Subfloor not level

Q: We are placing pergo flooring on on a 24×24 foot area. The floor is not level all over. There is about a half inch variance on the out area of floor. It seems to be a big job to level whole area. What would happen if we put in the flooring without leveling the whole floor?

A: The floor may not be level, but is it flat? Any dips could be evened out with building paper or such. However, if there is a hump, you won’t be able to install this type of rigid flooring until the hump is removed.

Related Q: I moved into an old house and took up the tile flooring. The original subfloor is there, but it is uneven in some places and has a lot of gouges in it. Do you recommend a new subfloor? If yes, can it be placed over the old one? If I put a new subfloor in, will it still be uneven if the problem has not been fixed?

A: If the unevenness is not severe, you could plain down the high spots. Low spots could be raised by laying 1/8 masonite strips in the void, then screwing plywood sheeting over it.

Related Q: I have a room just re-finished with pre-finished oak hardwood floor. The highs and lows have 3-4mm difference. Is that normal?

The room is roughly 16’x16′ and we just installed with new prefinished 2-1/4″-wide red oak hardwood floor. The new floor is unfortunately uneven. When we walk across planks, our feet can feel the highs and lows. See pictures for the amount of unevenness. What can we do to fix?

A: The sub floor is not 100% flat. Nothing really to be done with it now. Prefinished floors are themselves not perfectly flat either. This is why the edges and ends of the boards have a bevel on them.

Follow-up Q: Just curious, is this something that could be (easily?) prevented while nailing down the floor? Since the floor have been nailed down, is it possible to remove a few planks and re-do it? Fix it from crawl space? Sand it and re-finish it?

A: If you have a crawl space you may want to go and look at the joists. Some of those joists may not have been installed level with the rest. In a worse case scenario, you could have dry rot happening and they are all falling apart.