Screw Gauge for OSB Subfloor/Underlayment

Discussion in 'Hardwood and Laminates Q&A' started by jhigh, Nov 7, 2006.

  1. jhigh

    jhigh Charter Member

    Screw Gauge/Length for OSB Subfloor/Underlayment

    Hi everyone!

    Project Background:
    I'm installing a floating laminate floor in the kitchen and family room. I'm building up the subfloor in the family room to match the kitchen by adding 3/4" tongue and groove OSB over an existing 3/4" OSB subfloor. Due to the direction of the light source and an adjoining long and narrow hallway, I will be installing the laminate floor parallel to the joists. Because of this, I'm putting down the new OSB panels perpendicular to the existing subfloor.

    My question is: what would be the appropriate gauge of 1 3/4 inch screw to secure the new OSB panels to the existing subfloor? Would #6 be sufficient or do I need to go up to a #10 gauge screw?

    Thanks in advance!
    Last edited: Nov 7, 2006
  2. Chris Mha

    Chris Mha Charter Member Senior Member

    I have a screw gun and I use #8x1-3/4 screws that are fed through the gun on a plastic strip. A #10 is a lot of screw. I also prefer a square head vs. a phillips. /seem to drive better. Stay away from the #6 screws. These are drywall screws and you will find that you will snap the heads off of a lot of them.
  3. Floorguy

    Floorguy The Living Dead Charter Member Senior Member

    I don't know, I've never looked at that.

    I just buy a couple of big boxes of decking screws, coarse tread.

    I wouldn't use 1¾ as they will be sticking out the bottom of the existing decking and will lose most of their holding power.

    1¼ is a much better length to attach the OSB to the decking. Your not going into the joists, just mating the OSB to the decking.
  4. jhigh

    jhigh Charter Member

    Thanks for the replies!

    Just one point of clarification. The screw length that I was planning on using was actually 1 5/8" not 1 3/4".

    Is the 1 5/8" an acceptable length, or is 1 1/4" safer for this type of mating? (there's also a layer or two of 15 lb roofing felt in between the new and old OSB).

  5. Flip

    Flip Charter Member

    Perry's got it right. By the time you sink the 1-1/4" screw into a double thickness of 3/4" board you have all, that's ALL of the grab those screws have to give. I like deep thread screws for composit board and nibs cut clean counterinks. The screw costs a little more than a dry wall screw but the performance is much, much better. #8 X 1-1/4 particle board screws with type 17 points and nibs are super in wood to wood. I get mine a a fastener supply that sells a lot of cabinet makers.
  6. Chris Mha

    Chris Mha Charter Member Senior Member

    I wasn't thinking when I replied. I was under the assumption you were going into the joists. I agree, if your not hitting the joists a 1-1/4" is all you want. As Floorguy said with the screw penatrates the subfloor it looses holding power.
  7. jhigh

    jhigh Charter Member

    Thanks again to all for the replies!

    As a quick followup, what is the recommended screw spacing for mating two 3/4 inch OSB layers in my situation? All the guidlines I have seen talk about installing OSB as the main subfloor, not as a second layer/underlayment.

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