Solid hardwood subfloor question

[6Gtdf]

Not a garage specific question but the folks here are always super knowledgeable so figured I would ask if anyone had any advice.

I am going to be installing ~1400sq ft of 2 1/4” solid red oak in the next few weeks. Given the way my home/windows lay out I have decided that I want my floor to run parallel to my joists.

My current subfloor/joist setup is 3/4” t&g OSB over 16” on center joists.

My understanding is that NWFA says if I want to run parallel than I need at least 1 1/“4 total subfloor height, so I plan to add 1/2” ply perpendicular to the existing subfloor. I am seeing some conflicting advice on the best way to go about adding this layer, or if the new layer should even be called another subfloor layer or an underlayment.

My questions are:
- Should fasteners in this second layer hit joists? I am seeing some folks say no so the layers can move independently.
- Should I use any adhesive and/or any sort of felt paper between the layers to avoid squeaks?
- I want to use subfloor screws, but seems like I would need 1 1/4” subfloor screws which I can’t really find. Mostly 1 5/8”. Any issue with using those screws that are too long?
- Anything else I should be thinking of/doing?

 

[duneslider]

The second layer should be installed perpendicular to the floor joists and joints should be a minimum of 4" offset from lower joints.
Squeaks are almost always a result of wood rubbing on metal (nails). I highly recommend that you screw down the 3/4" first before adding the new layer.

Once you screw the 3/4 down I would check for any proud nails and pound them in or remove them and then if your seams are raised at all hit them with the belt sander to smooth out the ridges. You want the first layer as "good" as possible.

Clean the floor up really good, vacuum, maybe even mop. Cleaner the better. Then add 15lb felt and start installing the second layer. Be mindful of the first layers joints, you can't see them anymore...

Stagger end joints by at least one joist, I try for more typically. If the new ply isn't T&G then leave an 1/8" gap on the 4 foot ends. 8' sides can be tight. Leave a 1/4" gap at walls.

Fasten the new layer just to the layer below and not into joists. You want something that can just penetrate the lower subfloor. I would shot for 1 1/2" but 1 5/8" might be easier to find. I am pretty certain I use #9 1 1/2" deck screws all the time (GRK). I think that is what I used in my house to make sure I didn't have any squeaks. I think I used several thousand of them.