The Difference Between Timber and Dimension Lumber
Timber is the raw ingredient from which dimensional lumber is made. In the process of taking a growing tree and turning it into a 2-by-4 or other board, timber is an intermediate phase. In some cases, rough cut timber is used in its rough form for construction. The main differences revolve around texture and sizing.
From upright and growing to cut down and sectioned into logs, wood is known as timber. These timber logs are cut on large rip saws, often known as mill saws. These saws use large circular blades, or in some cases, band saw style blades, to remove bark and to square up the edges of the raw logs to create pieces that can be further refined. These pieces are then sliced into standard thicknesses and dried in large heated kilns to "season" or dry the wood so that it will be less likely to warp, swell or shrink.
Nominal lumber is the next stage toward dimensional lumber and is named according to thickness and width, such as 2-by-4, 1-by-4 and the like. Once they are trimmed into dimensional lumber these sizes are not exactly as labeled. All lumber labeled as 2-by, which indicates the thickness, is actually 1 1/2 inches thick and 1-by is actually 3/4 inch thick. All dimensional lumber is 1/2 inch narrower than its dimensional label. This discrepancy is due to the amount that is removed in the planing process when timber is cut down into dimensional lumber.
From the kiln-dried timber, the lumber is cut into board sizes using large surface planers that remove up to 1/4 inch with each cut to create a smooth, uniform board surface. This is dimensional lumber. From here it goes through a sorting process to separate it into the various "grades" or quality ranks that are then sold as project lumber in lumberyards and home improvement centers for use in building and remodeling.
Posts and Beams
Timber posts and beams are some of the most common timbers that are used in their rough state. The surface texture is slightly rough, with a somewhat shaggy appearance. The corners are often rounded, since the planing process is where the crisp corners are created, and saw marks are often visible along the length of the material. This additional thickness adds to its structural integrity. This type of lumber is not ordinarily used where a finished surface is required.