We have squeaking sub-flooring in our hallway upstairs. We have had the carpet pulled back and we are ready to make the floor squeak repair by puting screw nails through the sub-flooring into the floor joists. How long should the screw nails be?
Squeaking floors can be super annoying. Floor squeaks happen for a number of reasons but most commonly because the sub-floor gets loose. That flooring is often put down with a rosin coated nail called a "cooler". The idea is that when the nail is driven the friction melts the glue coating making it less likely to pull out. But in reality, the nails do move and because they are coated by rosin, create floor squeaks that can drive you nuts!
In your question you refer to "screw nails." There's really no such thing. You should be buying case-hardened drywall screws, that are at least 2 1/2 inches thick for the floor squeak repair. Once that carpet is pulled up, I'd screw down each and every sheet of plywood, using 4-5 nails for each floor joist running under each sheet of plywood. If the subfloor is installed on 16" centers, that means you'll need at least 28 screws per sheet! The good news is that they can be easily installed with a drill-driver. Be sure to screw down every single sheet that you can get to because I can guarantee that as soon as you fix one squeak and put the carpet back - another one will immediately pop up!