The general term lumber refers to both “Dimension lumber” and “Timber”. Dimension lumber means a piece of lumber 38 to 102 mm inclusive in smaller dimension. Timber means a piece of lumber 114 mm or more in smaller dimension.
Almost all formwork, regardless of its type, uses some wood. Although the wood species, grades and sizes vary with every supplier, the design of formwork can always be made to fit the material available. Any wood that is graded, straight and structurally sound can be used in the building of formwork.
Lumber is designated by its nominal dimensions that are the sizes of the cross-section prior to finishing. Lumber surfaced in a planing machine to attain smoothness of surface and uniformity of size is called Dressed Lumber. The finished dimension of the lumber cross section will be less than its nominal dimension, and it is the finished dimension that shall be used in the design. The finished dimensions are sometimes referred to as “surfaced“, “dressed” or “net” sizes.
Lumber as supplied to the site comes in three categories; finished on all four sides designated as S4S, finished on two edges designated as S2E, or it may be used as it comes from the sawmill with no finish, designated as Rough.
Formwork lumber members shall be assumed to be S4S, unless stated otherwise on the falsework drawings. If the use of rough–cut material is required by the falsework design, the actual member size must be verified prior to use.
Surfaced Dimension lumber can be either Surfaced Dry or Surfaced Green. Surfaced dry Dimension lumber (S–Dry) is dressed at a moisture content of 19 percent or less, and is designated according to its actual finished size.
Green Dimension lumber is defined as the lumber having a moisture content greater than 19%. Surfaced green Dimension lumber (S–Gm) is designated by its anticipated dry size at 19% moisture content, and will be approximately the same as S–Dry Dimension lumber.
Timbers are always surfaced green. Surfaced green Timber is designated by its actual green size. Tables 1 and 2 give standard sizes of Dimension lumber and Timber. Metric sizes are rounded to the nearest millimetre.