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philipb

07:52AM | 05/19/08
Member Since: 03/04/03
44 lifetime posts
Bvflooring
(I posted this on the Carpentry section, forgive the cross post but afterwards I thought it should be posted here.)

I gutted my kitchen and dining room, down to the rafters/studs and under layment.

The existing sub floor is one layer of 1/2" (15/16") plywood over 2x8s 16" on center. The sub floor is very soft, bouncy and in some areas feels like you can fall through if you step hard or jump. The 2x8s are not bouncy, just the plywood.

Before I cover with finished floor - which will be 3/4" solid hard wood (Brazilian Cherry) 4" tongue-in-groove is dining area, 1/4" durock covered with 12x12 tiles on thinset in kitchen - I plan on covering the existing sub floor with a 2nd layer of 4x8x1/2" fir plywood. Was planning on nailing the fir sheeting down with 3" full round head glue tipped nails dispensed from a pneumatic framing nailer.

Ok, that's the background, now for the question.

To avoid squeaks, should I use construction adhesive between the existing and new layer of sub floor sheeting? I think this would stiffen the floor by bonding the two layers of sub floor together, and avoid any squeaks down the road.

Any experience/thoughts on this is appreciated.

Thanks,

Phil

scuttlebuttrp

07:14PM | 05/19/08
Member Since: 08/29/07
56 lifetime posts
Can't speak about the wood, but 1/2" ply won't cut it for tile.

What is the length of the span of the joists?

If they can pass Deflection, then you need 3/4" minimum ply run perpindicular to the joists.

philipb

04:43AM | 05/20/08
Member Since: 03/04/03
44 lifetime posts
The room they span is only 13', and there is no deflection.

I realize the existing, builder installed 1/2" plywood sub floor isn't enough, which is why I stated I am covering it with another layer of 1/2".

The question is, is construction adhesive either necessary or beneficial between the existing 1/2" plywood sub floor, and the new layer of 1/2" I am going to install?

Thanks,

Phil

scuttlebuttrp

04:04PM | 05/20/08
Member Since: 08/29/07
56 lifetime posts
Adhesive isn't necessary. If you use it though, make it full spread.

Span has nothing to do with the size of the room your tiling. It's unsupported span underneath supporting those joists.

3/4" T&G ply as a first layer is necessary.

Tileguybob

04:18PM | 05/20/08
Member Since: 07/03/05
283 lifetime posts
By the book, you dont glue the one layer to the other. And you dont hit the joists when nailing the second layer down (per Tile Council of North America handbook for Ceramic Tile Installation, Method# F150-07. "Underlayment fasteners should not penetrate joist below"). By unifying the two layers with glue, if the joist moves it will pull the bottom layer with it and the top because of the glue. The two layers should be isolated from each other, other than the nails to hold he second layer down.

philipb

05:06PM | 05/20/08
Member Since: 03/04/03
44 lifetime posts
I can't see how "Method# F150-07" can work.

What if there was only one layer of underlayment - let's say 3/4" or more, it would have to be fastened to the joists below, no other way to secure it. So then, if the joists deflect, so does the underlayment, as well as the mud or durock layer, and tile.

I would guess the term "underlayment" in "Method# F150-07" applies only to the tile backer board or substrate, and does not refer to the actual subfloor. The subfloor has to be attached to the joists, and two subfloor layers glued together or just nailed through to the joists, is no different than one thicker subfloor layer.

Tileguybob

05:22PM | 05/21/08
Member Since: 07/03/05
283 lifetime posts
What is it you dont understand or what is it I dont understand about your statement? The OP asked if the 2nd layer of plywood should be glued to the first and should he use 3" nails. I answered it based on what he stated. Method F150 pertains to the situation where there are two layers of plywood. He made no mention of substituting cement board for the underlayment layer of plywood.

If he had asked that I would have said it cant be done over 1/2" plywood subflooring. It would have to be a minimum of 5/8" and be tongue and grooved ply. The nails or screws would be at the direction of the CBU manufacturer
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