I've seen some posts here that say to either staple or screw in 1/2" CDX plywood for adding an underlayment to an existing plywood subfloor. I also saw that Gary once suggested gluing the underlayment in addition to screwing.

My question is: how do you go about gluing the underlayment, i.e. what type and how much glue do you use?? Also, I will avoid overlapping the joints from the existing subfloor, but I assume that I will still be screwing the underlayment into the joists?

Thanks for your help
dave

I am a DIY'r... by no means a Gary or Floorguy.

I just finished laying 1/2" ply underlayment in several rooms and used glue, staples and screws. I cut each board to required dimensions and used either liquid nails or pro 400 subfloor adhesive and a putty knife to spread it evenly. There might be an easier gluing method but I wasn't looking for one.

I set and aligned each ply board, stapled it (a bunch) and applied deck screws into all joists and solid cross bracing. I ended up with the solid, tied in, even floor effect I wanted.

You stated:
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Also, I will avoid overlapping the joints from the existing subfloor, but I assume that I will still be screwing the underlayment into the joists?
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I don't understand this part of your question. What are you saying here?

Dave T

You answered the part about screwing into the joists (i.e. vs just screwing into the subfloor). As for the joints, I'll avoid overlapping the existing plywood joints with the new joints for the underlayment.

I don't understand why you'd use both staples and screws. Are these flooring staples that you put in with a flooring stapler?

-dave

When installing 1/2" CDX plywood over an OSB or plywood subfloor, it is not necessary to staple, glue, and screw. All you you are attempting to do is add more rigidity to the subfloor and supply a superior nailing base. Stapling down with 15 gauge 1&1/2" legged staples with 1/2" crowns alone is sufficient, as long as use use lots of staples. I recommend stapling 4" around the perimeter and every 6" in the field in a grid pattern. It isn't needed to screw into the floor joists as long as your existing subfloor is well attached. You should renail any loose areas before installing the 1/2". You can glue if you want but it isn't normally needed and just takes more time and money. You want to be sure you do not create a bubble in the underlayment. Start your stapling working from the middle out to the edges while stepping on the ply to ensure it is tight to the subfloor. I always do my underlayments this way and NEVER have problems. One last tip. Avoid marrying seams together and allow at least a 1/16" or more between 1/2 ply panels for expansion.

Gary: Relative to your answer, I'm doing the same thing, putting 1/2" underlayment over 1/2" sub floor, but I will only be deck screwing the new 1/2". Is it still necessay to put screws every 4" on perimeter & 6" in the field. I would think with so many screws, it would come back to bite you when nailing the 3/4" hardwood. I was going to just screw into the joists with the new 1/2", every 8". Your thoughts on this, when using only screws.
Thanks, FJK

The problem with this is you will only be fastening the new 1/2" ply every 16". If you want to use screws, go ahead, they're fine (slow and expensive though). But screw in an 8" grid. That would be to the joists and a row in between. With screws, 6" around the perimeter should be fine. Keep in mind, you're attaching the new ply to the old ply, not necessarily to the joists. Do a few sheets and see what I mean. If you go in 16" rows, the new ply will be moving in between.