If you’re planning a construction, home renovation, or garden decking project in the UK, you’ve likely come across the terms C16 and C24. Both are structural graded softwoods, but choosing the wrong one can lead to failed building inspections, sagging floors, or unnecessary overspending. In this guide, we break down the key differences between C16 and C24 timber, how they are graded, and exactly when building regulations require C24.
What Does the “C” in C16 and C24 Mean?
The “C” stands for Coniferous (softwood, typically European Spruce, Pine, or Fir). The number that follows indicates the bending strength of the timber measured in Newtons per square millimetre (N/mm²):
- C16 Timber: Has a bending strength of 16 N/mm². It is the standard structural timber commonly used for internal partition stud walls, non-loadbearing frameworks, and basic rafters.
- C24 Timber: Has a bending strength of 24 N/mm² — representing a 50% higher load-bearing capacity than C16. It is sourced from slower-growing trees from colder climates, giving it tighter growth rings, fewer knots, and superior structural density.
Key Differences: C16 vs C24 at a Glance
While both timbers can be kiln-dried and regularised with eased (rounded) edges for smooth handling, their performance characteristics differ significantly:
- Strength & Density: C24 is noticeably heavier, denser, and capable of supporting heavier structural dead and live loads.
- Defects & Knots: C16 allows for larger, more frequent knots and slight grain deviations. C24 has strict grading rules: very few knots, minimal sapwood defects, and virtually no wane or twist.
- Span Capacity: Because C24 is stronger, joists can span longer distances without requiring mid-span support walls or intermediate pillars.
- Aesthetic Finish: Because C24 has fewer visual flaws, it is far more visually appealing if left exposed in timber frames or carports.
When Must You Use C24 Timber?
UK Building Regulations (BS EN 14081) often specify C24 grade timber for high-stress structural applications. You should always specify C24 for:
- First-Floor Joists & Loft Conversions: Where spanning wide rooms without bouncing or deflection is essential.
- Heavy-Duty Garden Decking Sub-Frames: Especially raised decks, commercial boardwalks, or terraces supporting hot tubs.
- Roof Rafters & Trusses: Particularly on pitched extensions and large timber outbuildings where snow and tile loads must be carried.
- Load-Bearing Stud Work: External structural walls that support upper floors and roof loads.
If you are working on internal non-loadbearing partitions or light shelving, C16 is typically adequate and budget-friendly. However, for anything bearing structural weight, investing in C24 graded timber provides long-term peace of mind and full regulatory compliance.
Why Kiln-Dried & Regularised Matters
At Ctimber, our C24 Timber Carcassing is precision kiln-dried (KD) to reduce moisture content below 20%. This minimizes the risk of twisting, warping, or shrinkage once installed inside heated homes. It is also regularised with planed faces and rounded edges, making it clean, splinter-free, and dimensionally accurate for trade contractors.
Buy Certified C24 Timber Online in Surrey & London
Whether you need 47x100mm (4×2), 47x150mm (6×2), or 47x200mm (8×2) C24 timber, Ctimber stocks certified lengths ready for fast delivery across Surrey, Berkshire, London, and the South East. Explore our full range of structural timber supplies or collect direct from our Ottershaw trade yard.