The successful installation of a new floor is dependent on the condition of the sub floor. The sub floor is the material either wood or concrete that serves as the bottom layer of support for the flooring. Homes built from the 1970's generally have plywood floors and homes before this have 1ft X 8ft boards as sub flooring.
Fixing Low Spots
A floor should be smooth and level but over time houses shift and low spots on the floor sometimes develop.
Check for low spots
Do a quick visual inspection to check for any significant low spots.
Lay and lift the level in various places over the entire floor and get down on your hands and knees and look for spots where the sub floor and level don't touch. If light passes between the sub floor and level indicate a low spot.