floorscrewssubfloor
I have removed the old bathroom lino and cleaned the parts that adhered to the floor. I want to screw down the existing plywood subfloor because the nails are old (45yrs) and sparse.
What kind of screws should I use and how close together should they be?
I'd question how waterproof you need this to be. Will there routinely be water sitting on the floor? If not, you may be over-engineering things.
That said, off the top of my head, one option for waterproofing the subfloor would be to paint on Redgard:
It's a 'paint on' waterproofing membrane that you'd typically use behind tiles on cement board and the like.
It will create a mess if the particle board is not predrilled because the screws will at least break-out the backside of the particle board there by preventing the screw from ever drawing the PB flat against the plywood ... especially if the PB is not high density (heads will pull through) ... and especially if the screws are going into something as thin as 5/8" (low grade) subfloor grade plywood.
When predrilling, a depth stop will be essential to keep from drilling into the plywood because over-drilling by a mere 1/8" will leave less than 1/2" of plywood for the screw to grip. A small chunk of wood can be used as a positive (failsafe) depth stop. It'd be best to layout the joist locations on the 3/8" PB and aim the screws to grab into the joists.
If adhesive from a caulking gun is used, the floor will end up wavy, not flat, if you were to put a straight edge to it afterwards.
Regardless, I can't see the job coming out well using 3/8" particle board. Sorry for the pessimism. Is 3/4" not an option?
Best Answer
It should be noted that the above linked screw is a sheetrock screw, not a subfloor or underlayment screw, which you will probably find in real life labeled something like "deck screw" if you're working on the cheap.
Sheetrock screws are brittle and prone to breaking under the flexing/shifting of a floor. DO NOT USE sheetrock screws.
My apologies to the OP for not viewing this page 18 months earlier.
If you're installing tile, or doing anything involving concrete (like self leveler), you'll want to look into screws that are alkali resistant. The cement board that you'll grout to the latex primered (better bonding, moisture barrier) subfloor will also be attached with these screws, and the fiberglass tape you'll use to "mud" the seams between the cement board (just like installing drywall) will also be of an alkali resistant variety. In both cases of subflooring and cement board, leave a 1/16" gap between sheets for building shift/expansion.
One final note, if you're adding plywood subflooring over an existing subfloor, screw the original sub to the joists anywhere it's loose, and screw the new sub to the old sub but NOT to the joists. This will help the floor float during the inevitable expansion and contraction of temperature and humidity swings.