If you're buying structural timber in the UK, you'll come across three terms over and over: C16, C24, and CLS. They're not interchangeable, and choosing the wrong one can cost you in materials, time, and structural performance. This guide explains the difference, with the actual strength figures, span tables, and finished sizes you'll need before you order.
At Nottage Timber Merchants we only stock C24. Here's why.
What do C16 and C24 actually mean?
Both are strength grades for softwood structural timber, defined by BS EN 338, the European standard. The "C" stands for "coniferous" (softwood), and the number is the grade's bending strength in N/mm² (also written as MPa).
- C16 — bending strength 16 N/mm². Lower grade. Suitable for light structural work, DIY, and temporary structures.
- C24 — bending strength 24 N/mm². Higher grade. Required for most structural applications under UK building regulations.
The 50% jump in bending strength is the headline number. It also translates into better tensile strength, better compression strength, and tighter restrictions on knots, splits and other defects.
C24 vs C16: side-by-side comparison
| Property | C16 Timber | C24 Timber |
|---|---|---|
| Bending strength | 16 N/mm² | 24 N/mm² |
| Tensile strength (parallel to grain) | 10 N/mm² | 14 N/mm² |
| Compression strength (parallel to grain) | 17 N/mm² | 21 N/mm² |
| Moisture content (kiln-dried) | ≤20% | ≤20% |
| Permitted defects | More tolerant of knots, wanes | Stricter — fewer defects allowed |
| Stability | Moderate | High — less twist, warp and shrinkage |
| Typical use | Light DIY, non-structural framing | Joists, rafters, trusses, stud walls, beams |
The CLS question: is CLS the same as C16 or C24?
One of the most common questions we get is "do you sell CLS timber?" The short answer is no — and once you understand what CLS actually is, you'll see why it matters.
CLS stands for Canadian Lumber Standard. In terms of grade, CLS falls under C16. So when you see CLS on a timber yard's website, you're essentially looking at a C16 product with planed-rounded edges.
The "3x2 illusion": why finished sizes matter
This is where most customers get caught out. A piece of timber sold as "3x2" can be wildly different sizes depending on the grade:
- 3x2 CLS (C16) finishes at 63mm x 38mm
- 3x2 C24 finishes at 70mm x 47mm
Both are sold as "3x2" but you're getting noticeably less timber with CLS. That's a real difference in cross-sectional area — about 31% less timber, before you even consider the strength gap.
So when someone tells you they can get "3x2 CLS" cheaper down the road, the comparison isn't fair. You're paying for a smaller, lower-grade piece of wood. Compare it like-for-like with our C24 range and the value picture changes.
C24 span tables (domestic loadings)
One of the practical advantages of C24 is that it spans further than C16 at the same dimension. Here's a quick reference for maximum spans of C24 timber under typical domestic loadings:
| Timber size (mm) | Max joist span (m) | Max rafter span (m) |
|---|---|---|
| 47 x 100 (4x2) | 3.0 | 2.5 |
| 47 x 150 (6x2) | 4.0 | 3.5 |
| 47 x 200 (8x2) | 5.0 | 4.0 |
| 47 x 225 (9x2) | 5.5 | 4.5 |
These are guidance figures — always check building regulations and span tables for your specific load case before ordering. Our team can advise on bespoke projects.
Why use C24 over C16
1. Fewer members or smaller cross-sections
Because C24 is roughly 50% stronger, you can often achieve the same load-bearing performance with fewer joists, wider centres, or smaller dimensions. On a typical floor, C24 at 450mm centres can replace C16 at 400mm centres — saving timber and labour.
2. Greater spans
C24's strength translates into longer spans without intermediate support. That's a real benefit on loft conversions, open-plan extensions, and any project where you want to avoid extra load-bearing walls or columns.
3. Better dimensional stability
The stricter grading process means C24 has fewer and smaller defects. Less likelihood of warping, twisting, or cupping after installation. That matters for engineered floors, tight tolerances, and anywhere movement would show.
4. UK building regulations
Most UK structural applications assume C24 as the minimum grade. Using C16 where C24 is required isn't just a quality issue — it can fail building control inspection.
Typical C24 applications
- Floor joists and ceiling joists
- Roof rafters and trusses
- Stud walls and timber framing
- Structural beams
- Decking subframes and load-bearing garden structures
- Loft conversions and extensions
Better value than the price tag suggests
C24 typically costs 10-15% more than C16 per linear metre. But that premium is often offset by:
- Fewer joists or rafters needed at wider centres
- Smaller dimensions achieving the same load capacity
- Less waste from twisting and warping
- No callbacks for squeaky floors or uneven surfaces
For most projects, the total cost difference is smaller than it looks — and the structural margin is significantly higher.
Our C24 timber range
We stock C24 in the standard UK sizes:
- 47 x 50mm (2x2)
- 47 x 75mm (3x2)
- 47 x 100mm (4x2)
- 47 x 125mm (5x2)
- 47 x 150mm (6x2)
- 47 x 175mm (7x2)
- 47 x 200mm (8x2)
- 47 x 225mm (9x2)
- 47 x 250mm (10x2)
All kiln-dried to below 20% moisture content and regularised for consistent dimensions. Pressure-treated options available for outdoor or ground-contact use.
Browse our full C24 timber range →
Summary: which grade should you use?
- C24 — the right choice for any structural application: joists, rafters, studs, trusses, beams. UK building regulations assume C24 as the minimum.
- C16 / CLS — fine for light DIY, non-structural framing, or temporary work. Don't use it where loads matter.
If you're not sure which you need for your project, get in touch — we'll talk you through it before you order.