Replacing damaged parts of a plank floor that is 7/8" thick. Need a level surface to lay another 1/2" plywood subfloor layer. Can I 'glue and screw' a two thinner pieces of plywood (1/2" and 3/8"), or do I need to source 7/8" plywood? I would use pl premium and screws for compression between the layers. My other idea was to add 1/8" hardboard strips on top of the joists in those sections and just using 3/4" instead. Any help would be appreciated.
I wouldn’t try to laminate two pieces of plywood together.
Group 1 type plywood is rated for various spans. Using 1/2” plywood to span 16” will span the distance without failing, but will have too much defection. That is to say, it will be springy. It’s designed for 2 span installations. If it’s a one span situation, it will be noticeable.
Adding additional plywood layers does very little to decrease the deflection problem and if the adhesive is not installed correctly (and cured correctly) it could create other problems, like 1) bulging, 2) warping, 3) delaminating, etc.
I’d use 3/4” plywood with the wood strips you suggest. 3/4” plywood will be stiff enough that you could use in a single span situation and still be acceptable for ceramic tile floor, etc.
Also, remember to install with the face grain perpendicular to the joists. (Screw and glue...adhesive... to insure no squeaks.)
Also, this is for “subfloor” only. You still need an underlayment. Check with the manufacturer of your finish floor material to see what they require for an underlayment. (It’s different for ceramic tile, vinyl, etc.)
Your plan should work just fine, assuming that the layers are bonded as you describe. I don't know that I'd bother screwing between joists, though, or maybe do so sparingly just to ensure a snug bond.
You'll want to apply the adhesive with a controlled bead (maybe 3/6" in a consistent pattern) or v-notch trowel to prevent waviness. The 3/8" layer is vulnerable to bulging due to excess adhesive.