In my experience for the most part, 7 out of 10 sub-floors will need a small to moderate amount of leveling, 3 out of 10 will need a significant amount and occasionally one will be so bad that it becomes cost prohibitive to fix the subfloor and so the choice is made to go back to carpet.
Ignoring the need for sub-floor levelling to save a little money is not a good idea. Remember, once the floor is installed the subfloor cannot be corrected.
The results of laying your new hardwood floor over an uneven sub-floor will be:
- Excessive movement which can lead to premature failure of the flooring
- Squeak and creaks
- Gaps and separation
- Veneer delamination for engineered flooring
- Loss of wood flooring manufacturers warranty
If you install a glue down floor over high and low areas it can release under stress
Squeaks and creaks can develop when excessive movement is present in the floor. Note: that a small amount of movement is normal and to be expected with a floating floor because the underlay beneath the hardwood is supposed to compress. The problem of movement comes when the hardwood flooring is allowed to bridge two high spots in the sub-floor leaving the floor suspended over a low spot.
The flooring is not designed to support wait in this way and so is easily pressed down into the low spot when someone walks over it. If the floor deflects more than 1/8" or 3mm this may lead to a problem in the future.
Leveling adds time and money to the whole project. A moderate amount of leveling will take a day or two to complete in a 500 square foot space. Estimating how many hours of labour and how many bags of concrete leveling mixture will be needed is like asking 'how long is a piece of string'. Therefore, I always price floor leveling by the man hour plus materials because it is the fair and equitable way for both parties.
Subfloor levelling is equally important for nail down, glue down and floating floor installation methods.
Good luck with you installation.