Both C16 and C24 timber are types of kiln-dried softwood, which are graded to show their strength and quality. The higher the number, the stronger the wood. C24 timber offers high strength and premium quality. C16 may have more defects and is slightly less strong, but it’s still a durable wood for construction.

The ‘C’ stands for conifer, which can include woods such as pine, fir or spruce. The number shows its strength grade, set by BS 5268 (structural use of timber). C16 or C24 timber for construction can also be known as Canadian Lumber Standard timber (CLS), which is a certain style of kiln-dried softwood which is planed and pre-treated.

The kiln drying process ensures a moisture level of less than 20%, to prevent the wood from warping, making it extremely hard wearing.

How is timber graded?

Softwood timber’s strength class can be graded by visual inspection, either by a person or a machine. There are several considerations which affect a wood’s grading:

Knots - naturally, more knots mean less strength
Shakes or splits along the grain of the wood
Slope of the grain
Uniform appearance
Woodworm holes or insect damage
Wane (want) - bark or a lack of wood fibre on the edge of a piece of lumber

It's then graded to one of four strength classes:

C14 - lower strength and quality, usually the best value, low/non load bearing such as framing
C16 - medium to higher strength and quality, ideal for medium load bearing such as partitioning and some domestic floor joists
C18 - high strength with higher quality, great for medium to heavier loads such as roof battening or commercial decking, more expensive than C16
C24 - highest strength and premium quality, ideal for strong load bearing such as balconies, or where sleek aesthetics are needed; usually the most expensive grade

C16 vs C24 timber comes down to strength and quality. As mentioned, C16 or c24 timber grades are the most commonly used in the UK.

Hardwoods are graded slightly differently and put into one of six strength classes - D24, D30, D40, D50, D60 and D70 (with the 'D' standing for ‘deciduous’ wood).

What is C16 timber?

C16 timber is a kiln-treated softwood which is ideal for construction projects, as it offers a certain amount of strength and durability but at a lower cost than C24 grades. It may appear slightly lower quality when you inspect it, with some knots and imperfections, but it’s still a perfectly acceptable choice for partitioning, joists and stud work during the first fix.

Is C16 timber suitable for outdoor use?

C16 wood is recommended for indoor structural use, but if it is properly treated and sealed it’s possible to use it successfully for exterior projects such as decking and other landscaping projects.

Is C16 waterproof?

Although some C16 wood may come pre-treated for strength and protection against insects and rot, this does not make it waterproof. The kiln drying process removes a lot of the moisture, but in order to make the wood waterproof it will need an additional waterproofing product to be applied.

Is C16 load bearing?

C16 timber may be a better value alternative to C24, but it is still strong and resilient, despite its defects. It is suitable for load-bearing up to a certain point, such as for some domestic floor joists. It is recommended to use C16 timber for loads not exceeding 1.5 kN/m². For higher footfall areas or larger loads, C24 is preferred.

Is C16 OK for decking?

Although it’s the preferred choice for internal structural work, C16 timber can be used for decking and exterior landscaping when properly treated and sealed. C16 graded wood is the minimum standard for decking above 600mm high, and is required by Building Regulations Part A for raised level structures.

What is C24 timber?

C24 timber is the highest quality of the softwood grades. It is a premium quality wood and is considered the strongest class, suitable for heavy load bearing or when a knot-free finish is needed. C24 wood is kiln dried, planed and can be available untreated or pre-treated. Naturally, with such quality and strength comes a higher price tag than most other grades.

Can you use C24 timber outside?

C24 timber can be used outside, providing that it’s been properly treated and sealed to prevent warping, rot and damage over time from insects and weathering. It may also need maintenance each year to keep it looking its best.

Does C24 timber rot?

Although it’s specifically engineered to be the strongest and most resilient softwood, C24 timber can still rot when used outside without being treated beforehand. Pressure treated C24 wood can usually stand up against the elements for many years without rotting or degrading.

How much stronger is C24 than C16?

C24 timber is stronger than C16, but both can be suitable for load bearing. For comparison, according to BS 5268-2:2002, the sheer strength value of C16 is 1.8 MPa (megapascal), and C24 is 2.5MPa. The actual weight each beam can support depends on the size and function of the wood. C16 vs C24 timber comes down to both strength and quality.

C16 vs C24 Timber: FAQs

Which is better C16 or C24 timber?

Although C24 timber is of higher quality than C16, offering more strength overall, the best timber for your project depends on what it’s being used for. Whilst C24 is recommended for heavy load bearing carcassing, it can be more expensive, so C16 may be better for joists and decking in some domestic properties.

What is C16 and C24 timber?

Both C16 and C24 timber are types of kiln-dried softwood, which are graded to show their strength and quality. The higher the number, the stronger the wood. The ‘C’ stands for conifer, which can include woods such as pine, fir or spruce. The number shows its strength grade, set by BS 5268 (structural use of timber).

Is C24 timber stronger than C16?

C24 timber is stronger than C16, but both can be suitable for load bearing. For comparison, according to BS 5268-2:2002, the sheer strength value of C16 is 1.8 Mpa (megapascal), and C24 is 2.5MPa. The actual weight each beam can support depends on the size and function of the wood.

Is C16 cheaper than C24?

C16 timber is usually cheaper than C24, which makes it a popular choice for internal structural work. It’s still strong and hard-wearing, yet it does have imperfections which make it cheaper. C24 is often imported, adding to the cost.

Is C16 or C24 better for decking joists?

C24 is often the timber of choice for decking joists, due to its strength and high quality finish. Depending on the function of the decking, C16 may be a more cost-effective option. C16 is less strong than C24, but is suitable for domestic decking when treated properly. For commercial or high footfall decks, C24 is recommended.

Disclaimer: The information contained on this page is intended as an overall introduction and is not intended as specific advice from a qualified professional. Travis Perkins aims to avoid, but accepts no liability, in the case that any information stated is out of date.

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