What precisely do we mean by ‘dowel laminated timber’ and how is it made?

To avoid confusion, dowel laminated timber (DLT) is the generic name used in English for a dowel-connected non-glued solid timber laminate product. There are, however, different types of DLT manufactured in Austria, Germany and Switzerland, where the bulk of this material is currently produced.

The simplest form – Brettstapel - is an arrangement of stacked boards in which the grain of each runs in the same direction. It can be used to form floors, roofs (Brettstapeldecke) and walls (Brettstapelwand). The latter panels are installed with the boards arranged vertically. In most instances the dowels are inserted perpendicular to the board stack, but one manufacturer uses a patented method (DiagonalDübelholz) that inserts the dowels at 45˚ to the board stack to improve the panel’s shear performance.

Some manufacturers produce DLT in cross laminated timber form (Brettsperrholz), i.e. similar to conventional CLT but as a non-glued product. It can be used to form floors and roofs (Brettsperrholzdecke) and walls (Brettsperrholzwand). The dowels are inserted perpendicular to the lamellae lay-up.

There are also large manufacturers that produce their own patented forms of non-glued cross laminated timber which include a diagonal layer of boards. These panels tend to be much thicker and use more timber but have distinct thermal and sound attenuation benefits.

Alongside the various lamellae arrangements in these different types of DLT, the dowels used can also vary depending upon the manufacturer. The simplest and most commonly used is a fluted or ridged hardwood rod (usually beech) which has a slightly larger diameter than the drilled hole into which it is inserted. The moisture content in the rod is lower than that of the softwood lamellae, and thus expands when the two reach a state of atmospheric moisture content equilibrium, locking the boards firmly into place.

A variation on this is a screwed hardwood rod which requires a threaded hole to be drilled into the softwood lamellae. This method is thus not wholly dependent on the interaction between the differing moisture contents of the rods and boards. The patented automated process screws the rod into place, locking the panel tightly.

Two other types of dowels have been extensively researched but are not available from any of the mainstream producers. These are densified softwood dowels which are compressed to a smaller diameter than the drilled holes. On insertion, the dowels expand to match the moisture content of the softwood and lock the panel lamellae tightly together.

The other method that has still to achieve economically viable commercial production is high speed rotation welding in which the dowel is hydraulically fired into the boards at incredible velocity. The heat generated by the speed and rotation causes the lignin in the wood to create a ‘glue’ bond between the dowel and the lamellae boards, again forming a strong lock in the panel.

Timber Design Initiatives Ltd has explored the suitability of each panel and dowel type for fabrication in a mobile press and is currently working with collaborative engineering to design an automated prototype that is fully transportable as part of the Build Back Rural mobile factory.