I don’t play a wood scientist on TV, but I have been on TV with a real wood scientist. Click here to see the show. I have been making a living working with wood since 1992.
I have noticed over the years that there seems to be quite a bit of confusion regarding the specification of timbers, myself included. Let’s start with the easy stuff.
Species: The vast majority of our projects are built using Douglas Fir timbers.
- It typically machines well
- It accepts stains, glues, and finishes well.
- It is widely available.
- I did a search on sustainability and according to the IUCN (International Union for Conservation of Nature) it’s status is considered “least concern” and its population trend is considered “stable”.
- Free of Heart Center timbers (FOHC). These tend to be more stable than boxed heart.
I can find no instance where boxed heart and free of heart are taken into consideration regarding the grade of timber. I believe it is for purely aesthetic reasons. Using FOHC reduces the chance of checking and twisting. Someone (I think it was my wood scientist friend) once told me to think of it this way: The rings are trying to straighten/flatten as the bound water (more on this later) leaves the timber.
OK, let’s do grading. That starts getting a little tricky. Not a lot, just a little tricky.
If there is a specification for lumber in the drawings it does not apply to the timbers. Lumber is defined as material 2 to 4 inches thick, 2 inches and wider. Our material is called timber: 5 inches and thicker, 5 inches and wider. Just to make it more interesting there are 2 sub-classifications of timber. They are separated by a ratio of thickness and width.
- Posts and timbers: (I’m not sure why it has two names): They have a nominal dimension of 5″ and thicker and the width is not more than 2″ greater than the thickness. These are usually posts like an 8″x8″ or 8″x10″. The majority of our posts are square. We have used a post that is rectangular (ie 8″x10″) when we have a knee brace coming into the post. Usually for aesthetic reasons more than structural, unless the brace has a large load on it.
- Beams and stringers: (? two names again): They have a nominal dimension of 5″ and thicker and the width is more than 2″ greater than the thickness. These are usually beams, for instance, an 8″x12″.
Once we have the timbers classified as posts or beams here are the criteria for visual grading.
- Knots and holes: Quantity, size, and location.
- Wane: this will come from timber cut from the outer wood. It’s the rounded cambial portion of the wood where the bark meets the sapwood.
- Checks and splits: these generally form during the drying process.
- Machining defects
- Slope of grain: reduces the strength of mechanical fasteners as the fibers are not parallel to the edges.
The timber grades are broken down into the following from best to worst: dense select structural, Dense No. 1, No. 1, and No. 2.
Our typical projects are made from Doug Fir #1 FOHC. The next part of this discussion is regarding moisture content of timbers. There is a lot of hoopla about that and I believe it’s a lot of “smoke and mirrors” Look for that topic in: “I am still not a wood scientist. Part 2”