Glue and Screw Underlayment or just screw?

12 years ago

We are in a situation similar to An August 11 post. We need to add APA rated 3/4" BC exterior plywood underlayment to our 3/4" OSB subfloor, two adjoining rooms totaling 1000 sq. ft. The subfloor was glued and screwed with 3.5" deck screws to the 2x10, joists, 16" OC, six years ago so the framing lumber should be stabilized by now. The floor was not finished but was in like new condition, before it was lightly sanded last week, to eliminate high spots. It is very dry and always has been. Checking with an 8' level, it is fairly flat in all directions, including circles. There are small dips here and there but none are greater than 1/8" and most are 1/16". Finish flooring will be 4" wide, random length White Oak planks nailed with L shaped cleat nails and tarpaper underneath the oak flooring. The underlayment will be laid in the same direction as the subfloor, perpendicular to the floor joists but with long edges offset halfway across the sublfloor panels and the 4' wide underlayment ends offset from subfloor ends by two joist bays plus 4" to prevent any seams from lining up and for max floor stiffness.

The insulated crawl space is very well vented, exceeding code, and there is a 6 mil vapor barrier in the crawl space. All wet work is done and eaves troughs are in place.

Our big problem is if we should glue and screw the underlayment to the subfloor the same way the subfloor was installed, only with more glue, so the underlayment acts as an extension of the subfloor? Or, should we just use 1-1/4" screws throughout with tarpaper in the low spots to prevent underlayment movement over the small 1/8" pockets?

There may be a small amount of tile near an entrance way to control mositure at the entrance. No more than 100 sq. ft. at one oend of one of the adjoining rooms. Not in both rooms.

Comments and suggestions are hoped for. It is a big problem for us right now and is holding up the flooring installation. Worried about squeaks and movement of the underlayment.



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