In my experience, roughly 6 out of 10 sub-floors will need a small to moderate amount of leveling, 3 out of 10 will need a significant amount and one will be so bad that it is cost prohibitive to fix the sub-floor and so the choice to go back to carpet is made.
Leveling adds time and money to the whole project. Ignoring the need for sub-floor leveling to save money is not a good idea because once the hardwood is installed the sub-floor cannot be corrected.
The result of installing hardwood flooring over an uneven sub-floor can be:
- Excessive movement which will lead to premature failure of the flooring product
- Squeaks and creaks
- Gaps and separation
- Veneer delamination (for engineered flooring)
- Voided manufacturers warranty
The problems that come with an uneven sub-floor become evident once the floor is installed. If you install a floating floor over an uneven surface then you will notice soft spots where the floor dips when walked on.
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 arises when the hardwood flooring is allowed to bridge two high spots in the sub-floor, say 5' apart, leaving a low spot in between. The flooring is not designed to span high spots and cannot support weight in this way. Subsequently the floor deflects down into the low spot when walked on. If the floor deflects more than 1/8" (3mm) in 10' then the industry standard has not been met.