Why does my floor squeak?

Usually this is due to a poor sub-floor like those found in many older homes.

The old 3/8" strip flooring reacts to the humidity changes by expanding and contracting, sometimes causing a cupping or crowning effect on the strips. When you walk on the floor, the pieces might flatten out and cause a rubbing effect that gives a creaking sound. It is best to avoid using any solid 3/8" flooring if possible.

In newer floors it can sometimes be a result of shifting of your floor because of changing humidity. This can be avoided by maintaining an equal humidity level throughout the year.