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Last edited 16 Feb 2021
|Workers instal a subfloor for a home being built as part of the Southeast Missouri Farms Project, 1938.|
Others may have a solid concrete foundation that serves as the subfloor. Concrete slabs that form the ground floor of a building may be either supported on beams (called a suspended slab) or supported directly on the subsoil (via hardcore, for example) called a ‘ground-bearing slab'.
Additional structure is sometimes installed over concrete slab subfloors in order to provide a level foundation that’s suitable for the installation of underlayment and flooring. This type of structure may be used when bearing conditions require it or groundwater is present.
Placing a solid subfloor over the joists, covering it with underlayment and then layering that with flooring provides a more stable surface. It also helps to assure that the joists support most of the load of the floor.
 Types of subfloor
- High strength.
- High panel shear.
- Moisture resistance.
- Chemical and fire resistance.
- Impact resistance.
The thickness of a plywood subfloor depends on the distance between joists. Thinner plywood can be used when joists are close together, but slightly thicker plywood is recommended when there is more than 40cm between joists.
Subfloors can also be made from oriented strand board (OSB). OSB has similar properties to plywood and is suitable for load-bearing applications. It can be more cost-effective than plywood. Impermeability to water can be achieved through the use of additional membranes.
Another type of subfloor is high-performance engineered panels. Like plywood and OSB, these panels are designed for load bearing applications. They have built in moisture resistance and are manufactured with special resins to reduce other issues - such as swelling - associated with some subfloors.
 Problems with subfloors
Issues with subfloors can arise due to problems with installation, use of unsuitable materials or damage caused by dampness. These problems can be minor (resulting in squeaking or sagging) or they can be major (resulting in significant movement or total failure).
- Sunken sections.
- Smelling damp.
- Shifting or bouncing floors.
- Rocking or loose fixtures and fittings.
- Cracking tile flooring.
- Dipping or buldging.
- Bubbling linoleum floor.
A vapour or moisture barrier installed over the subfloor can sometimes help to reduce seasonal fluctuations brought about by changes in humidity and heat, but serious moisture issues may require more significant remediation measures including subfloor replacement.
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