Hello, I'm new to this board and have a head-scratcher of a situation. I stumbled into 1500sqft of very old fir flooring that has never been installed. The pieces are 2-1/4 x 5/4 (true 1" thick) random length 12" to 24" (apparently end drops from longer floor boards). The pieces are all groove-less on the bottom. The grain is extremely tight, clear, and straight. The pieces are abnormally dense (for fir).

The tongues and grooves are very loose, typically 1/16" of slop. I don't know if it was the original milling or shrinkage over time. These could be very old pieces. I'm at least the fourth owner of this pile of several thousand blocks of wood.

I am getting ready to start laying the floor down and am finding that when fastened down to my (very flat) test plywood sheathing, these stubby little blocks are still rocking 1/32" up and down. The house is new construction. The floor area is about 1200sqft. The subfloor is 3/4" CDX plywood on top of TJI engineered joists on 24" centers. The floor is insulated R38. The basement below is well insulated, but unconditioned (a recent cold snap into the teens and the basement below stayed at 45deg).

The fastening at present is 2" Bostitch staples, pneumatically driven. Because the standard shoe doesn't fit the 5/4 boards, I have arrived at a tapered shoe that shoots the nail at about 42 or 43 degrees (which cuts the board rocking up-and-down to the 1/32" range).

I'm at a bit of a loss as to what to do to take the rocking out and/or prevent squeaks, etc. It looks like I may need to:
A) lay a course, put down a bead of Liquid Nails (or other adhesive?) under the heal of the next board (at the groove end).
B) swab a layer of glue in the tongue with each course of boards, or
C) I don't know, any suggestions.

Any of the above will really slow down the install.

Also, the subfloor has been painted with an emanel floor paint as it was expected that a proper floor would be added in about a year... until this crazy deal on this ancient fir came along. Unless it interferes with the fastening/adhesive securing method selected, I am planning on using waxed rosin paper inbetween the fir floor and the subfloor.

I would greatly appreciate any input that you may have as regards this loose tongue and groove problem. Thank you in advance.

Id run a bead of tightbond glue in the tongue and it should stop.

Thank you Kevin for the helpful response. The glue on the tongue is workking great. The test on the first few rows yields zero rocking of the heal of the board at the tongue/groove (gap) interface.

The only issue now is the variation of the widths of the boards which have obviously shrunk over the years, coupled with their short length (12" to 24"). I would love to know the whole story of the life of these boards since they were milled decades ago!