he importance of sustainable construction. Wood is the only primary structural material that is renewable and grows naturally.
Julius Natterer, a famous Swiss timber engineer, re-introduced the concept of NLT (known as brettstapel, literally “stacked elements”) to Europe in the 1970s. Natterer saw NLT as a mass timber product that could be produced by anyone, and encouraged manufacturing throughout Europe. However, the nails inside NLT meant that CNC machining of these panels was impossible, and manufacture by hand was laborious.
History of Dowel Laminated Timber
In the early 1990s, DLT/dübelholz was developed by Alois Tschopp (Tschopp Holzbau) with Pirmin Jung in Switzerland. They saw this product as a superior product to NLT/brettstapel in every way – it used only wood, it was CNC machinable, and production of the panel was possible with automated machinery. They proceeded to create the first automated machinery line for DLT.
In Europe, DLT is a well-known and well used mass timber product. Although both products were developed around the same time, CLT has developed a larger market share in Europe, as the big glulam manufacturers saw CLT as a glued product which would be easy to expand into. DLT remained the realm of smaller manufacturers – the largest manufacturer produces around 15,000m3/year. Interestingly however DLT is often cheaper than CLT in Europe, and is gaining more interest, due to DLT being 100 per cent wood.
Recent larger and taller wood buildings in Europe have used DLT as floor and wall panels (E3, Berlin). There are more than 15 manufacturers of DLT in Europe, primarily located in Switzerland, Germany and Austria.
DLT in North America
In 2017, StructureCraft installed the first DLT production plant in North America. This high capacity, fully automated DLT machinery line is the fastest and largest capacity worldwide, and is intended to introduce a new cost-competitive mass timber product to the rapidly growing market in North America.
DLT panels are made from softwood lumber boards stacked like the boards of NLT, but friction-fit together with hardwood dowels instead of nails. The dowels hold each board side-by-side, forming a stiffer and stronger connection than the nails in NLT. Each board lamination in a DLT panel is finger-jointed, creating a stiffer and stronger panel than NLT as it eliminates the board splices and butt-joints which are characteristic of NLT.
DLT panels may be processed using CNC machinery, unlike NLT panels (due to the nails). This creates a high tolerance panel which can also contain pre-integrated electrical conduit and other service runs.
Panelized Building Elements
DLT panels are prefabricated in sizes of up to 12 feet wide and 60 feet long. Each panel is put through a panel planer to ensure a dimensionally accurate and planed surface. Prefabricated panels can be factory finished with sealers or stains.
Finish and Geometric Possibilities
DLT is a versatile product, and naturally lends itself to creating unique aesthetics on the exposed face of the panel. Each of the laminations are run through a profile molder, meaning many different profiles are achievable, from notches and reveals to flowing curves.
Curved DLT panels can be created by milling custom profiles into each lamination, creating a flexible panel which, like an accordion, can accommodate curves perpendicular to the span direction of the panel. The radius for these curves is limited only by the bending stiffness of the dowels, which are located centrally in the panel. These panels are created initially flat and then curved into shape on site.
Fluted panels (e.g. 2×4-2×6-2×4) can create a unique soffit aesthetic if exposed, and allow the running of electrical conduits or sprinklers in the gaps.
Unique to DLT as a mass timber product, acoustic profiles can be integrated directly into the bottom surface of a panel. This can help a designer achieve acoustic objectives while keeping the wood exposed and allowing for a wide variety of surface finishes.
Any wood species incorporated in the International Building Code (IBC)