What is C16 and C24 timber – and which should you use?

Charlotte Jewell
Aug 5
2 min read

At Youngs, we stock C16 and C24 structural timber across our Kent branches because each has a role. But what’s the difference - and how do you choose the best one for your build?

What do the C16 and C24 grades mean?

In England and across Europe, softwood timber is graded by strength under the BS EN 338 standard (strength class).

C16 means the timber has a minimum bending strength of 16 N/mm²
C24 means 24 N/mm² - stronger, straighter, and with fewer knots

These strength classes ensure the wood is fit for purpose - whether it’s joists, beams or stud walls - helping prevent sag or failure over time.

How are these grades used at Youngs?

We stock both C16 and C24 treated carcassing timber. C16 is typically used for internal stud work, rafters, ceiling joists and decking joists. C24 is used when structural strength or appearance matters - like long-span rafters, heavy load-bearing frames, or exposed beams.

Both grades are kiln-dried and planed, making them dimensionally stable and easy to work with.

C16 vs C24 - which one suits your project?

Use Case	Recommended Grade	Why it Works
Internal stud walls, partitioning	C16	Cost-effective and adequate
Decking joists (up to ~600mm high)	C16 (treated)	Meets Building Regs for low decks
Long-span rafters, loft joists	C24	Stronger and straighter for structural use
Exposed framework or visible finishes	C24	Cleaner look and fewer knots

By choosing C16 where strength isn’t critical, you save money - only use C24 where necessary.

Why it matters

Kent's climate brings damp soil and changeable weather.

Timber in contact with the ground, used externally, or supporting heavy loads needs to meet both strength (like C24) and durability (like UC4 treatment).

Using untreated or incorrectly graded timber risks deflection, warping - or even failure.

In summary...

C16: Cost-effective, suitable for many indoor projects and lower-loading outdoor builds
C24: Higher strength, straighter profile, suited to load-bearing or visible structure
Make sure either grade is treated to UC4 standard if used outdoors or in ground contact
Need help choosing the right grade or size? Pop into your local Youngs branch—we’ll help work it out

Whether it’s decking joists, internal frames, or rafter setups for a garden building, choosing the right timber makes all the difference.