What is C24 Timber? The Differences Explained between C24 vs C16

When planning any structural project—whether you’re building a raised deck, fitting a new roof, or framing a garden room—the quality and strength of the timber you choose is critical. One of the most commonly searched terms among DIYers and trade professionals alike is “What is C24 timber?” Closely followed by “C16 vs C24: what’s the difference?”

In this guide, we’ll explain the meaning of C24 timber, how it differs from C16, and which type of timber you should use for deck joists, floor joists, roof framing, and other load-bearing construction tasks. Whether you’re a homeowner looking to build a garden structure or a tradesperson sourcing reliable structural timber, this article will help you make an informed decision.

What is C24 Timber?

C24 timber is a high-strength, graded softwood used for structural applications such as deck joists, roof joists, and timber framing. It is machine-graded (or sometimes visually graded) and kiln-dried to reduce moisture content, ensuring maximum strength and stability.

The “C” in C24 stands for “conifer,” and the number refers to its bending strength in N/mm². In this case, 24 N/mm². That means C24 joists can handle higher loads, longer spans, and more demanding structural applications compared to standard C16 timber.

C16 vs C24 Timber – What’s the Difference?

The key difference between C16 and C24 lies in their strength, stiffness, and visual quality.

Feature	C16 Timber	C24 Timber
Strength	Lower (16 N/mm²)	Higher (24 N/mm²)
Knot Size & Frequency	More knots and defects	Fewer knots, higher quality
Load-Bearing	Suitable for basic structures	Ideal for structural spans
Appearance	Coarser, more visible defects	Smoother, more refined
Applications	Budget sheds, basic garden projects	Deck joists, roof joists, framing timber

C24 timber is stronger, more visually appealing, and can span longer distances without bending or flexing. This makes it the preferred option for structural builds that require reliability, longevity, and compliance with building regulations.

Why Choose C24 Timber for Decking?

If you’re building a decking subframe, the timber you choose can make or break the project’s long-term success. C24 timber joists are ideal for decking because:

They’re kiln-dried to reduce movement and twisting
Stronger than C16, so they can span longer gaps between support posts
Better resistance to load from furniture, people, and weather

To get the most out of your decking project, we recommend pairing C24 timber with joist flashing tape for moisture protection and ForgeFast Timber Screws for secure fixings. For timber deck boards, you’ll find a wide range in our decking category.

Is C24 Timber Better for Roof and Floor Joists?

Yes—C24 timber joists are the ideal solution for roof structures and floor builds where strength and compliance are critical. Roof and flooring applications often require timber that can handle constant load over time. Here’s why C24 roof joists or floor joists are often specified:

Stronger under load – supports greater weight without deflection
Better dimensional stability – reduces warping or sagging over time
Certified and compliant – meets structural regulations in most domestic and commercial builds

Using C24 timber joists ensures peace of mind and reduces the need for mid-span support, which also saves time and material cost in certain cases.

Which Size of C24 Timber Should I Use?

The size of C24 timber you need depends on the application, span length, and load. Here’s a quick guide:

100mm x 47mm C24 Timber – Light framing, shelving, general garden use
150mm x 47mm C24 Timber – Most popular size for deck joists and framing
200mm x 47mm C24 Timber – Ideal for larger decks, roof joists, or raised floors with long spans

You can browse our full range of C24 timber sizes here, all pressure treated and kiln dried for exterior and structural use.

What Does Pressure Treated C24 Timber Mean?

All C24 timber sold at Clarke Fencing is pressure treated, meaning it’s treated for ground contact and long-term outdoor use. The treatment process infuses preservatives deep into the wood fibres, protecting against rot, fungal decay, and insect attack.

However, even with pressure treatment, we recommend limiting direct ground contact where possible to further extend the lifespan of your framing. Elevating your timber using concrete deck support posts or similar alternatives helps prevent moisture exposure and reduces the risk of premature rot, especially in damp or shaded garden environments.

This approach not only boosts durability but also improves structural integrity over time—making it an essential step for deck joists, floor frameworks, and other load-bearing projects using C24 timber.

Is C24 Timber Worth the Extra Cost?

In short—yes. While C24 timber is typically 10–15% more expensive than C16, it pays for itself in performance, longevity, and fewer headaches down the line. With fewer knots, straighter cuts, and longer spans, you’ll need less bracing and enjoy a more professional result.

For builders, landscapers, or homeowners who value quality, C24 framing timber is the smarter long-term investment.

Where Can I Buy C24 Timber Near Me?

Clarke Fencing stocks a wide selection of C24 timber in various lengths and sizes, available for delivery across Suffolk and beyond. Whether you need a single 3.6m joist or a full load of 4.8m lengths, we can supply what you need quickly and affordably.

Shop the full C24 timber range online here or visit us in store for trade quantities.

Summary: Should You Choose C24 Timber?

If your project demands strength, stability, and long-term performance, C24 timber is the clear winner over C16. It’s the preferred grade for decking joists, roof structures, floor frames, and outdoor buildings. Though slightly more expensive than C16, the benefits far outweigh the cost—especially for larger builds.

If you’re still unsure which timber grade to choose, read our full guide:
👉 What is C24 Timber? The Differences Explained between C24 vs C16

Or feel free to call us or visit Clarke Fencing for tailored advice on your project.

Jason Clarke

Ask them a question by emailing [email protected]

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What is C24 Timber? The Differences Explained between C24 vs C16