Screw 2nd layer of subflooring to joists or not?

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  #1  
Old 03-25-06, 08:38 PM
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Screw 2nd layer of subflooring to joists or not?

Hi all,

I have been reading through all the messages regarding subflooring and underlayment and saw conflicting advice on how to attach a 2nd layer of subflooring. I am remodeling my kitchen/dining area (12 x 17 ft) which I have recently gutted. I will be putting down tile eventually. I currently have 1/2 inch subflooring on my 12 foot long 2 x 8 joists which are set 16" apart. I have just sistered these with additional joists to strengthen the support system (thanks again, Jack the carpenter!). I will be adding another layer of 5/8 inch plywood on top of the 1/2 inch that is down now (then 1/4 inch cement board).

Do I screw the 5/8 inch down to the joists or do I just attach it to the 1/2 inch subfloor avoiding the joists (I have read this) and if the latter, why avoid the joists? Any other advice appreciated.


Thanks,

Mark
 
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Old 03-27-06, 10:43 AM
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Screw the 5/8" ply to the subfloor only, no glue. Long edge of the 5/8 plywood should cross the joists, not run parallel. Short edge of the plywood underlayment should end about 2" past the joist and not on it. 1/8" gap between sheets, 1/4" gap around the perimeter. Fasten every 6" around the perimeter and every 8" in the field, fastening in straight rows so any bowing is removed from the plywood. It all has to do with limiting the transfer of movement of the suppotring structure and joist curvature under load.

When it comes time to tile, post your questions in the tile section of the flooring forum. Your backer board, so you know, needs to be set into a bed of unmodified thinset and screwed down to the plywood. Same gapping requirements. Every 8" over the sheets of cbu which is 58 backer screws per board.
 
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Old 03-28-06, 10:45 AM
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Tilebri,

Thank you for taking the time to reply to my question. So every 8 inches in a grid pattern across the plywood as long as I avoid the joists, correct? I have been using exterior screws for attaching the subfloor down to the joists. Are these ok? I have also been using spiral nails at an angle in various areas also just to really make sure that sucker is down tight. Just one other thing, would 1 1/4" screws be the correct size to use for attaching the 5/8" to the 1/2"?

Thanks,

Mark
 
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Old 03-30-06, 07:46 AM
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All that above sounds good. Proceed.
 
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Old 04-22-06, 03:52 PM
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A slight twist on the original thread about subfloors

Tilebri -

Getting ready to install 8" square quarry tile in a 5 X 8 bathroom. Due to poor original construction materials (30 yrs ago), I have decided to replace the original 5/8" plywood subfloor.

Here's my question. Am I better off installing 5/8" B-C plywood as a subfloor, covered by 3/8" A-C plywood or cement-board as an underlayment......or a 3/4" B-C subfloor covered by 1/4" A-C plywood or cement-board? As you might imagine from the questions, I am attempting to keep the original finished floor height the same.

Steven
 
 

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