TIMBER
will be more than sufficient for most general structural applications where they are hidden from view.
GET SUPPORT WITH C16
C
16 and C24 are the two main strength classes of structural softwood timber available in the UK. They are just two of the 12 strength classes used throughout Europe, as defined in British Standard BS EN 338.
All softwood structural timber 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 16N/mm₂ and C24 for 24N/mm₂. The structural properties defined for each strength class can be found in Table 1 of BS EN 338.
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 efficient because everyone knows that if these strength classes are specified, they will be available off the shelf at cost-effective prices anywhere in the UK.
But the timber supply chain also has a portfolio of other structural timber products available to meet your needs.
26
In general terms, C24 timbers have fewer 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 also choose 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 so may be worth the price premium where appearance is important to buyers.
AND C24 TIMBERS
Many merchants report being uncertain of exactly when they should be recommending and selling C16 and C24 timbers. TDUK Chief Executive David Hopkins explains what merchants need to know to better support their customers.
Which should I recommend? To make the most efficient use of our wood resources we need to use the whole range of products available, and that means both C16 and C24 timbers. Not only is this great for the planet, but it is also cost effective. Trees are a natural product and their properties vary depending on the climatic conditions where they are grown. 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 grow more slowly, coming to maturity in 60+ years. They produce a higher proportion of timber meeting the properties of strength class C24 - which makes great longer-span floor joists or rafters.
Most C16 and C24 timber sold in the UK is kiln-dried to reduce its moisture content and surfaced finished to make each piece stable and easy to handle. This means all structural timber
Are C16 timbers less durable than C24? The specification of a strength class does not infer any specific level of durability. If a structural timber is being 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, your customers can specify naturally durable species of timber which meet their requirements.
Keep span tables close at hand Span tables allow you to calculate the size of a timber member of a particular strength class required for a given span – as well as the maximum spacing between each section or timber member. They help you to choose an appropriate size and stress grade to achieve spanning requirements.
Timber Development UK has just published a suite of span tables for C16 and C24 timber grades, which can be downloaded for free from the TDUK website.
These new tables provide guidance on the loadbearing capacity of timber members to give adequate support to floors, ceilings, and roofs in domestic dwellings. BMJ
• Download your free copy at
www.timberdevelopment.uk/ resources/c16-span-tables
www.buildersmerchantsjournal.net October 2023
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