Thursday, 16 October 2014

Lumber or Timber?

You may have heard the terms lumber or timber used to describe wood construction materials, but perhaps you've wondered what the difference is. Well, outside of North America, there's no confusion because timber can be used to refer to all wood building materials and the term lumber isn't used. But here in Canada both terms are used and there are subtle differences between them.

If you consult Webster's dictionary, lumber is "wooden boards or logs that have been sawed and cut for use" and timber is "wood suitable for building or for carpentry." Both terms can describe the same thing, but notice that the definition of timber makes no mention of sawing. In North America, it is common to use lumber to refer to the wood products that have come from the mill and timber to refer to the logs or standing trees prior to milling.
North Americans might call this stuff "timber."

Americans and Canadians typically call this stuff "lumber."

Seems simple enough? Well, things get a little more confusing when you get into the technical definitions. The definition of lumber starts to get long-winded¹, but essentially doesn't differ from Webster's dictionary definition.

¹According to the old American Institute of Timber Construction, lumber is "a manufactured product derived from a log in a sawmill, or in a sawmill and planing mill, which, when rough, shall have been sawed, edged, and trimmed at least to the extent of showing saw marks in the wood on the four longitudinal surfaces of each piece for its overall length, and which has not been further manufacture other than by cross cutting, ripping, resawing, joining crosswise and or endwise in a flat plane, surfacing with or without end matching, and working."

According to the Canadian design standard for wood structures (CSA Standard O86-09: Engineering Design in Wood), timber is defined as "a piece of lumber 114 mm or more in its smaller dimension." Put another way, the word timber is reserved for pieces of wood that are 114 mm × 114 mm (4½" × 4½" if you're unfamiliar with the International System of Units) or larger. Those are actual dimensions, so we're talking about a piece of wood that is nominally a 5×5 or larger. Typical nominal 4×4 or 4×6 (89 mm × 89 mm or 89 mm × 140 mm actual dimensions) fence posts are lumber, but once you go to a 6×6 (140 mm × 140 mm actual dimensions) post it's technically timber

The confusion doesn't end with ordinary sawn wood. There are also glued-laminated (glulam) and cross-laminated wood products that are referred to as timber. These laminated wood products are manufactured by gluing sawn wood boards together to make larger panels, beams, or posts. Generally speaking, these products meet the size criterion to be called timber because the thinnest cross-laminated timber panels available here are 114 mm thick and most glulam beams and posts are at least 130 mm wide. That said, standard glulam beams are available in 80 mm width and you can still refer to them as timber.

Some pretty awesome designs are possible with glulam.

There's yet another exception to consider: parallel strand lumber (PSL). PSL is made from strands of wood clipped from thin sheets called veneers. The strands are all oriented parallel to each other and then glued and pressed together to make a beam or post. The product is not referred to as timber, even when the width exceeds 114 mm (PSL is available in 130 mm and 180 mm widths).

To summarize, lumber is a general term that describes building materials made by processing wood into boards, planks, posts, beams, etc. Timber generally refers to lumber that exceeds 114 mm in the smaller dimension. Glulam and PSL products are exceptions to the size criterion, where glulam can still be called timber even at 80 mm wide and PSL is still lumber even at 180 mm wide.