Any good timber project starts with selecting the right timber. At first glance this should be fairly straight forward, but there are several important details that can make or break a project.
Species:
Most all timber frame projects utilize one of two species of wood, Douglas fir or Pine. Both of these species are soft woods, cut clean and easily, and are fairly stable through periods of changing moisture content. There are slight differences in the hardness, color, and structural strength, but the major difference is where each species grows. Douglas fir is prevalent on the west coast while Pine is more prevalent on the east coast. This locational difference is typically the determining factor for what species is selected for any given project.
Since we are located in the great PNW, we use Douglas fir on almost all of our projects. The remainder of this discussion will be thru the vantage point of Douglas fir, even though most everything will apply to any wood species.
Let’s dive into some of the most important variables to consider when selecting a Douglas fir timber.
Moisture Content
For simplicity sake, we will bucket timber into two categories: dry and green. Green timbers are fairly fresh cut timbers that have not had a chance to expel the amount of moisture present in live trees. A fresh cut tree will have about 40% moisture content (MC). There are numerous ways to dry a timber including kiln dried, natural airing, radio-frequency drying, and more. A timber is deemed dry when the MC is 17% or less, as measured 1” in from an outside face. As a general rule, only dry timbers should be used for building. One very practical element of green versus dry timbers is the behavior of sap. Green timbers will commonly continue to expel sap for years whereas sap expelling from a dry timber is rare.
Cut
This variable does not get enough attention. The two buckets in this category are box heart and free of heart center (FOHC). Is the center ring of the tree present in the finished timber? If yes, this is box heart. If no, this is free of heart center. The practical matter at hand is the severity of the checking. Box heart timbers will have a more severe check (i.e. crack), almost always right to the center ring. A FOHC Timber will have smaller, more random checks.
Example of box heart with checking
Example of free of heart center (FOHC)
Grade
The common grades, from highest quality to lowest quality, are Structural Select, #1, and #2. Depending on the state that the timber is produced it may or may not have a grade stamp on each timber. Before each log is cut it will be inspected by a scaler who will determine the best way to cut the log in order to optimize the output as well as the grade of the cut timbers. The main variables that determine grade are the slope of the grain, quantity of pitch pockets, and quantity/size of knots. Not only will the grade of each timber have different visible attributes, but each grade will also have different structural capabilities. For this reason, it is common for plans to specify a specific grade in order to make sure correct engineering assumptions are used.
Other Characteristics and Acronyms
All the variables above will ultimately affect the performance and quality of your timbers. With most plans and order forms you will likely see a long list of stated acronyms. Other common acronyms include FOW (free of wayne), LTNW (light to no wayne), S4S (surfaced 4 sides), and RSW (resawn). While it can seem overwhelming, always make sure you understand the acronyms before moving forward or work with a professional that can ensure that your desires will be met with the specifications ordered.
At Northern Timber Crafters, we facilitate the planning and craft custom timber features. Our expertise will ensure you select the right timber to make your project exceptional.
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