Why stairs squeak before floors do
A staircase is the hardest-working assembly in a house. Every trip between floors puts a concentrated load on a small number of joints, and unlike a floor there is nowhere for that load to spread out. A traditional wood staircase is built from treads and risers fitted into notched stringers and held with glue blocks and wedges driven in from behind. Over time the glue fails, the wedges dry out and back off, and the tread starts to move a fraction of a millimetre against the riser or the stringer every time somebody steps on it. That movement is the squeak. Because the components are wood, the same seasonal shrinkage that loosens a subfloor fastener also shrinks the wedges holding a stair together, which is why stair noise usually gets worse once the heating season starts. The good news is that a staircase is a repairable assembly. In almost every case the fix is refastening or re-wedging the specific joint that has moved, not rebuilding the stairs.