When choosing structural timber, builders, carpenters and DIYers are often faced with three common options: C16, C24 and CLS. All three are used across construction, but they are not equal — and choosing the wrong grade can cause issues later, especially with strength, straightness and long-term stability.
If you want the strongest, straightest and most reliable option, C24 is the clear winner. Here’s the factual breakdown of why.
1. C24 Timber Has Higher Strength — Proven by Grading
Every structural timber in the UK is stress-graded.
The “C” grades relate to compressive strength, bending strength and density.
Strength Comparison (simplified)
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C16 – Minimum structural grade allowed in the UK building regulations
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C24 – Higher structural grade, typically imported from slower-grown timber
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CLS – Not a strength grade (usually falls within C16 unless specified otherwise)
Why this matters:
C24 timber can withstand higher loads and longer spans.
That means:
✔ Fewer supports needed
✔ Less bounce in floors
✔ Better long-term structural stability
If you’re building anything load-bearing — floors, joists, decking frames, roof timbers — C24 is the superior choice every time.
2. C24 Timber Is Straighter, Cleaner and More Stable
To be graded as C24, the timber must meet stricter requirements:
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Fewer and smaller knots
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Straighter grain
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Higher density
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More consistent moisture content
This makes C24:
✔ Less likely to twist or bow
✔ Easier to work with
✔ Better looking for exposed structural areas
✔ More reliable over long lengths
C16 and CLS simply don’t have the same restrictions, which is why they can vary more in appearance and stability.
3. C24 Reduces Wastage and Installation Problems
Because C24 is straighter and stronger, you waste fewer lengths to:
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Excessive knots
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Warping
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Crooked grain
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Splits
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Strength defects
Contractors consistently report that C24 creates:
✔ Faster installations
✔ Less site sorting
✔ Fewer callbacks
✔ Better finish quality
The reliability of C24 saves labour time — and labour, not timber, is the most expensive part of any job.
4. C24 vs CLS: Why They Are Not Interchangeable
CLS is commonly used for studwork, partition walls and internal framing.
It is planed with eased corners, making it cleaner for handling and boarding.
However:
CLS is not a strength grade.
It is usually graded to C16, not C24.
This means:
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It is not suitable for many structural applications.
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It cannot replace joists, decking frames or load-bearing components.
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It is designed for non-structural internal uses.
If you need strength, stiffness or long-term performance, C24 is the step up.
5. C24 Performs Better Outdoors and in Challenging Conditions
For treated structural timber used externally — decking, pergolas, sheds, raised beds, etc. — C24 is far more reliable.
Because of its density and lower knot content, C24:
✔ Resists movement better in changing weather
✔ Holds fixings more securely
✔ Performs better over long spans
✔ Keeps its shape longer
C16, especially at longer lengths, is far more prone to twist, bow and cup.
(Important note: outdoor structural timber must be UC3 or UC4 treated — C24 describes strength, not treatment.)
6. C24 Is Simply a Higher-Quality Timber
Because it is sourced from slower-grown species (often Scandinavian or Baltic), the growth rings are closer together. This gives:
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Higher strength
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Better rigidity
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Improved holding power
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Better machining properties
In short:
C24 is engineered by nature — then verified by grading.
C16 and CLS cannot match that performance.
7. The Price Difference Is Surprisingly Small
Most customers assume C24 is “expensive.”
In reality, the price difference is usually small, especially when weighed against:
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Fewer failed lengths
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Less wastage
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Faster installation
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Better long-term performance
When the structure matters, the value of C24 massively outweighs the cost difference.
Summary: Which Timber Should You Choose?
| Timber Type | Best Uses | Limitations |
|---|---|---|
| C24 | Structural work, joists, decking frames, floors, roofs, outdoor structures | None — the highest standard commonly available |
| C16 | Basic structural tasks, small internal studwork | More prone to twisting, lower strength, less reliable |
| CLS | Internal studwork only | Not suitable for load-bearing uses |
Final Word: Why We Recommend C24
At Nottage Timber Merchants, we strongly advocate using C24 for structural projects — not because it’s more expensive, but because it performs better in every important category:
✔ Stronger
✔ Straighter
✔ Fewer defects
✔ More stable
✔ Better for long spans
✔ Better for outdoor structures
✔ Better long-term value
If you want peace of mind, durability and reliability, C24 is the best choice you can make.