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C24 and C16 timber: What’s the difference?

Understanding the differences between C16 and C24 can be tricky. Our guide should help answer some of your questions to make it clearer!

Ultimately, it depends on your needs, but there’s no denying that having an understanding of each type can help you make a better decision.

C16 is usually used in areas that need moderate load bearing, like floors and walls in outbuildings or for repairs in existing homes. C24 offers higher load-bearing capacity typically used in larger projects, new builds and for achieving building regulation requirements.

So here’s a round-up of the key differences, and some key information about what to consider and look for when choosing structural timber for your next project.

What is C16 and C24 timber?

structural timber

C16 and C24 are types of softwood timber that have been structurally graded. They are used across a wide range of structural applications including the main shell, and frame of skeletal structures of buildings. This includes the construction of floor joists, stud walls, partition walls and roofing rafters.

Essentially it’s structural timber milled from graded softwood that has been kiln-dried to reduce the moisture content to 20% or below.

Both can be pressure treated, and this is where they differ. C16 is pressure treated to Use Class 3, also known as UC3. C24 on the other hand only achieves only UC2 treatment certification. Due to the density of the timber, the treatment does not penetrate enough to achieve UC3.

What is UC3 treated?

The UC3 tanalising process itself can be broken down into five main steps:

The timber is loaded into a large treatment tank and put under a vacuum. The tank is then completely flooded with wood preservatives. The tank, whilst full, is then put under hydraulic pressure, forcing the preservative deep into the timber.

So C16 is better for outside projects, or applications where it might be exposed to moisture, but is not suitable for below-ground conditions.

What does the ‘C’ in C16 and C24 stand for?

Wondering what the C in C16 and C24 stands for?

The letter ‘C’ actually stands for conifer, referring to the type of tree that the timber has been produced from. This includes a range of tree species, such as fir, cedar and pine.

What does ‘16’ and ‘24’ mean?

structural timber
Whilst the letter refers to the type of tree that the timber has come from, the number denotes the strength. The higher the number, the stronger the timber.

Set by British Standards, there are 12 strength grades of timber deemed suitable for construction. The most common two are C16 and C24 timber.

How is timber graded?

To be suitable for structural purposes, timber must have a grading stamp.

This stamp is an important seal of approval that means the timber’s strength and integrity has been tested and approved as safe and strong enough for structural use.

When timber is graded, certain criteria set out by BS5258 must be tested and met to get the golden stamp of approval.

The criteria is there to assess the structural integrity or strength of the wood. Of course, if you’re using timber for building purposes, it must be strong and safe.

When timber is graded for structural purposes, the following criteria is assessed:

  • Woodworm holes
  • Knots
  • Splits along the grain
  • The slope of the grain

Timber with few defects will be graded higher, and timbers with several defects will either be given a lower grading, or it won’t make the cut as being graded as structural timber.

What is C16 and C24 timber?

The key difference between C16 and C24 graded timber is strength.

Because C24 has a higher grading than C16, it is stronger. But what makes it stronger?

C24 has a higher bending, tension, shear and compression parallel to the grain. It also has a high compression perpendicular to the grain and has a higher density and modulus of elasticity.

Whilst this all sounds very technical, the upshot is that because C24 has fewer defects, it is stronger and more resilient.

However, that’s not to say that C16 isn’t a strong timber. In fact, C16 is highly sought after in the construction industry for a wide range of applications due to its strength and density, and pressure treated to the higher UC3.

It’s important to note that as soon as you re-saw the timber within the cross-section, the strength of the grade may be compromised, and if pressure treated will need Ensele™ applied to the cut end.

The key differences between C16 and C24:

C16 is more cost-effective than C24. There are a number of reasons for this. C16 has more defects, meaning you can expect more visual defects.

What’s more, whilst both C16 and C24 have been cut from conifer trees, C16 is usually made using trees grown and cut in the UK in comparison to C24 which is usually produced from conifer trees grown overseas where the climate allows the tree to grow at a slower rate, thus producing a denser log with fewer imperfections.

This, combined with shipping costs, means that C24 has a higher price tag per meter. C16 is a great choice if you’re on a budget or not concerned about the visual appearance of the timber.

Which grade to choose?

Not sure whether you need C16 or C24 for your next project? For some things, there really isn’t a definitive answer for which to choose. However, here is a list of relevant applications for these two structural timbers to help you to decide which timber is right for you.

Uses for C16 timber

C16 is strong enough for most projects, but it might not meet the requirements for building control.

When treated, it can also be used externally for decking projects and will outperform C24 against rot and decay.

Uses for C24 timber

C24 can be used for all applications, and may be a preferred option for any timber that is likely to be on show as it has fewer visual defects than C16. You should consider C24 over C16 when structural integrity and load bearing is important. C24 is a timber of choice when seeking a structural timber suitable for roofing, floor joists, and structural stud work. .

What to look out for when buying C16 and C24

c24 timber
One of the most important things to look for when buying timber for structural applications is the stamp of approval. All C16 and C24 timber that has been graded to British Standard 5258 should have a stamp on it. If it doesn’t have the stamp, you certainly shouldn’t buy or use this timber for a structural purpose.

Shop C16 and C24 here

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