7x3 Timber Treated C24

Understanding 7x3 Treated Timber

75mm x 175mm sawn treated timber, traditionally known by the nominal 7x3 inch measurement, is a high-mass carcassing material designed for applications where standard 47mm widths lack the necessary bearing surface or stability. These sections are kiln-dried to improve dimensional stability and are strength-graded to C24 standards. This ensures a high-density wood with minimal knots and defects, providing the mechanical resistance required for safety-critical structural elements that must support significant permanent and imposed loads.

To ensure performance in humid or exposed environments, the timber undergoes a high-pressure preservative treatment process. This forces biocides deep into the timber's cellular structure, creating a robust barrier against fungal rot and wood-boring insects. This makes the 75mm x 175mm profile particularly suitable for structural work that may be subject to moisture before the building envelope is fully sealed.

The timber is regularised to a finished size of approximately 70mm x 170mm. This precision finishing includes eased edges, which facilitates safer manual handling and ensures a consistent height across floor or roof planes. The 75mm width offers a generous surface for fixing sub-floors and roof decks, while the 175mm depth provides excellent resistance to deflection over medium-to-long spans.

Material Range

• Individual 75mm x 175mm Lengths: Heavy-duty structural pressure-treated lengths available in 3.6m, 4.8m, and 6m sizes to meet diverse engineering specifications for floor and roof construction.
  
  

Performance Benefits

• Superior Compression Strength: The 75mm thickness provides a wider bearing surface, reducing the risk of crushing at support points and improving fixing stability.
• Enhanced Durability: Pressure-treated with advanced fungicides and insecticides to prevent decay and rot, ensuring the longevity of the building’s skeleton.
• High-Level Structural Grading: Delivered in a C24 grade to offer superior tensile strength and stiffness compared to standard domestic framing timber.
• Dimensional Consistency: Regularised through a machining process to ensure uniform depth and width, essential for level floor and ceiling finishes.
  
  

Typical Applications

• Primary Floor Joists: Supporting internal flooring systems across wide spans in residential and light commercial builds.
• Heavy-Duty Roof Rafters: Forming the main sloping framework for high-specification pitched roofs.
• Structural Timber Studwork: Constructing load-bearing external wall panels and internal partitions that require extra thickness.
• Decking Substructures: Providing a robust, rot-resistant base for large elevated timber decking and platforms.
• Flat Roof Joists: Supporting the deep structural layers required for insulated warm or cold flat roof designs.
  
  

Specification & Standards

• BS EN 14081-1: Strength graded structural timber with rectangular cross section.
• BS EN 335: Durability of wood and wood-based products - Use classes.
• BS EN 350: Classification of the durability of wood to biological agents.
  
  

Related Materials and Construction Uses

These 75mm x 175mm sections are frequently utilised by contractors requiring premium structural wood with superior density. In many large-scale builds, these wide sections are used in conjunction with other structural timber for joists to create a rigid framework that minimises bounce and vibration in flooring systems.

To maintain the protective integrity of the material, any on-site cuts, notches, or drill holes must be retreated with a suitable end-grain preservative. These heavy-duty sections should be installed using appropriately sized galvanised hangers and high-tensile fixings to ensure the long-term structural safety of the assembly.