Construction timber: strength classes and CE marking
C18, C24 and the old T designations — how to read strength-graded timber
Wood is a natural material, and strength varies from plank to plank depending on ring width, knots, cracks and grain angle. To be able to calculate a load-bearing structure you must know the wood's strength with certainty. Therefore structural timber is sorted into strength classes, either mechanically or visually, and each class has well-defined calculation values for bending strength, stiffness and density.
§C-classes and the old T designations
Strength classes are specified with a C followed by a number — C14, C18, C24 and C30 — where the number corresponds to the characteristic bending strength in newton per square millimetre. The classes are defined in the European standard EN 338. Previously, T designations were used, and the two systems correspond to each other: C14 was T0, C18 was T1, C24 was T2 and C30 was T3. You still meet the old designations on older drawings and among experienced craftsmen.
| C-class | Old designation | Typical application |
|---|---|---|
| C14 | T0 | Lighter, non-critical purposes |
| C18 | T1 | Common building timber, the most commonly used quality |
| C24 | T2 | Load-bearing structures: rafters, beams, straps |
| C30 | T3 | Heavily loaded beams and girders |
§C18 or C24 — which do you choose?
C18 is the most commonly used quality in Danish construction and is used for ordinary structural timber where there are no stringent load-bearing requirements. When timber needs to carry load — rafters, beams, girders and joists — C24 is normally used, which is strength-graded and CE-marked. Choosing too low a class for a load-bearing task means the structure cannot meet static requirements; choosing unnecessarily high means you pay too much for the timber.
§CE marking and documentation
Wood used for load-bearing purposes — rafters, columns or load-bearing beams — must be CE-marked and strength graded according to DS/EN 14081-1. Mandatory CE-marking of load-bearing structural timber has applied in Denmark since 1 January 2012. The CE-marking documents that the timber is graded according to an approved method and that calculation values are provided which the designer can base their calculations on. Without CE-marking you cannot legally incorporate the timber into a load-bearing structure.
§Læs stemplet
- 01Strength class — e.g. C24, which tells the characteristic bending strength
- 02Standard – reference to DS/EN 14081-1 and the sorting method
- 03Manufacturer or sorting company — so the wood can be traced
- 04Moisture condition — often dried to around 18 % upon delivery, marked as dry wood
Being able to read and select the correct construction timber is a basic skill in the wood trades' construction education. It's not just about following a drawing, but about understanding why that specific class was chosen — and catching it if the wrong quality has been delivered before it's built in.