Note: Glulam Perspectives is a quarterly series on Unalam's Wood Times blog. In each article we will take an in-depth look at a project or issue facing the timber industry from our unique standpoint as a glulam manufacturer. These are our opinions and we invite constructive discussion. Next time, we will focus on a completely different topic, so stay tuned.
Before getting into the specific grade characteristics, it is important to point out that wood is a natural product with inherent variation. In fact, designers often choose to build with wood because of its grain, warmth, and natural beauty. No two trees are exactly alike. As such, the standard allows all of the appearance grades to show natural growth characteristics, in other words, grain.
The standard describes four appearance grades for glulam: framing, industrial, architectural, and premium. We will focus on industrial, architectural, and premium, because in practice it is quite rare to produce framing grade glulam. Each grade has different requirements for growth characteristics (ie. knots, and knot holes,) void filling (we use clear wood plugs but wood-tone filler is also an option,) and surfacing (planing.)
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Industrial- Except on timber truss members, loose knots and open knot holes are filled. The material needs only to be surfaced on the two sides of the member, not the faces. (Note: member "sides" are where the glue lines appear, "faces" are a solid piece of wood.) Low plies and wane are allowed to an extent and up to ¼" deep. A low ply is where an individual board is offset from the rest of the beam enough that planing doesn't reach it. Wane is missing edges on lumber cut near the outside of the tree, that would appear as a small missing wedge in the finished beam. Industrial grade is used most often in things like road bridges, salt storage sheds, and utility poles.
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Architectural- The vast majority of our jobs are specified Architectural grade. Having manufactured glulam for over 50 years, our opinion is that Architectural appearance achieves the best balance of beauty and practicality for most building projects. In this grade, we fill voids over ¾” on exposed surfaces with matched clear wood plugs. Loose knots and open knot holes are also filled. As we mentioned earlier, the standard also allows color matched wood-toned filler. We primarily use plugs, because we feel that they look better and last longer.
On Architectural grade glulam, all exposed faces are surfaced and no low plies or wane are allowed. We also ease the exposed edges of the member.
Premium- Premium grade shares many of the same standards as Architectural grade but they are more stringent. For example, in Premium grade, on exposed wide faces of the lamination knot size is limited to 20% of the net face width of the laminations. And, no more than two maximum-sized knots are allowed in a six foot length. What does this mean? It means that Premium grade requires clearer material and more plugging than Architectural grade. This adds material and labor expense to the job. Although we work to match plugs with the wood grain, plugging visually breaks up the natural grain and tone of the wood. This is important to take into account when deciding between Architectural and Premium grades for a project.
It is important to remember that appearance grades are only concerned with the visual appearance of surface defects. They have no bearing on laminating procedures or strength properties of the finished product, design values will be the same regardless of which appearance grade you choose. Other specifications, like stain, surface finishes, wrapping or other protective coverings during shipping, are all independent of the appearance grade.
When ordering glulam for a project, you should have a good understanding of how your manufacturer or supplier (if you are ordering glulam from someone other than a manufacturer) deals with stain and wrapping. Do they make sure stain matches? This can be an issue if a supplier sources material from different manufacturers for the same job. How is the glulam handled at the supplier’s or manufacturer’s facility? Is it stored inside or outside before shipping? Is it wrapped for shipping, and how is it wrapped? Depending on the end use and desired look of the project, these details can have a major impact.
A project designer has to balance the budget and aesthetic concerns of the owner and we hope that this description of appearance grades offers guidance. We feel it is important for the designer and glulam supplier to communicate well about appearance expectations and realities. Our staff is happy to answer questions; please contact us.
