From 
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1. Slide a pair of shims into the gap for a snug fit. Draw a line on each shim to mark the depth. Don’t wedge the gap wider.
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2.Add a bead of construction adhesive to both sides of each shim. Shove shims back into the gap. Align to the depth line.
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3. Score the excess shim two or three times with a sharp utility knife and snap it off. Keep off the floor for four hours while the adhesive hardens.
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1. Measure the length of the subfloor gap for 2x4 blocking. Extend the block about 1 ft. on each end. Cut away protruding nails with diagonal cutters.
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2. Predrill 3/16-in. clearance holes every 12 in. along the 2x4 at a slightly upward angle for the screws. Then apply construction adhesive liberally to the top and the side of block, press tightly to the subfloor and drive 2-1/2 in. wood screws into the joist.
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3.Squeeze a thick bead of construction adhesive into the crack along both sides of the squeaky joist and subfloor. Apply adhesive to adjacent joist/subfloor joints as well.
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1. Cut 2x8 blocking to fit snugly between joists. Add construction adhesive to the top and slide it into place.
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2. Predrill angled pilot holes with a 1/8-in. bit. Drive 3-in. wood screws to force the block snug against the subfloor seam. Drive an additional pair of screws (or 16d nails) through the joist into the block on each end.
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3.Mark the depth of the pilot hole on a 1/8-in. drill bit with tape. The depth should be 1/2 in. less than the floor thickness. Drill pilot holes 4 to 8 in. apart. Drive No. 8 wood screws flush to subfloor.
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Was there an answer to darne18's question of a squeeky floor on an upper level? Do I have to go in from the top, tearing up the flooring?
I'd like to know how to stop a sqeaking floor on the second floor of a two-story house. You can't access the joists. So the method shown here doesn't work.