C16 vs C24 Timber — What's the Difference and Which Do You Need?
If you've ever ordered structural timber and been asked whether you need C16 or C24, you're not alone. It's one of the most common questions we get at Nick's Timber Store, and it's worth understanding properly — because getting it wrong can mean either over-spending on your materials or, more seriously, using timber that isn't up to the job.
This guide explains exactly what the two grades mean, how they differ, and how to decide which one your project requires.
What Do C16 and C24 Mean?
The C stands for conifer — both grades are softwood structural timber, typically European redwood or whitewood, grown in managed forests in Scandinavia or the UK.
The number refers to the strength class as defined by the European standard BS EN 338. The higher the number, the stronger and stiffer the timber. So C24 is stronger than C16 — it has a higher bending strength, higher stiffness (modulus of elasticity), and higher density on average.
Both grades are visually graded or machine stress graded during production, and each piece is marked with its grade, species, and the grading standard it meets. When you buy from Nick's Timber Store, all our structural carcassing timber is clearly marked so you always know what you're getting.
What Are the Actual Differences?
The key structural properties that differ between C16 and C24 are:
Bending strength (MOR) — C24 has a characteristic bending strength of 24 N/mm², compared to 16 N/mm² for C16. This is where the grade numbers come from.
Stiffness (MOE) — C24 is stiffer, with a mean modulus of elasticity of 11,000 N/mm² versus 8,000 N/mm² for C16. Stiffer timber deflects less under load, which matters for floor joists and rafters where bounce or sag is undesirable.
Density — C24 tends to be denser and therefore heavier. The characteristic density is 350 kg/m³ for C16 and 410 kg/m³ for C24.
In practical terms, C24 can span further for the same size section, or carry more load for the same span. This means you can sometimes use a smaller C24 section where you'd need a larger C16 section, which can offset the cost difference.
When Should You Use C16?
C16 is the most widely used structural timber grade in domestic construction in the UK and is suitable for the vast majority of standard applications including:
- Stud walls — partition walls and load-bearing stud walls in standard domestic construction
- Floor joists — for standard residential spans up to around 4m, C16 is generally adequate when correctly sized to the span tables
- Roof rafters and ceiling joists — again for standard domestic spans and pitches
- Flat roof joists — in most domestic applications
- General carcassing — noggins, trimmers, lintels over small openings, sole plates
For most house extensions, loft conversions, and new builds following standard timber frame specifications, C16 will be the grade specified by the structural engineer or building inspector. It is widely available, cost-effective, and perfectly fit for purpose in these applications.
View our C16/C24 treated carcassing range.
When Do You Need C24?
C24 is required when the structural demand is higher than C16 can comfortably meet. The most common situations where C24 is specified include:
- Longer spans — if your floor joists need to span more than around 4–4.5m, C24 allows you to use a smaller section timber for the same span, or gives you greater confidence in a larger section
- Building regulations specifications — some structural engineers and building control officers will specify C24 in their calculations, particularly for engineered designs or where loads are higher than standard
- Timber frame construction — many timber frame house manufacturers specify C24 throughout for consistency and structural confidence
- Exposed or feature structures — pergolas, oak frame style structures, or any application where the timber is visible and structural
- Scottish building regulations — Scotland has historically been more likely to specify C24 as a default grade than England and Wales
If your project has a structural engineer's specification, always follow it. If building control has issued a requirement for C24, use C24. Never substitute C16 where C24 has been specified without checking with your structural engineer first.
Can You Tell the Difference by Looking at Them?
Not reliably — and this is important. C16 and C24 timber can look very similar. C24 tends to have fewer and smaller knots, tighter grain, and a slightly denser feel, but you cannot reliably grade timber by eye. The only reliable way to know the grade is to check the stamp on the timber itself.
All structural timber sold in the UK should be marked with its strength class (C16 or C24), the grading standard (BS EN 14081 for machine grading), the species or species group, and the grading organisation's mark. If timber isn't marked, don't use it for structural applications.
Is C24 Worth the Extra Cost?
C24 typically costs around 10–20% more than equivalent C16. Whether that's worth it depends on your application:
- If C16 meets the specification — there's no structural benefit to paying more for C24. Save the money.
- If you're on the borderline — for example, a joist span that sits right at the limit of C16, upgrading to C24 gives you a meaningful margin of safety and may allow you to use a smaller section, partially offsetting the cost.
- If C24 is specified — it's not optional. Use it.
For trade customers who are ordering regularly, it's worth having a conversation with us about your typical applications so we can make sure you're always ordering the most cost-effective grade for the job.
Treated vs Untreated — Does the Grade Affect This?
No — C16 and C24 are strength grades and are independent of treatment. Both are available in treated and untreated versions. For any structural timber that will be used in ground contact, near the ground, or in exposed external conditions, you should always use pressure-treated timber regardless of grade.
Our treated carcassing uses UC2 green tanalith treatment for above-ground use and UC4 for ground contact applications such as fence posts. The treatment does not affect the structural grade.
View our untreated carcassing range.
Quick Reference Guide
| C16 | C24 | |
|---|---|---|
| Bending strength | 16 N/mm² | 24 N/mm² |
| Stiffness | 8,000 N/mm² | 11,000 N/mm² |
| Typical use | Standard domestic | Longer spans, engineer spec |
| Cost | Lower | ~10-20% more |
| Appearance | More knots typical | Tighter grain typical |
| Building regs | Often acceptable | Required when specified |
What We Stock
At Nick's Timber Store we hold large stocks of both C16 and C24 treated and untreated carcassing in a full range of sizes — from 47x50mm battens up to 47x225mm joists. All our structural timber is clearly grade-stamped, sustainably sourced, and kiln-dried for dimensional stability.
Common sizes in stock include:
- 47mm x 75mm — stud work and noggins
- 47mm x 100mm — standard joists, rafters, and studwork
- 47mm x 125mm — medium span joists and rafters
- 47mm x 150mm — larger span floor joists and rafters
- 47mm x 175mm — longer span applications
- 47mm x 200mm and 47mm x 225mm — for the longest domestic spans
View our full treated carcassing range or untreated carcassing range.
Still Not Sure Which Grade You Need?
Call us on 01452 300159 and our team will help you work it out. If you have a structural engineer's spec or building control drawing, we can read it with you and make sure you order exactly the right grade and size. We'd rather spend five minutes on the phone getting it right than have you order the wrong thing.
We deliver within 75 miles of Gloucester — covering Cheltenham, Bristol, Stroud, Swindon, Hereford, Oxford, and the Forest of Dean — with free delivery on orders over £750 + VAT. Collection from our Gloucester depot is also available same day on stocked items.
Nick's Timber Store, Canada Wharf, Bristol Road, Gloucester GL1 5TE. Rated 4.9/5 on Trustpilot from over 96 reviews.
