Do You Need Subflooring Under the Carpet?

Definition of Subflooring

• The online Merriam-Webster dictionary defines a subfloor as “a rough floor laid as a base for a finished floor." The International Standards and Training Alliance for Floor Covering Professionals describes subflooring as the floor or surface of the structural foundation or the surface upon which a carpet is placed. Although definitions vary a bit, the concept remains the same -- the subfloor is literally the solid floor upon which your flooring material is placed. Whether the subfloor is actually the structure's foundation or an additional layer of floor placed over the foundation floor is dependent upon the requirements of the flooring manufacturer and the needs of the homeowner.

Types of Carpet Subflooring

• In residential construction, there are two choices for subfloors that are foundations for a carpet installation: wood and concrete. Wood in the form of plywood, plank or oriented strand board — a compressed wood chip sheet — is nailed to a series of parallel beams called joists to create the subfloor. Concrete is poured as a slab and allowed to cure before carpet is installed over it. Moisture that remains in the concrete can interfere with the adhesives used to apply carpet. Therefore, all concrete should be tested for residual moisture before laying carpet.

What Carpet Requires

• Carpet can be placed directly on either type of subfloor — wood or concrete. Wood requires a debris-free surface and the removal or repair of nails, screws and sharp objects that might damage the carpet backing. Concrete subfloors require testing and preparation before installing the carpet. This Old House recommends testing whether a vapor barrier — placed by the original builder -- exists between the slab and the ground before laying carpet. A simple test for moisture requires taping a 12-inch square sheet of plastic to the slab and leaving it for 24 to 48 hours. If condensation shows up on the plastic within that time, there is no vapor barrier or the barrier is compromised. In this case, providing a vapor barrier as an underlayment to the carpet is necessary before installing over the concrete.

Using a Substrate

• Substrates, sometimes referred to as underlayments, are additional layers of material applied between a level concrete or wood subfloor and carpet. Substrates include backing, padding, moisture barriers and adhesives. In some cases, plywood is used as a substrate in carpet installations. Although not necessary for a normal carpet installation, the wood substrate is used to raise the finished floor surface to meet other flooring surfaces. Padding and additional moisture barriers are provided per the homeowner's personal requirements. Substrates that are moisture barriers prevent liquids from seeping through the carpet to the subfloor below, as in the case of spilling a drink or toppling an open container.