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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?

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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.

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The type of screw I recommend is one where the thread ends before the head about the thickness of the subfloor. This increases the pull of the screw head on the sheet downwards onto the joists. This also allows you to use a battery operated drill/driver without the screw thread shredding your finger tips. Also use PAZ screws (passivated). As mentioned previously use course thread. Use a cable/pipe detector to precisely mark your subfloor for screw placement if you aren't lifting the existing floor.

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Depends on how thick the underlayment is. If you can in time find out, the rule of thumb is a minimum of 1" embedment into the lower substrate. So if you underlayment is 5/8" or 3/4" thick you can use 1 5/8" coarse thread screws. You could use the same screws for thinner underlayment, but the risk of hitting something unseen below the floor grows.

The typical screw spacing for underlayment, NOT sheathing is 4", +/- on the edges and 6", +/- in BOTH directions in the middle. It sounds a little overkill for thicker underlayment material, but for the thinner stuff, say 1/4" thick, it is needed.

  • Aren't subfloor and underlayment two different things? Doesn't underlayment go on top of the subfloor? Does the same rule of thumb apply for screwing in the subfloor as you have posted for the underlayment? – Pᴀᴜʟsᴛᴇʀ2 Feb 7 '15 at 15:15
  • Yes they are, and yes it does. No it is not possible to TOTALLY have the same rules. The only rule of thumb applies for the depth of screw, not spacing. Subfloor is typically screwed 6" +/- on the edges- which there is only 2. The edges that are perpendicular to the joist cannot. The center of the subfloor as a rule without using adhesive is 8" in the field and the joist span dictates the dimension the other way. Because the underlayment goes over the subfloor therefore having something to screw to in all places, the underlayment can be screwed in any direction, in any increment. – Jack Feb 7 '15 at 16:58
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    Don't get me wrong, this is very good information, but I'm curious as to how it answers the OP's question of what kind of screws should be used for subflooring? – Pᴀᴜʟsᴛᴇʀ2 Feb 7 '15 at 17:43
  • The link connected to the words beginning in the second line of my answer "coarse threaded screws". It shows a picture of the screws I recommend and a written description. – Jack Feb 7 '15 at 18:36
  • You might want to edit your answer to specify this then. You are only specifying for underlayment, not for subfloor. As it stands, this could be boiled down to a "link only answer". Just trying to help. – Pᴀᴜʟsᴛᴇʀ2 Feb 7 '15 at 18:48

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