If you buy or sell structural softwood, C16 and C24 are the two grades you will meet most often. They look almost identical on the stack, but they are not interchangeable on a structural drawing. Here is what separates them.
What the code actually means
The grade follows the European standard EN 338. The letter tells you the species group — C for coniferous (softwood), D for deciduous (hardwood). The number is the characteristic bending strength in newtons per square millimetre. So C24 has a characteristic bending strength of 24 N/mm², and C16 of 16 N/mm². Higher number, stronger and stiffer timber — which means a given load can be carried with a smaller section, or a longer span with the same section.
C16 vs C24, side by side
- C24 is stronger, stiffer, and typically straighter, with tighter limits on knots and wane. It is the default for joists, rafters, and most engineered floor and roof designs.
- C16 is a lower grade with more permitted defects. It is perfectly sound for lighter framing, studwork, and non-critical structural uses where C24 would be over-specified.
- Because more of each log meets the looser C16 limits, C16 can be cheaper or more available in some markets — but across much of Europe C24 is so dominant that it is often the easier of the two to source.
How timber gets its grade
Strength grading is done in one of two ways, both controlled under EN 14081:
- Visual grading: a trained grader assesses knots, slope of grain, wane, and distortion against a written standard.
- Machine grading: each piece passes through a machine that measures stiffness and predicts strength, allowing more consistent and often higher yields.
Structural timber placed on the European market must be CE marked to EN 14081, which links the piece back to its grade and the mill's audited process. The grade stamp should show the strength class, the standard, the species, and the grading body.
Specifying without over-paying
- Take the grade from the structural engineer's drawings — do not substitute C16 for C24, or vice versa, without sign-off.
- Confirm species and origin: 'C24' covers several species (spruce, pine, fir) with different appearance and moisture behaviour.
- State the moisture condition (kiln-dried vs fresh) and any treatment separately; the strength class does not cover them.
Strength class describes mechanical performance only. Appearance, moisture, durability class, and certification (FSC/PEFC) are separate specifications — set them explicitly on the order so the mill knows exactly what to cut.
Timberhub sources C16, C24, and higher classes across its DACH, Nordic, and Baltic network, machine- or visually graded and CE marked. Send us the structural spec and we will match it to a mill that runs it as standard.