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:
- I can't remove the existing planks, for historic preservation reasons. (The house is Grade II Listed).
- I don't have a lot of extra height to give up, as the ceiling is already rather low.
- The existing planks are too thin in places to use them without covering them.
- 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?