Floor joists and support beams may sag over many years of settling and load changes. Creaky floors may be a sign that floors are starting to deform.
The outer perimeter at ground level, or 'rim joist', may show signs of deterioration if water infiltration has happened. Watch for stains and obvious signs of decay.
Older stairs may show the effects of sagging floors - cracks open up in the joints at stringers, risers and treads
Floors in kitchens and bathrooms would see the most deterioration, for the presence of water.
Rotting of floor structure is a serious problem that happens when water collects around the foundation.
Hand hewn hemlock timber rim joists - Eastern Ontario