What Does C24 Mean? Timber Grading Explained
If you've been browsing timber for a decking frame, floor joists or roof timbers, you’ve probably come across the terms C16 and C24. And if you’ve assumed that C24 means it’s pressure-treated—you're not alone!
But here’s the thing: C24 is a strength grade, not a treatment. So let’s clear things up and help you choose the right timber for your job.
? What Does C24 Actually Mean?
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C24 is a strength classification used for structural timber—like joists, rafters, lintels, and studwork.
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It’s kiln-dried and stress-tested to meet building regulations.
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C24 timber is stronger, denser, and straighter than C16—often sourced from slower-grown timber like Scandinavian whitewood.
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It’s commonly used in structural builds where higher load-bearing capacity is needed.
? And What About C16?
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C16 is still structural, but it’s the entry-level strength grade.
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It may include more knots and slightly less stability under heavy load—but it’s still totally fine for most general building work.
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Often used in internal frames, wall studs, and areas with lighter loads.
? So... Is C24 Treated?
Not necessarily!
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C24 and C16 are grading terms, not treatment types.
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Some C24 timber is treated (usually green)—especially when used outdoors.
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But not all of it is! You’ll often find C24 timber in its natural state—perfect for internal structural work where treatment isn’t needed.
If you’re building outside or in contact with moisture, always make sure you’re using treated timber, regardless of the grade.
? Which One Should I Use?
| Use Case | Recommended Timber |
|---|---|
| Garden decking frames | C24 treated timber |
| Floor joists or roof rafters | C24 (treated or untreated, depending on exposure) |
| Stud walls or light frames | C16 untreated is fine |
| General indoor structural work | C16 or C24 (untreated) |
| High-load structural areas | C24 preferred |
? Quick Tip: Check Both the Grade and the Treatment
When choosing timber, check:
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Is it C16 or C24? (strength)
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Is it treated or untreated? (longevity)
If you’re unsure, we’ll always help point you in the right direction.
Need Structural Timber?
We stock both C16 and C24 graded timber at Wood Green Timber—available in treated and untreated options depending on your needs. Whether you’re building a garden frame or fitting a roof, we’ll make sure you leave with the right gear for the job.
Pop into our Wood Green branch, give us a ring, or browse online—we’re here to help.
