How to flatten an uneven plank subfloor when the existing planks can't be removed?

Question

The upper-floor master bedroom in my c. 1700 house has a plank floor laid across the joists which is quite old, if not original. The planks were apparently once 1/2", but in some areas of the room they're worn down to 3/8", and even 1/4' in a few places. (There are knots which are proud by 2 or 3mm. It's kind of amazing to see that amount of wear on interior wood, but I suppose it's an imperceptible amount of wear per annum over three centuries.) The plank floor is wavy both across the planks and along them, though much more along them---there's maybe a 30mm difference between the highest and lowest points along the planks and about half that across them. This is in a room which is about 5m square.

I discovered just how wavy and thin the planks were after taking up the carpet in preparation for putting down a hardwood floor.

Constraints and observations:

1. I can't remove the existing planks, for historic preservation reasons. (The house is Grade II Listed).
2. I don't have a lot of extra height to give up, as the ceiling is already rather low.
3. The existing planks are too thin in places to use them without covering them.
4. The plaster and lath ceiling of the room below is attached directly to the underside of the planks.

I'd still like to lay a hardwood floor. The thinness and unevenness of the existing planks appears to rule out nailing the hardwood directly to them. If I could remove the existing planks and lay plywood across the joists I'd have a planar surface to work with, but I can't remove the existing planks.

How can I make this work?

Answer 1

Though the ceiling height is a constraint I would recommend laying down 3/8" 8'x 4'plywood sheets and shimming them up to level. Lay the hardwood floor on top of that.