What is a subfloor?
A subfloor is the structural layer installed directly over floor joists or a concrete slab that provides a base for finish flooring material like hardwood, tile, or laminate.
The subfloor sits between the floor framing and the visible floor covering you walk on. In residential construction across the Greater Toronto Area, subfloors are most often made from plywood (typically 3/4 inch) or oriented strand board (OSB). In concrete-slab homes or basements, the subfloor may be a layer applied directly to the concrete itself. This layer carries the structural load and must be rated to handle both weight and moisture.
Leveling and proper preparation of the subfloor directly affects how finish flooring performs. An uneven subfloor causes finish materials to move, crack, or separate. When hardwood or laminate planks sit on a wavy base, they shift underfoot and fail prematurely. Tile on an uneven subfloor becomes a cracking hazard. For this reason, contractors check subfloor condition early in a renovation and may sand, shim, or replace sections before laying any finish material.
Moisture matters too. Plywood and OSB swell when damp, which leads to buckling and cupping in wood floors or delamination in laminate. Concrete slabs require a moisture barrier. Damaged or soft subfloor wood signals a leak that must be found and fixed before any new flooring goes down.
A qualified flooring contractor will assess your existing subfloor and determine whether it can support the flooring you want or needs repair first.