First post here.

I am installing 3/4" maple 4" wide hardwood on the second floor of my home. I have been looking at a lot of information including the NWFA standards. I have 1x8" pine planks that run at a 45 to the underlying joists. NWFA, and most everything else I find, state the maximum width is 1x6" for a solid wood subfloor. I am not 100% opposed to putting 1/2" plywood down but don't understand why from a structure standpoint. The floor is sound and I have replaced several boards that had long splits. I also used 2 deck screws in every board every 16" into the joists to make sure I have no squeaks. Originally there was 5/8 particle board above this floor which I removed for a nail down installation of the 3/4" maple. A reputable installer I was going to have do the work looked at the solid subfloor and said it needed nothing else (flatness or structure to make sure I had a good bid). I am a little worried now that best practices state 1x6 is the maximum.

Thanks for your insight...

You will be fine with your sub-floor. The reason for the 1x6 is to make sure that there won't be expansion/contraction issues after the floor has been down for a while which can cause cupping and looseness. Usually only two fasteners are required for each board. When the boards get wider it becomes more important for three fasteners to be used but is not required. The screws you used for extra security will help keep the floor tight to the floor joists. To get the most value from adding screws, pre-drilling through the pine using a bit that allows the screw to touch the joist before the screw starts to twist down keeps the screw from screw jacking where the screw is tight against the pine, but there is still a gap between the pine and the floor joist.

Thank you for your insight Pete. I have taken time to remove any boards that creaked or bent due to having a knot or a split. I also lightly planed and sanded out edges that were higher than the next board (floated out the sharp edge). I will go back and make sure I got them all. Thank you for the tip on pre-drilling - I didn't do that and had to run the screw multiple times on some boards to stop the jacking. Now I am off to decide on staples or cleats (seems both would work since I have solid subfloor although Oklahoma can have pretty high humidity) and the underlayment that would be best for a second floor application for noise reduction. More learning...

The national Hardwood Floor Association may have the name and number of a floor contractor in your area. They have lists of certified installers.