Constructional Carcassing Timber is generally used for structural purposes and hidden from sight. C16, for example, can be used for construction projects such as roof rafters or floor joists and Treated Sawn is perfect for outdoor garden projects, provided it is treated to the right specification for the job. C16 is generally recommended for most applications and is the most cost-effective option if the correct spans are used. For more demanding, higher loads or wider span applications then C24 will be required. C24 graded structural timber has fewer/smaller knots than C16 and will carry a higher point load.