Hello all,

I am planning to install 3/4" x 3.5" hardwood floor on my 2nd floor.
The 2nd made up of 2x10 at 16" OC (span about 13.5') and sheet with 5/8" T&G plywwod.
Currently the floor is uneven (joists are high and low 3/8" in 8'), squeaky (nailed at about 2" OC), and bouncy. (bad construction)

I am undecided how I would like to place the floor. It will look the best if I install the hardwood floor parallel with the joists ( length wise) but will need to add addtional layer of 3/8" plywood. Which will be a lot of work, labor and costs, not to mention that the current floorboards are nailed every 2 inches or closer, I am scared it will split the joists. Plus having to climatize both the hardwood planks and the plywood which will take extra time. But the 3/8' plywood wont solve the uneveness rite? and it will be higher than the level of the tiles in the laundery room and bathrooms etc.

The easiest way will be to install the hardwood floor perpendicular with the joists without adding the plywood, but it wont solve the problem of uneveness. If i use self levelling compound, will I be able to nail/staple through the compound?

Please advise! Thanks in advance!

The solution to your situation is this:
1) sand the high spots down a bit. This will enable you to use less material when filling in the low places.
2) using 30 lb. asphalt felt and/or fiberglass roofing shingles, fill in the lowest areas to flatten out the subfloor, tapering the edges so it's not lumpy.
3) overlay the entire area with 3/8" minimum CDX plywood (1/2" is preferred).
4) Attach new plywood underlayment by stapling securely with 1&1/4 long construction staples 16 gauge with 1/2" crown. Staple every 2" around the edges and in a 6" grid in the field. Do not attempt or worry about stapling to the floor joists. You can use screws if you prefer.
5) Now you will have a flat, sturdy surface to install your flooring over. You can even sand the new underlayment some more if needed. And it will enable you to run the flooring parallel to your floor joists. Simply install a reducer to the tile or use thinner wood flooring. Do NOT attempt to use leveling compound then nail into it. It will crack, if not now, eventually. Plus it doesn't hold fasteners for beans.

Thank you Gary. I will add 1/2" struc 1 plywood, glue and and 1 1/4" drywall screws to attach to the existing plywood. Now I have to figure out how to adjust the height of the steps.