Regardless of whether you are producing glued-laminated timber (GLT), cross-laminated timber (CLT), or dowel-laminated timber (DLT), you require lumber in lengths far greater than are available from a mill run. These long lengths of lumber, known as lamellas, are comprised of graded structural lumber, finger-jointed into required lengths, often spanning 40+ feet in length These lamellas are the slices of bread of the mass timber sandwich – the bones of a mass-timber component.
So, what does it take to produce a good lamella.
Lamellas, or lamstock as they are colloquially known, start as lifts of dimensional kiln-dried structural lumber. This lumber is predominantly softwood in North America and covers all ranges of spruce, pines and firs. The selection of species is often geographically dictated, with manufacturers utilizing what is most readily available.
For example, much of the southern U.S. is dominated by southern yellow pine, whereas the northeast and eastern Canada utilizes a lot of black spruce and Douglas fir often reigns in the Pacific Northwest. Each species has its own intricacies, challenges and structural properties and there is no clear magic bullet when it comes to species. Regardless of species, there is one thing that all lumber destined for lamstock does have in common… That commonality is the requirement of grading.
Different grades play different roles within a mass timber element and are governed closely by certification bodies. Structural lumber grades are based on knot size, slope-of-grain, density, and other allowable defects. Manufacturers can manage these grades by visual grading of each board or by using modern technology to scan every inch of the board and perfectly assign the correct grade. Grading methodology and technology is a critical facet and a complex process, but that is a discussion for another day (or article).
Back to lamella production… Once graded and sorted, manufacturers are left with a variety of dimensional lumber, either in common mill-run lengths or short lengths, and must get them to the length of their desired mass timber component by process of finger jointing. Finger-jointing, the process of machining a “finger” profile and matching to a counter-profile of “fingers” into the ends of lumber and gluing them together, provides two critical facets when producing lamellas.
The first of which is the ability to produce an infinitely long piece of lumber (or as long as your finger-jointing line will allow). In a world where getting 20’ pieces of graded solid-sawn lumber is both costly and a struggle, finger-joint lines allow for the production of any long length lumber out of smaller incoming lengths.
Secondly, finger jointing allows for incredibly high lumber yield after grading, where potentially only a fraction of the lumber meets grade rules. Instead of the entire board being unusable due to shape or defect, the cutting out the defects - or parts of a board that don’t meet grade - allows for smaller lengths of usable lumber to be jointed into long lengths, maximizing total lumber yield (minimizing lumber waste).
These joint profiles can be cut horizontally across the width of the board or vertically across the thickness of the board, both of which can provide a structural joint. While horizontal jointing is most common in North America for mass timber applications, vertical jointing (which is the predominant method in Europe) can be preferable depending on the product to hide the “joint” on a visual grade product. While it may seem like a finger profile glued together would be the obvious “breaking point” of a lamella, when properly cut, glued and pressed, finger-joints are stronger than the grade-reducing knots of the grade.
Once jointed, the lamella must then be cured before it can be processed and laid-up into a mass timber component. When using structural polyurethane (PUR) glue for the finger joints, the lamella is fed into a storage buffer allowing the joints time to cure. These buffers can range from a simple chain deck to a vertical drying tower to a programmable storage system, as long as the lamella is allowed the proper time to cure ahead of further handling.
Alternatively, when using structural MF, PRF or EPI glues for finger-joints, these joints can be cured at high-speeds in a radio-frequency tunnel. At speeds of upwards of 500 feet per minute, these joints can be cured almost instantaneously while running in line – and achieve near their full strength within seconds. This radio-frequency curing process allows for immediate handling of the lamella, as well as structural proof loading in line with the finger-jointer. While not a requirement, proof loading allows each lamella and its joints to be strength tested immediately after jointing, eliminating certain certification rules (i.e. APA’s requirement for 6” spacing between finger-joints on face-glued components). Proof loading of lamellas immediately after finger jointing also ensures that if a joint fails, that the adequate length of a given grade of lamella can be replaced immediately.
Once cured, lamellas are almost always planed prior to lay-up. This planing process is most often implemented in order to clean glue squeeze out from the finger-jointing process and prepares the faces of the lamella for lay-up. As discussed in the previous article on GLT and CLT press types, both face and edge gluing are the backbone of a strong mass timber component and having a clean, uniform surface allows for optimal adhesion. However, surface preparation is not the only use for an in-line lamella planer. When manufacturing acoustic or decorative components, a popular application in DLT and NLT, using profile knives on planer heads allows for profiling a lamella into a required shape or design on the external facing edge of the lamella.
Whether you are edge gluing a CLT panel layer, preparing to dowel laminate a 60 foot long beam, walking across a GLT bridge, the first building block was a well built lamella.
