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Ripping up 3/4" particle board underlayment over 1/2" plywood subfloor. Running with the joists the floor is flat and level. But running perpendicular to the joists I have high spots every six to seven feet creating a sway of 2 to 3 eighths at its lowest point.

My question is, to level or more importantly flatten the floor, can I lay shingles and 30lb felt the length of the low spots, then screw down 3/4" ply on top in prep for vinyl plank flooring?

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Egads. Why would you not fix the framing properly? By leaving gaps between your new underlayment and the subfloor you allow much more flex (and possibly noise) than you have now.

I'd sister 2x4s to the sides of your joists on plane where needed. Run drylines across the floor at intervals, one pencil thickness above the high points in the joists. Make additional level adjustments if things are tilted. Set your new lumber using a pencil as a spacer.

Use construction adhesive and framing nails or gold screws to attach the lumber and prevent squeaks. Lay your new subfloor and underlayment. (I'd recommend 3/4" tongue-and-groove BC plywood or OSB, then 1/4" birch plywood.)

  • Over a 24 ft expanse three joists are higher(crowned?). Almost equal distance apart. Seems a lot to sister the rest. – Richard Ripoli Jun 14 '16 at 23:12
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    If it is just 3 joists, my temptation would be to plane them down. – Aloysius Defenestrate Jun 15 '16 at 1:05
  • Well that's new information now, isn't it? :) It also contradicts your post, which implies that the low spots are the exception. Now you sound like you'd lay shingles over most of the floor. That will probably get you the stink of asphalt and lots of crunchy sounds. – isherwood Jun 15 '16 at 13:42
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I bought an electric planer to do this in a bathroom of mine. For really low spots I also cut strips of wood and used construction adhesive to raise them a bit. I then used a laser level to determine the height of each joist along the entire length. It took some time but the floor is flat.

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