Nick Boulton, TDUK’s Head of Technical and Trade Policy, explains the fundamentals of these two strength grades.
I am being offered C16 and C24 structural timber, which one should I use?
There is a lot of marketing around this question. But the simple answer is: it’s all about the strength! C16 and C24 are the two main strength classes of structural softwood timber available to designers and construction buyers in the UK. They are just two of the twelve strength classes used throughout Europe, as defined in British Standard BS EN 338.
But what do the terms mean?
Softwood structural timber, whether it comes from the UK or is imported from Europe, all starts life as a coniferous tree like spruce or pine. Hence the “C” for “coniferous”. The number is just shorthand for the defined bending strength of that strength class: C16 for 16 N/mm₂ and C24 for 24 N/mm₂. Look up Table 1 of BS EN 338 for the other structural properties defined for each Strength Class.
Why are these two classes the most popular in the UK?
Having a structural timber supply chain based on C16 for general constructional applications and C24 where higher strength or longer spans are required is the most efficient for designers, contractors and suppliers. Everyone knows that if these strength classes are specified, they will be available off the shelf at the most cost-effective price anywhere in the UK. But, as variety is the spice of life, the timber supply chain always has, alongside these two strength classes, a portfolio of other structural timber products available to order to meet your needs.
Image © W Burton & Sons
Wouldn’t it just be easier for everyone to use C24, as it’s stronger?
To make the most efficient use of wood resources we need to use the whole range of wood products available. And that means fully utilising both C16 and C24 timbers. Not only is this great for the planet but it is also cost-effective for your construction budget.
Trees are our raw material and, as a natural product, their properties will vary depending on the climatic conditions under which they grow. Here in the UK, coniferous trees like spruce grow very fast, coming to maturity in around 40 years. This makes excellent structural timber at strength class C16, suitable for most general construction applications like structural framing and cut roof timbers. Coniferous trees in colder climates like the Nordic countries or the central European Alps grow more slowly, coming to maturity in 60 years or more. They produce a higher proportion of timber meeting the properties of strength class C24 – which makes great longer-span floor joists or rafters.
Does C24 have a better appearance than C16?
Most C16 and C24 timber sold in the UK is kiln-dried to reduce its moisture content and surfaced finished in order to make each piece easy to handle and stable for use in applications within the building envelope. This means all structural timber will have a good appearance – more than sufficient for most general structural applications that are hidden from view behind decorative finishes, up in the loft or under the
floorboards.
In general terms, C24 timbers will have less strength-reducing features like knots. This allows them to achieve the higher performance required by strength class C24, and in this way they may appear to be visually more appealing than most C16 timbers. Some merchants are also choosing to stock C24 timbers that have been specially selected for their visual appearance. These do not have a higher strength classification than other C24 timbers, but claim to present better and may therefore be worth the price premium where this is important to buyers.
Are C16 timbers less durable than C24?
The specification of a strength class does not infer any specific level of durability. Therefore, if a designer is intending for structural timbers to be used in anything other than Service Class 1 construction environments they will need to separately specify durability requirements for the intended service class. Such specifications may also be necessary for Service Class 1 roof spaces in geographical locations where the building regulations identify house longhorn beetle as a specific pest.
The easiest route to achieve such durability specifications will generally be through the use of factory-applied preservative treatments: usually Use Class 2 for all applications within the building envelope. Alternatively, designers can look to specify naturally durable species of timber which meet their requirements.
Does structural timber need to be marked?
Yes, absolutely. Every piece of structural timber carries a strength-grading mark to identify its strength class and source. This means everyone in the supply chain, including the building inspector, can identify that the specified strength class has been used correctly in the different parts of the construction project.
Any timber that is not marked on each piece to show its strength class should not be purchased for use in structural applications.
To sum up…
If you specify C16 you will receive structural timbers which achieve a bending strength of 16 N/mm.
If you specify C24 you will receive structural timbers which achieve a bending strength of 24 N/mm.
Whether they are better looking or not will depend entirely on the eye of the beholder.
For more information, see the TDUK website: timberdevelopment.uk/resources/timber-strength-grading-and-strength-classes/