Two factors determine the available sizes of softwoods and hardwoods: the size of the trees from which they are cut and the range of internationally agreed sawn sizes, which are set out in British Standards. With the development of forest management programmes, more trees are felled once they have reached a usable economic size; f ewer trees are left to reach their full height and girth. Long lengths (more than about six metres for softwood and perhaps five to six metres for some hardwoods) are difficult to obtain in most species and, when available, command premium prices. Although larger sizes, e. g.150 x 300, are still available in some species, both softwood and hardwood, when large sections are required, the use of glulam, PSL (parallel strand lumber) or LVL (laminated veneer lumber) is generally preferable. These products are a more efficient use of the natural resource and provide a more stable and stronger component available in sizes much larger than those from the tree. For internal joinery, veneer on a plywood or MDF base is an economic alternative for producing large panels.
Softwood is sawn to a range of sizes in accordance with BS EN 1313 and, for structural timber, BS EN 336 which has also introduced the term 'target size' to the UK specifier.
Hardwoods are less well regulated - sizes available being dependent on the provenance of the species and are generally traded in imperial dimensions that may not lend themselves ideally to metric conversion.
Metric availability is generally in 10mm width increments in thicknesses of 16, 19, 25, 38, 50, 63, 75 and 100mm.
Wood required for joinery necessitates removing at least 2 to 3mm for each planed surface. If planed (finished) sizes are specified which cannot be machined from the nearest sawn size, it is necessary to use the next available larger size, which may be more than one incremental size larger, wasting wood and increasing cost.