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Unread 01-07-2004, 09:59 AM   #1
grisby
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Join Date: Aug 2003
Posts: 69
plywood and cbu install ??

Ok, another 3 questions (sorry to be a pain) but when I put my 2nd layer of plywood on over the original(putting 1/2 ply on top of 1/2 ply) I know to stagger joints and go perpendicular, BUT after doing a search I am reading on some posts here that the 2nd layer of plywood should not be screwed/nailed into the joists. Why not?? Wouldn't you want that for rigidity? Afterall I will be putting a layer of thinset on top of that before I put the 1/2 inch CBU on which I know does not get screwed/nailed into the joists, but into the upper plywood layer.

Second ?? is where my CBU butts up to the drywall, do I use thinset to tape the joint or regular joint compound?? Tile is going over the drywall area also.

3rd ?? is when I put CBU on floor and it butts up to tub, do I leave a small gap(1/8" or so) or just butt up against tub.

As usual thanks in advance.
bill g
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Unread 01-07-2004, 10:50 AM   #2
bbcamp
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Hey, Bill!

Layering the plywood and keeping the fasteners for the second layer out of the joists is a holdover from when tile was routinely set directly on plywood. This ws to create a de-coupling effect, where the layers would be able to slide just enough to relieve stress cause by structure movement. If you use an underlayment like CBU, you don't want to screw into the joists for that same reason. Using CBU or Ditra allows you to relax a little about screwing the second layer of plywood into the joists. So, try not to hit the joists, but don't stress over it.

Where CBU meets Drywall, you can use either drywall tape and compound or thinset, and CBU tape. Your choice. If it will show, use the drywall stuff, it will be easier to get the same texture after under the paint.

Where the CBU meets a wall or other structure, there should be a 1/4" gap. No thinset, but caulk if you want. Around the tub, the gap can be 1/8". You'll want to caulk there for sure.
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Unread 01-07-2004, 11:00 AM   #3
grisby
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bbcamp, ok, so its ok for me to nail the plywood into the joists. I understand the CBU does not, as this is the decoupling effect.as it should not be screwed into the joists. I am just a little dense here.

Thanks for the clarification of the joints
bill g
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Unread 01-07-2004, 11:09 AM   #4
bbcamp
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Yeah, you got it!
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Unread 01-07-2004, 11:13 AM   #5
grisby
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Thanks!!!!
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