If there are any large gaps, they should be filled or they may show through after vinyl mat is laid flat. If you are installing with adhesive, you will want to start with a clean, dry and smooth subfloor for the best adhesion. If you are not adhering the mat to substrate, you do not need to level the floor surface. The vinyl flooring mat is flexible and will contour to the floor to some extent. If you can start with a smooth, filled surface your end result will be better.