Sub flooring for installing carpet over plank flooring

Existing carpet and pad were installed directly over rough plank floor. Over time the plank irregularities are seen on the carpet. I like the idea of screwing down 1/8" hardboard on top of the planks before installing new carpet. I don't want to add much thickness to the subfloor. However, I am worried both that it might squeak, and also that since the planking is rough sawn, the hardboard could break down where there are gaps between the planks and the hardboard. Is there something flexible we could put down to level the planks and support the hardboard, or some other solution? Or should I worry? It is very heavy solid planking, BTW.

 

Welcome to the forums! If your subflooring is terribly irregular, nothing will help "smooth" it out. Normally a 1/4" underlayment grade plywood is laid down to offer a smooth surface for the carpet and/or linoleum. This is not "Luan", as luan has voids in it, so know the difference. I would install it with narrow crown 1" to 1 1/2" staples. They hold better. I would not use HDF as it is too brittle.

 

The problem is that the edges of the subfloor planks do not line up perfectly. We have thought about sanding them, but it would take quite a bit, and I think we would still want to lay plywood or something because of the small gaps between the planks.

 

With that happening, I would overlay with nothing less than 1/2" plywood, fastened to the planks only (not the joists) using 2" decking screws, no glue. The screws will tend to pull things together and the 1/2" plywood will make the floor more sturdy and flatter.