Sub-floor refers to the base by which a floor covering such as wood, carpet, vinyl or ceramic tiles, etc. is laid upon.
Sub-Floor vs. Underlayment
A sub-floor is a permanent part of the building structure and should never be removed unless damaged or in cases of remodeling. Underlayment is the layer between the sub-floor and floor covering installed to create a stable and smooth surface. It is also used to raise an area up to be even with another floor. Underlayments can be safely removed and are usually stapled but can also be nailed, glued or screwed.
Types of Sub-Floor
Concrete
A concrete sub-floor is a suitable base for floor installation methods that involve glueing, such as carpet or hardwood planks. The concrete must be fully cured and 100% dry in order for the glue to adhere properly and ensure that no moisture makes its way into the carpet or wood, causing irreversible damage to the flooring materials.
Wood
For wood sub-floors, a 1″ x 6″ boards installed diagonally across floor joists 16″ on center are commonly used for hardwood flooring.
Plywood
For plywood sub-floors, a 3/4″ or 5/8″ T&G plywood sub-flooring installed at 90° to floor joists 16″ on center is usually used for hardwood flooring.
OSB
Oriented Strand Board or OSB is also used as a subfloor nowadays. Its low cost makes it an ideal alternative for plywood or solid wood sub-floors. It is manufactured using wood chips heated and bonded together by resins. In terms of strength and durability, it is almost the same as plywood.