Gappy floorboards - ply or hardboard?

[Ouch77]

the floorboards on the first floor of my house are pretty knotty, gappy, and abused T&G without the tongue or groove in many cases where they've been lifted. We don't have exotic plans for flooring other than new carpet, and was wondering what was best to finish the floors before laying carpet & underlay. 3mm Hardboard, or 6mm plywood?

I'm mostly looking to prevent some poor soul in heels doing themselves and injury (and putting a hole in my carpet) should they step on a knot-hole wrong, though anything that helps reduce creak and drafts would also be good.

Thoughts anyone?

 

[lymmranger]

4mm ply is perfectly adequate If you are going to fit carpet.
You only need to fix it enough to keep it secure and flat - use ring shank nails sized so that they do not go through the floor and puncture any wires or pipes