Utilization of hardwood

The increasing proportion of deciduous tree species is opening up new opportunities and possibilities for the increased use of hardwood as a building material

Forest conversion in Germany – the proportion of hardwood is increasing. © Canva

Our forests are changing – a change that also requires a rethink in the use of wood in the construction sector. The increasing proportion of deciduous tree species is opening up new opportunities and possibilities for the increased use of hardwood as a building material.

Forest conversion and future opportunities for hardwoods

Last week, the results of the fourth National Forest Inventory were presented, which provides a comprehensive overview of the state of Germany’s forests every ten years. The survey shows that the forest area in Germany has remained stable and has even grown slightly. However, the effects of the climate crisis are unmistakable: Our climate protector forest is becoming a source of carbon, as the incorporation of carbon is lower than the release for the first time in decades. The main reasons for this are calamities such as drought and pest infestations, which have caused a significant loss of spruce stocks in particular. On the positive side is the increase in deciduous trees and the growing diversity of species in our forests. The measures taken since the 1990s to convert conifer-dominated monocultures into climate-resilient and near-natural mixed forests are having an effect. The hardwood area has grown by 7 % compared to 2012. The increase in deadwood also has a positive impact on the promotion of biodiversity and the stability of ecosystems.

The most common tree species are pine (22 % of the wood floor), spruce (21 %), beech (17 %) and oak (12 %). The remaining 28 % is distributed among a further 47 tree species and tree species groups. Today, weak hardwood is mainly used for energy generation through combustion or for pulp production. In contrast, stronger, sawable hardwoods are mainly used in furniture and veneer production as well as in interior fittings. In addition, some of the deciduous trees that show signs of damage remain unused in the forests as so-called biotope trees in order to support the natural forest ecosystem. Hardwood is not yet a fully-fledged substitute for softwood, which is still mainly used in the construction sector, particularly for load-bearing structures. Due to its positive properties, however, it could gain in importance in the future. However, there are still challenges that need to be overcome. With advancing technical development and targeted use, the use of hardwood has the potential to play a more important role, particularly in the context of sustainable construction. The decisive factor in the use of wood is the selection of sustainably managed and certified sources. The basis for this is responsible and near-natural forestry that ensures the preservation of ecosystems and guarantees the long-term availability of the resource.

Systematization of the hardwood stock under the aspect of timber utilization. © FNR

Historical use of hardwood in the construction sector

Historical buildings show that hardwoods were once frequently used in load-bearing structures such as columns and beams. Oak wood in particular, known for its durability, proved to be an excellent construction timber, even for outdoor use when exposed to the elements. For centuries, foundations made of oak piles played a central role – cities such as Venice and Amsterdam were largely built on these stable pile grids. In the Middle Ages and early modern times, hardwood was also used in half-timbered houses, bridges and as load-bearing elements in buildings. In addition to its structural use, hardwood, especially noble hardwood species, was widely used in interior fittings, as it still is today. With industrialization and the introduction of softwoods in the construction industry, which grow faster and more evenly, produce a significantly higher yield compared to hardwood and are also easier to process, the use of hardwood in construction declined.

Properties of hardwood and optimization of manufacturing processes

The manufacture of construction products from hardwood poses several challenges, as the wood species-specific properties vary greatly, which makes standardization difficult. While there is already extensive experience and established treatment and processing methods for softwood, these need to be specially developed and researched for hardwood. So far, the processes for hardwood have been technically more complex and associated with higher costs. In addition, the use of hardwood in construction requires careful planning, as the specific properties of each type of wood must be taken into account in the construction process. During processing, properties such as crookedness, drying behavior, stronger swelling and shrinkage behavior and the low durability of some hardwood species prove to be particularly challenging. The disadvantage of short sawn timber lengths due to the crookedness can be overcome, for example, by using glued products instead of solid wood. This can also reduce the often pronounced shrinkage and swelling behavior of the material. Studies have also shown that specially adapted bonding technologies must be used for hardwoods, as proven bonding systems are not equally suitable for softwoods. Hardwoods such as beech, oak and ash are characterized by a higher bulk density and strength compared to softwoods, which makes them particularly robust and resilient. These properties are particularly advantageous for load-bearing structures, as they make it possible to create modern, delicate and aesthetically pleasing load-bearing structures. In addition, hardwoods offer appealing surface qualities, making them attractive for a variety of architectural applications. Some hardwoods, especially oak, are naturally very resistant to weathering and fungal attack, which allows them to be used outdoors without additional protective measures. Hardwoods also generally emit fewer VOCs than softwoods, which, combined with their availability, is an advantage for the wood materials industry for the production of particleboard or fiberboard.

