Reinforcing sub floor

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Old 06-01-15, 05:54 PM
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Reinforcing sub floor

I recently removed the carpet from the bonus room over the garage in my house built in 1986. There was 5/8 inch particle board on top of a 1/2 inch plywood subfloor. I have removed the particle board and want to place new plywood down on top of the 1/2 in plywood sub floor. I don not have height issues and can replace with 5/8 or 3/4 inch T&G plywood. Looking for some advice on the best way to proceed. Should I run the new plywood perpendicular to the old? Should I secure the new plywood to the 1/2 plywood sub floor only, or to the joists as well? Glue or felt? I appreciate any helpful advice or recommendations. Thanks ( plan on installing carpet over finished project)
 
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Old 06-01-15, 06:54 PM
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Welcome to the forums! I would use 3/4" Advantech or equivalent, after you screw down the 1/2" to the joists, then screw the 3/4" to the field only without any glue nor felt. You could get by with 5/8" with the carpet, but the Advantech is T&G and is a stiffer underlayment.
 
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Old 06-01-15, 07:21 PM
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Only things I would add...

You should orient the plywood the same direction as the original 1/2" subfloor, with the long dimension of the plywood perpendicular to the joists. It is a good practice to offset the layout so that you don't ever have seams on top of seams, and as Larry mentioned, you don't want to screw into the joists. So if you offset your end (butt) joints 2" (actually 46" or 50" from the last butt joint... if you really wanted to stagger your new seams from the old) from the 1/2" subfloor (as recommended by the TCA handbook- Tile Council of America) you will avoid the joists provided you stick to 8" on center in the field and 6" on center on the edges. I know you're not putting down tile, but this is sage advice for any subfloor.

In my experience, I have always liked to predrill and countersink the holes in 3/4 underlayment in a situation like this. 1/2" plywood doesn't always give the screw enough of a bite, and because of that, your 1 1/4" or 1 5/8" deck screws may strip or at least not set... unless you predrill with a countersink first.

And be sure you don't use drywall screws to screw your subfloor!!!
 
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