Subfloor
![]() |
Workers instal a subfloor for a home being built as part of the Southeast Missouri Farms Project, 1938. |
Contents |
[edit] Introduction
The subfloor is the initial layer of solid material placed just above the joists or other structure of a building.
Some structures may have solid hardwood flooring that is attached directly to the joists. This surface essentially serves as the subfloor.
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.
[edit] Types of subfloor
Plywood is the most common material used for subfloors. Plywood has several properties that make it a useful and popularfor this purpose, including:
- High strength.
- High panel shear.
- Flexibility.
- Moisture resistance.
- Chemical and fire resistance.
- Impact resistance.
- Insulation.
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.
[edit] 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).
Moisture is one of the most common problems associated with subfloors. Signs of subfloor moisture problems include:
- Sunken sections.
- Squeaking.
- 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.
[edit] Related articles on Designing Buildings Wiki
Featured articles and news
Boiler Upgrade Scheme and certifications consultation
Summary of government consultation which closes 11 June 2025.
Deputy editor of AT, Tim Fraser, discusses the newly formed society with its current chair, Chris Halligan MCIAT.
Barratt Lo-E passivhaus standard homes planned enmasse
With an initial 728 Lo-E homes across two sites and many more planned for the future.
Government urged to uphold Warm Homes commitment
ECA and industry bodies write to Government concerning its 13.2 billion Warm Homes manifesto commitment.
Places of Worship in Britain and Ireland, 1929-1990. Book review.
The emancipation of women in art.
CIOB Construction Manager of the Year 2025
Just one of the winners at the CIOB Awards 2025.
Call for independent National Grenfell oversight mechanism
MHCLG share findings of Building Safety Inquiry in letter to Secretary of State and Minister for Building Safety.
The Architectural Technology Awards
AT Awards now open for this the sixth decade of CIAT.
50th Golden anniversary ECA Edmundson awards
Deadline for submissions Friday 30 May 2025.
The benefits of precast, off-site foundation systems
Top ten benefits of this notable innovation.
Encouraging individuals to take action saving water at home, work, and in their communities.
Takes a community to support mental health and wellbeing
The why of becoming a Mental Health Instructor explained.
Mental health awareness week 13-18 May
The theme is communities, they can provide a sense of belonging, safety, support in hard times, and a sense purpose.
Mental health support on the rise but workers still struggling
CIOB Understanding Mental Health in the Built Environment 2025 shows.
Design and construction material libraries
Material, sample, product or detail libraries a key component of any architectural design practice.
Construction Products Reform Green Paper and Consultation
Still time to respond as consultation closes on 21 May 2025.
Resilient façade systems for smog reduction in Shanghai
A technical approach using computer simulation and analysis of solar radiation, wind patterns, and ventilation.