The most common reasons are either a basement or other concrete based surfaces or a remodel that has various levels of subflooring.
Building a subfloor on uneven concrete.
I have listed a couple of forgiving flooring materials carpet and vinyl to use over uneven concrete floors.
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You have a few options.
A good carpenter will install sleepers using shims and once the sleepers are in place and leveled you can install the plywood overtop.
The result is a subfloor that protects finished flooring from cold and damp concrete based floors.
There are many reasons that a floor surface or subflooring can be uneven.
Fixing an uneven floor is just more than leveling.
You need a dry clean and flat surface before you can install finished flooring in a room built on a concrete slab.
One of the main reasons for installing a sleeper system subfloor is to deal with un eveness of the concrete slab.
Place 16 inch planks perpendicular or diagonal to direction of flooring.
Alternate subfloor system use a minimum 3 4 inch cd exposure 1 plywood sheathing 4 foot by 8 foot sheets.
In a perfect world all the old flooring and subflooring would be removed and have everything leveled with a brand new subflooring surface.
One of the simplest options is a sleeper based subfloor.
As a result the material for the subfloor will acclimate to the temperature and moisture content of the room expanding or contracting before the installation rather than after.
Grind the floor use a self leveling compound or use a subfloor to get it pretty close to even as possible.