Subflooring vs Joists vs Underlayment: What’s the Difference?

When constructing or renovating a home, understanding the different layers of flooring is crucial for ensuring durability, stability, and comfort. Among the essential components are subflooring, joists, and underlayment, each serving a distinct purpose within a building’s structure. Despite their interconnected roles, these elements are often confused. In this article, we will explore the differences between subflooring, joists, and underlayment, examining their functions, materials, and importance in construction.

Joists: The Structural Foundation

Joists are the foundational elements that support the entire flooring system. They are horizontal beams, typically made of timber, steel, or engineered wood, that span across a building’s foundation, walls, or supporting beams. Joists act as the primary structural support for the floor, bearing the weight of everything above, including furniture, occupants, and additional flooring layers.

Joists are usually spaced at regular intervals—commonly 400mm or 600mm apart—to provide even weight distribution and prevent sagging. The material and spacing depend on the load-bearing requirements and building codes. If joists are improperly installed or weakened due to moisture, termites, or excessive loads, they can compromise the entire floor’s integrity, leading to uneven surfaces or structural failure.

Subflooring: The Load-Bearing Layer

Subflooring is the layer that sits directly on top of the joists, acting as a base for all other flooring materials. Its primary function is to provide a stable and even surface for finished flooring while helping to distribute weight efficiently across the joists. A well-installed subfloor prevents squeaking, bouncing, and structural damage caused by movement or shifting.

Common materials used for subflooring include:

  • Plywood – A widely used option due to its strength, affordability, and ease of installation. Plywood subfloors are typically 18mm to 25mm thick.
  • Oriented Strand Board (OSB) – A budget-friendly alternative to plywood, OSB is made from compressed layers of wood strands and adhesives. While strong, OSB is more susceptible to moisture damage than plywood.
  • Concrete – Used in multi-storey buildings, basements, or commercial properties, concrete subfloors provide excellent durability and moisture resistance.

Ensuring the subfloor is properly secured to the joists with screws or nails is vital to prevent movement that could lead to flooring defects. Gaps or weaknesses in the subfloor can result in noise issues or an uneven finished floor.

Below is an example of plywood

 

Underlayment: The Comfort and Protective Layer

Underlayment is an additional layer placed between the subfloor and the final flooring material, serving as a protective and cushioning component. While not always required, underlayment enhances comfort, soundproofing, and moisture resistance, depending on the type of flooring being installed.

There are various types of underlayment, each designed for specific flooring needs:

  • Foam Underlayment – Commonly used under laminate or engineered wood flooring, foam provides a soft cushioning effect and minor soundproofing.
  • Cork Underlayment – An eco-friendly option that offers excellent sound absorption and thermal insulation, often used under hardwood or tile floors.
  • Rubber Underlayment – Ideal for high-impact areas or commercial spaces, rubber underlayment enhances shock absorption and reduces noise transmission.
  • Felt or Fibreboard Underlayment – Used under hardwood floors, felt underlayment helps to smooth out minor subfloor imperfections while providing insulation.
  • Cement Board or HardieBacker – A moisture-resistant underlayment for tile installations, ensuring stability and preventing tile cracking due to subfloor movement.

The necessity of underlayment depends on the type of flooring being installed. For example, carpet installation often includes an underlay for additional comfort, while luxury vinyl flooring may feature a built-in underlayment.

Key Differences and Their Importance

Understanding the differences between joists, subflooring, and underlayment is essential when planning a flooring project. Each layer serves a unique purpose, and failing to install any one of them correctly can result in long-term structural and functional problems.

  1. Function:
    • Joists provide structural support, carrying the load of the entire flooring system.
    • Subflooring acts as a bridge between the joists and the final flooring material, offering stability and weight distribution.
    • Underlayment enhances comfort, soundproofing, and moisture resistance, serving as a supplementary layer.
  2. Materials:
    • Joists are typically constructed from timber, steel, or engineered wood.
    • Subflooring is commonly made from plywood, OSB, or concrete.
    • Underlayment materials vary depending on flooring needs, ranging from foam and cork to rubber and cement board.
  3. Installation Considerations:
    • Joists must be properly spaced and secured to maintain structural integrity.
    • Subflooring should be installed with screws or nails to prevent movement and noise.
    • Underlayment should match the specific flooring type to enhance performance and longevity.

Conclusion

Joists, subflooring, and underlayment each play a crucial role in the construction of a stable and durable floor. Joists form the foundational framework, subflooring provides a solid and level base, and underlayment adds comfort and protection. Whether you are building a new home or renovating an existing floor, understanding these components ensures a successful and long-lasting flooring system. Proper installation and material selection can prevent future issues such as sagging, creaking, or moisture damage, ultimately contributing to a comfortable and resilient living space.