I'd fill the low spots with a self leveler and then put on the plywood with screws and glue. Disoriented strandboard needs all the help it can get and you can never have too stiff a subfloor. Tarpaper really does nothing structurally. You certainly don't want any between the two subfloor layers. You don't want them to MOVE relative to eachother.

Hi Ronnatalie,
Thanks for the reply and the advice. Would you screw the underlayment through the subfloor and into the joists? Would that help prevent movement of the panels? Also, what glue would you use? I have heard you can't use Titebond II or III because it is a brittle glue and it makes "crackling sounds" when you walk on the floor. Is some other kind of glue recommended?

I use heavy duty construction adhesive, nails and special purpose screws for the subfloor. If you need access below, you can't pry it apart, but have to cut it out. Glue and screw the underlayment as well. Use professional quality hd adhesives.

Here is a link that might be useful: Professional quality construction adhesives

Make sure the screws penetrate into the joists at least 1" along with the adhesive advise above.

You guys are great. This is a wonderful site. I really appreciate all your advice and I might be able to get this done yet.

How would you apply the adhesive? If we use pro quality urethane like Titebond 811 and trowel it on with their recommended 1/4" notched trowel to cover the entire sheet, the factory says the coverage is just 30 sq. ft. per gallon, or about one gallon per sheet, so for 1000 sq. ft. it will cost us around $900 for the glue. Is there a less expensive way? Can we just use pro adhesive in a large caulking gun, like from Home Depot, and make zig zag patterns, then screw the underlayment down, or will that cause air spaces where there is no glue? Could those air spaces allow the underlayment to move slightly when the floor is walked on? How have you guys applied the glue?

We will definitely screw at least one inch into the joists.

Just a thought. When we built our home we screwed/glued the subfloor to the joists. This was over 20 years ago and the floor is still tight.

However, the underlayment we just screwed (not glued) to the subfloor and into the joists. We are glad because now 20 years later we are removing rugs, underlayment and putting down solid oak flooring. If the underlayment was glued down, this would of been a disaster to remove.

Just a thought for later down the road...

Casey

"http://www.driverdb.com/Sean McDonagh"

Hi Case,
Good point on the underlayment removal. Thank you for sharing that. Possible furture underlayment removal is a serious concern. Will you be installing new underlayment before the solid oak flooring is installed, or installing the oak flooring directly to the subflooring? Did your old underlayment, not your subfloor, ever squeak or move on you?

A major reason for installing our 3/4" thick underlayment is to prevent the 2" long flooring cleats from penetrating the vapor barrier. So we were looking at the underlayment as an extension of the subfloor. The only reason for the glue is to prevent future squeaks, especially in the field where we can't screw into the floor joists and need to use screws no longer than 1-1/4" so they do not penetrate the vapor barrier under the OSB. If you nstall you roak right to the subfloor you will probably be fine but if you are going touse underlayment again, how will you prevent it from moving or squeaking after the flooring is installed? This is our dilemma.

Hi Casey,
Good point on the underlayment removal. Thank you for sharing that. Possible furture underlayment removal is a serious concern. Will you be installing new underlayment before the solid oak flooring is installed, or installing the oak flooring directly to the subflooring? Did your old underlayment, not your subfloor, ever squeak or move on you?

A major reason for installing our 3/4" thick underlayment is to prevent the 2" long flooring cleats from penetrating the vapor barrier. So we were looking at the underlayment as an extension of the subfloor. The only reason for the glue is to prevent future squeaks, especially in the field where we can't screw into the floor joists and need to use screws no longer than 1-1/4" so they do not penetrate the vapor barrier under the OSB. If you nstall you roak right to the subfloor you will probably be fine but if you are going touse underlayment again, how will you prevent it from moving or squeaking after the flooring is installed? This is our dilemma.

The Titebond is for adhering the finished floor, not the subfloor.

For wood planks, the 3/4" underlayment is overkill; a 1/4" would do it. In fact, I used to put down hardwood directly on 5/8" ply on 2x10s @ 16" o.c. with never a complaint from buyers.

There is no logical reason to remove solidly adhered underlayment if you change the flooring later.

BTW, unvented crawl spaces are the preferred building method, particularly in hot humid climates.

Hopefully this is posted in the correct section. Building a two-car garage w/ two rooms and bath above. Using 12" engineered I-beam floor joists. Using 3/4" OSB for subflooring. Should a vapor barrier be installed between the floor joists and the subfloor. If so, what is recommended. Should I also glue and screw the subfloor? Located in north Alabama. Thanks, David.