C18 (EN 338) - Design Values
C18 (EN 338) is a dimensional lumber grade with reference design values to EN 338:2016. In Eurocode 5 terms its characteristic bending strength fm,k is 18 N/mm², compression parallel fc,0,k is 18 N/mm², and mean modulus E0,mean is 9 kN/mm² (EN 338 / EN 14080). Values are shown before adjustment factors; apply the relevant NDS C-factors or EC5 k_mod and γ_M for your load case.
| fm,k | Bending strength | 18 N/mm² |
| ft,0,k | Tension parallel | 11 N/mm² |
| ft,90,k | Tension perpendicular | 0.4 N/mm² |
| fc,0,k | Compression parallel | 18 N/mm² |
| fc,90,k | Compression perpendicular | 2.2 N/mm² |
| fv,k | Shear strength | 3.4 N/mm² |
| E0,mean | Mean MOE parallel | 9 kN/mm² |
| E0,05 | 5th-percentile MOE | 6 kN/mm² |
| Gmean | Mean shear modulus | 0.56 kN/mm² |
| ρk | Characteristic density | 320 kg/m³ |
| ρmean | Mean density | 380 kg/m³ |
Source: EN 338:2016. Reference values are shown before adjustment factors (NDS C-factors / EC5 k_mod, γ_M) - apply these per your load case in design.
Understanding these values
Under Eurocode 5 C18 (EN 338) is described by characteristic values: bending fm,k 18 N/mm², shear fv,k 3.4 N/mm², compression parallel fc,0,k 18 N/mm² and perpendicular fc,90,k 2.2 N/mm². These are 5th-percentile figures, not allowable stresses - divide by the partial factor gamma_M (1.3 for solid timber for dimensional lumber under the recommended values) and multiply by k_mod for the service class and load duration, so the usable strength is materially lower than the number shown.
For C18 (EN 338) the mean modulus E0,mean is 9 kN/mm², and the 5th-percentile modulus E0,05 used for stability is 6 kN/mm². Characteristic density is rho,k 320 kg/m³ with a mean of 380 kg/m³, which is the value that drives fastener and connection capacity rather than any of the strength figures - two grades of similar bending strength can give quite different screw and dowel resistances if their densities differ.
Within the dimensional lumber entries in this catalogue, C18 (EN 338) ranks 3 of 4 on bending at 18 N/mm²; the strongest listed is C30 (EN 338) at 30 N/mm². Grade choice is usually a trade of cost against section depth, so it is worth checking whether a lower grade in a deeper section is cheaper than a higher grade at the depth you started with.
The closest alternative in the same family is C16 (EN 338) at 16 N/mm² in bending, 13% below this grade. If availability drives the choice rather than capacity, that is the substitution to price first - but re-run the deflection check as well, because stiffness and strength do not always move together.