The Mechanism, in One Paragraph
A floor squeaks because a subfloor panel or plank has worked loose from the joist under it and rubs against the smooth shank of its nail each time you step there. For most of the last century subfloor was nailed down: board planks laid diagonally across the joists in older houses, plywood after the Second World War, and OSB in most new construction today. A nail holds a panel down but never draws it tight to a joist that sits slightly proud or slightly low, so a small gap exists from the day the house is built. Wood then takes on moisture through a humid summer and gives it up once the furnace runs. Over decades that cycle works the nail loose until the panel flexes into the gap under your weight, and the rubbing is the noise. Gluing subfloor to the joists is standard practice now, but it is not an Ontario Building Code requirement, so newer homes squeak too, just far less often than pre-1980 nailed floors. Fixing the flooring on top rarely holds. Closing the gap at the joist, with a shim and construction adhesive, a block fastened to the joist, or a screw that draws the panel down, is the repair that lasts in most cases.