflooring to help keep the joists vertical so that they can do their job better if they were to twist under load.

Other than that, use whatever buildup you need to get the overall height as long as you have two layers of subflooring for your stone tile, and they are installed properly. ANd, yes, your first layer should be T&G unless you want to add blocking to keep those long seams aligned. Not needed on the second layer.
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