A subfloor is the layer of material that is installed directly over the structural floor joists and serves as the foundation for the flooring.
It is essentially the base upon which the finished flooring is installed. The subfloor provides stability, support, and a smooth surface for the flooring material.
The subfloor is typically made of plywood, oriented strand board (OSB), or concrete, depending on the construction of the building.
Its primary function is to distribute the weight of the flooring and any loads placed on it evenly across the floor joists. It also helps to minimize movement, provide structural integrity, and prevent the flooring from sagging or flexing.