Wood timbers, timber products specification : 1. The relevant functional requirements for the timber to be used in the building or structure. 2. The grading of timber ensures that all timber is able to perform to a minimum performance requirement and to provide a reliable standard against which a designer architect can create a design and specification. 3. To ensure that timber is graded consistently across regardless of manufacturing source and location a number of quality guidelines and criteria have been established. 4. A number of resources exist to ensure that the correct size of timber is chosen for the design intent of the designer/architect. Span tables are critical to ensuring appropriate dimensions and performance is achieved. 5. Durability is one of the key performance factors used to assess the suitability of a timber species for a specific application. The durability rating of a species is based on the natural ability of the heartwood of that species to resist decay and insect pests. However, for many species, the durability of sapwood which is usually not durable may be increased by preservative treatment.