Floors are rarely perfectly level.
How to level floors for hardwood installation.
If the area is very large use a long straight 2 in 4 in 5 1 cm 10 2 cm board and set your level on top of it.
On the top side of the floor another fix it idea to lay down new hardwood over the existing floor.
If the floor is uneven lift up the board until it is level so you ll know how much flooring compound you ll need to even it out.
Before installing hardwood floors use a straight edge to find the humps and dips in the subfloor.
Clean it well with a wet dry vacuum.
Hardwood floor installation can be slowed by an uneven subfloor.
Then using a floor leveling compound or asphalt shingles shim the dips so the floor is level.
How to level a floor for installing hardwood before you install hardwood flooring follow the directions above for fixing high and low spots in a wood or concrete subfloor.
A plywood subfloor will bridge any minor waves in the existing floor and leveling compound would help too.
You will have to make sure your joists can handle the addition of considerably more weight from the plywood subfloor and any floor coverings.