What is DLT and how it differs from the CLT panel

Wood is valued worldwide as a building material because it is environmentally friendly and renewable. To achieve the strengths required in construction and eliminate weak parts, wood is processed in specialised factories. Laminated beams (glulam) or CLT panels are solid wood materials made in factories, which have been used in our country for years. However, there are also those who point out that these materials are not 100% wood, as the glue used to glue them together is also part of their composition. This is how DLT - Dowel-laminated Timber - or Dübelholzas it is known to German speakers. It is a material made from planks of timber fixed with dowels. Thus the resulting panel is 100% wood, without glue, nails or other metal connectors.

How and where the idea for 100% laminated wood panel came from

The idea originated in Switzerland in the early 1990s. To understand how the idea came about, we go back a long way, before the beginning of the 20th century. That's when wooden slat panels nailed together began to be used for the floors and ceilings of buildings. These have proved very durable over time, with some still standing in houses built in the early 20th century. Industrial development and the advent of cement and concrete led to the abandonment of this building material.

In the 1970s, when the issue of environmental protection had already arisen,Julius Natterer has revitalised this concept based on the idea that wood is a renewable and non-polluting source. The panels made of nailed wooden slats began to be manufactured industrially under the name of NLT (Nail-laminated Timber) or Brettstapel in German (BS panels).

This brings us to the early 1990s. In the meantime, CNC woodworking had gained momentum, but for NLT panels it could not be used because of nails. They were replaced with glue and so CLT was born. But for some keen to use wood in construction for environmental reasons, the presence of glue inside the panel meant chemistry and a departure from the natural concept. So the idea of using dowels (dowels or tipli, as they are called) came about, as they were used to assemble furniture. The two building materials, CLT and DLT, were enthusiastically received by those who wanted to replace the old materials considered polluting and were developed in parallel.

Over time, the development of CLT has taken off and it is now an impressive combined material. DLT has remained at the level of the average mills, but is still quite a popular material. The proof is the existence of more than 15 plants in Austria, Switzerland and Germany alone. It should also be noted that unlike CLT where the lamellas are laid perpendicular, DLT is parallel, making it less structurally strong than CLT panels, but much more resilient.

How to get DLT panels

Although glue is sometimes used to fix furniture elements with nails, DLT panels are made of nothing but wood. The blades are made of softwoodand the dowels are made of hardwood. The fixing is due to the difference in moisture content between the two types of wood. The transfer of moisture from the dowels to the drier blades causes them to expand and fix very well.

The production process has 4 main stages:

• removal of defects in the saw blades
• the teeth jointing of the blades to reach the desired length
• blade profiling
• fixing with dowels

I do not insist on the process of eliminating defects, it has been discussed extensively in the articles on obtaining massive panels and joining in teeth. Profiling of panel elements is an operation that gives DLT an advantage. Thanks to this operation a wide variety of patterns can be obtained, making DLT panels very aesthetic. Profiles can also be used to improve the acoustics of rooms where they are used.

Dowel fastening has several stages:

• pressing the blades
• drilling
• introduction of dowels
• moisture balancing

The panel is not obtained from the beginning at the desired width. Groups of slats are glued together with dowels, which are then clamped together to form the final panel. Panels can be 20 m long, 4 m wide and 200-260 mm thick. For extra security, some manufacturers also put the dowels on the diagonal.

Where DLT panels are used. Advantages and disadvantages

DLT panels are used for the construction of floors, ceilings and interior walls of wooden buildings. They are built in the factory to the required dimensions so that only the prefabricated elements are assembled on site. In addition to CNC machining, it is also possible to finish the wood in the factory.

The advantages of using such a material have been mentioned throughout the article. However, I will summarise:

• material 100% solid wood, without glue, nails or other metal connectors,
• the lack of metal elements makes it easy to process by cutting or milling. Its processing can be done on automated production lines,
• very versatile material, the profiling of the elements results in a wide variety of designs. Soundproofing materials, electrical conductors or other such elements required for construction can also be fitted into the profiles from the production phase,
• It makes structures much lighter than using concrete or metal structures, which results in a smaller foundation and therefore lower costs,
• From the data found, production costs appear to be up to 20% lower than those for obtaining CLT.

The most important disadvantage of DLT panels is their sensitivity to moisture. Unlike CLT, DLT panels are not totally stable and can have dimensional variations when humidity increases or decreases greatly. Another disadvantage is the lower structural strength than CLT panels.

Industrialised timber construction has led to the development of new materials to meet the strength requirements of ever larger and taller buildings. In future articles I will tell you a little about MHM and WLT panels.