21 Jul

C16 & C24 Timber Properties & Strengths

C16 and C24 are the most common used softwood timbers within the UK. They are widely used in both domestic and commercial projects, but with the two timbers ideally suited to different and specific loads, it can be difficult to know whether you are using the best timber for your construction. Here we have a rundown of the qualities of the two timbers to help you evaluate the whether C16 or C24 timber selection for your project.

Grading Timber – Properties & Strengths

The strength of timber is evaluated after the wood has been kiln dried to below 20% moisture content. The timber goes through an automated grading line where various measurements are taken that indicate the strength of the wood, with physical defects such as knots and pinworm holes lowering the strength of the timber. The ‘C’ grading simply stands for conifer, the variety of tree the timber is made from while the number indicates the strength, with the specific measurements allowed outlined in the table below:

Strength & Properties Class       C16          C24  
 Bending parallel to grain (N/mm2)  5.3  7.5
 Tension parallel to grain (N/mm2)  3.2  4.5
 Compression parallel to grain (N/mm2)  1.8 7.9
 Compression perpendicular to grain (N/mm2)  2.2-1.7  2.4-1.9
 Shear parallel to grain (N/mm2)  0.67  0.71
 Modulus of elasticity mean (N/mm2)  8,800  10,800
 Modulus of elasticity minimum (N/mm2)  5,800  7,200
 Characteristic density (kg/m5)  310  350
 Average density (kg/m5) 370 420

If you would like more information on the guidelines for the grading of timber, the standard BS5268-2:2002 has all the information you need.

C16 Timber

C16 graded timber allows for a number of defects that may reduce the overall strength of the wood, such as grain deviations and wanes, as well as an unlimited amount of superficial defects that do not affect the strength of the wood, such as blue sap stains and the like. C16 is the most widely used timber in the UK as it is strong enough to be used in most applications and is the most cost effective timber option.

C24 Timber

C24 boards have fewer defects than C16 graded timber and the timber is therefore stronger and more resilient than C16 boards. You will also find that C24 boards have a more uniformed and neat appearance as there are fewer physical imperfections on the surface of the board. C24s are not as widely used as C16 as the timber is much more expensive because the demand for high standard boards means that there are a lot of boards rejected, so you gain less timber from the same amount of wood as you would C16 boards. Generally, C24 boards have to be imported too, which can drive the price up.

C16 or C24 timber: Which is better?

It is impossible to state categorically that one timber grade is better than the other as the preferred timber for your project comes down to what features are important to you.

With regards to strength and load carrying, C24 boards are clearly superior to C16, however, C16 timber is strong enough for most projects and will certainly meet your performance requirements at a much cheaper cost than C24 boards.

If the aesthetic appeal of your timber is important – if you would like to leave the boards unvarnished, for example – then again, C24 is more beneficial than C16 boards. The main benefit of C24 over C16 boards in instances where both boards are adequate is that C24 boards can have a wider rafter centre or a smaller deck board section, giving you more choice in the finished aesthetic.

In general, C16 boards are used more often in circumstances where both timber types are suitable, with C24 timbers mainly being reserved for projects such as balconies and bridges, where aesthetics and strength are important.

Hitchcock & King have high quality C16 and C24 timber in stock at all of our 8 branches across the Capital in many different dimensions, perfectly suited for projects of all kinds. Get in contact today to locate your nearest branch; we’ll be happy to help in any way we can.