Understanding Strength Classes

Do you know the difference between C16 and C24?

C16 and C24 remain the two principal strength classes of structural softwood timber used in the UK. They are part of a wider system of 12 strength classes used across Europe and defined by British Standard BS EN 338. Softwood structural timber comes from coniferous species such as spruce and pine, which is where the “C” designation comes from. The number relates to the characteristic bending strength assigned to each class: 16N/mm2 for C16 and 24N/mm2 for C24.

For the UK construction market, using C16 for general structural applications and C24 where greater strength or longer spans are required makes good practical and commercial sense. Making the best use of the timber resource means recognising the benefits of both grades and specifying each where its properties are most appropriate.

This is the message at the heart of the current Trust UK C16 Timber campaign, which is encouraging the construction industry to have greater confidence in UK-grown C16 and use it wherever the specification allows.

Coniferous trees such as spruce grow relatively quickly in the UK climate, producing good-quality structural timber that commonly achieves C16 and is well suited to many everyday applications, including structural framing and cut roof timbers. In colder climates, coniferous trees tend to grow more slowly, resulting in a higher proportion of timber that meets the requirements of C24. Its greater strength makes C24 particularly suited to applications such as longer-span floor joists and rafters.

Most C16 and C24 structural timber in the UK is kiln-dried to reduce its moisture content and surface finished to make is easier to handle and provide greater dimensional stability in use. Both grades offer an appearance that is more than adequate for most structural applications, particularly where the timber will not be on show. While C24 typically contains fewer strength-reducing characteristics and can have a cleaner visual appearance. This does not give it a higher structural strength class, but may command a premium where appearance matters.

Strength and durability should not be confused. A C16 or C24 classification relates to the structural properties of the timber and does not indicate how durable it will be in a particular environment or application. Every piece of structural timber must carry the appropriate strength-grading mark identifying its strength class and source, and unmarked timber should not be purchased or used for structural applications.

Used appropriately, C16 and C24 give the UK construction sector two complementary structural timber options. Choosing the right strength class for the job helps make better use of the available timber resource and, as the Trust UK C16 Timber campaign is highlighting, creates more opportunities to use home-grown timber rather than automatically specifying a higher strength class where C16 can comfortably meet the structural requirements.

You can learn more about differences between C16 and C24 at TDUK’s website www.timberdevelopment.uk