I am installing subfloor in a old house attic. I am having a hard time getting the tongue and groove all the way in.

Will I fail inspection with gaps this big?enter image description here

  • 1
    Better question might why you are having trouble. Depending on why they are are not fitting in might lead to problems after with a finish floor.
    – crip659
    Jan 21, 2022 at 21:49
  • 1
    @crip659 I think I figured out why. It's because I've been nailing too close to the groove. Jan 21, 2022 at 21:52
  • I am curious about the inspection, by whom? You are doing this for someone else by contract?
    – r13
    Jan 21, 2022 at 21:53
  • @r13 no I am adding a master suite to our attic. Our city requires a building, plumbing, and electric inspection. Jan 21, 2022 at 21:54
  • 1
    Your problem is not the inspection, is how to reduce the gap, so the flooring is adequately supported. Good luck.
    – r13
    Jan 22, 2022 at 3:07

2 Answers 2


That's not a gap--it's a nearly complete lack of engagement. While T&G OSB will not go completely tight on the surface (that's how it's designed), you need it mostly tight. If it isn't you'll have noise and/or flex, things the T&G arrangement is meant to prevent.

I've never taken the approach that Jack proposes. It could help, but I haven't found it necessary. We'd typically nailed a row down completely before laying the next. That said, we always used BIG hammers--six pound sledges--along with a "beater block" consisting of a 3-5' 2x4. In that I certainly agree with Jack.

I also agree that you need to start out straight. A zigzagging first row will result in larger gaps in subsequent rows.

It takes a lot of force to overcome the friction of that rough wood when inserting tongue into groove. There's no way around that.

If you've started out badly or for other reasons can't get your joints tight, you'll have to float backing to tie the sheets together. 24" lengths of two-by floated between the joists, perpendicular to the joint, and fastened to each sheet would do. Use construction adhesive and make sure they're pulled up tight.

  • A bigger hammer also helped a lot. May 15, 2022 at 15:51

You are correct on your comment. The last nail to the edge should be no closer than 6", to allow the tongue to ease up the groove if need be. The remedy should be as simple as pulling the nail out. You could pull it out partially, but that does make a mean trip hazard.

Besides inspecting each tongue and groove before setting the subfloor in place to be driven in, have a real heavy hammer, something like a 5 lb sledgehammer works well for this. To drive it in, keep a 2 by 4 ft long or more to protect the groove side that you are driving against.

When I am setting subfloor alone, I keep my screw gun, 2" screws and some small scraps of plywood handy. When the tongue does not want to align with the groove readily, I screw down a block onto the piece that is already down where the next piece has lifted up, this will draw it down to line it all up. Usually 2 blocks are the most you need, sometimes only one.

I also assume you chalked a line to have a guide to go by for the first row....


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