There are many variables but the most common are saggy beams and floor joists caused by wood rot or high levels of moisture in the floor joists. It could also be caused by improper installation when the home is constructed. VA building code states you can’t span a floor joist over 16 feet because it will droop. Older homes didn’t have this code resulting in the bouncy spongy, or squishy feeling of the floors in your home.