Possible areas of application in the construction sector

Structural hardwood products

In recent years, wood research institutes have intensified their efforts to develop glued hardwood products, as these make it possible to specifically determine the structural properties by sorting and gluing the boards. In the construction sector, two main product groups can be distinguished: solid wood products in the form of bars and solid wood products in the form of panels.

Rod-shaped solid wood products

This product group primarily comprises glulam, glulam hybrid beams, solid structural timber (KVH) and additional products such as battens and formwork. Glulam, one of the central products, consists of several glued layers of wood and has a high load-bearing capacity.

In recent years in particular, there has been increased work on the development of glulam made from beech. This material and some glulam products made from oak, sweet chestnut and poplar have been approved by the German Institute for Building Technology and can therefore be used for load-bearing structures in buildings.

Glulam of beech without and with red heartwood. © R.Rosin

Panel-shaped solid wood products

The most important panel-shaped solid wood product in structural applications is cross-laminated timber. It consists of several cross-laminated layers of boards, which give it excellent load-bearing capacity and high dimensional stability, making it ideal for use in wall, ceiling and roof elements. Another innovative material in this area is laminated veneer lumber (LVL) made of beech, which is particularly advantageous due to its high strength, load-bearing capacity and dimensional stability and can be used as a substitute for LVL made of softwood and solid wood.

Laminated veneer lumber as a panel and as a support. © R.Rosin

From tree to foam

At the Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institut (WKI), a foam material has been developed that consists of 100% wood fibers and other lignocelluloses. The strength is achieved by the natural bonding forces of the wood, which is why no synthetic adhesives are used. The result is a material that is light and has an open-pored structure with a low bulk density. Foams made from beech wood, for example, can be produced specifically in a density range between 50 kg/m3 and 200 kg/m³. The foam boards can be used, for example, as insulation boards or as a component of hybrid sandwich materials with a foam core. The open-pored structure also gives the wood foam a high level of sound absorption. The first wood foam boards are due to be launched on the market from 2026 and are also fully recyclable.

Wood foam is a one hundred percent natural product made from renewable raw materials.© FraunhoferWKI, Manuela Lingnau

Wooden nails

Hardwoods are particularly suitable for use as wooden nails due to their higher density and strength, as they offer greater mechanical stability and load-bearing capacity. Hardwood nails are particularly impressive in load-bearing structures due to their durability and resistance. Nowadays, collated wooden nails are available that have general building authority approval for load-bearing timber connections. Compared to conventional steel nails, wooden nails not only offer ecological advantages, but also technical benefits such as lower thermal conductivity, which prevents thermal bridges at certain points, and high resistance to corrosion.

‍Relevant files & links

https://www.bundeswaldinventur.de/vierte-bundeswaldinventur-2022/vorwort

https://mediathek.fnr.de/broschuren/holz/laubholz-produktmarkte-aus-technisch-wirtschaftlicher-und-marktstruktureller-sicht.html

https://www.lwf.bayern.de/mam/cms04/forsttechnik-holz/dateien/a98_bauen_mit_laubholz_bf_gesch.pdf

https://www.baunetzwissen.de/daemmstoffe/tipps/forschung/vom-baum-zum-schaum-8442074

https://www.nbau.org/2022/12/19/circular-globe-label-fuer-beck-und-awards-fuer-lignoloc-holznagel-mit-kopf/

Source

natureplus e.V., press release, 2024-10-16.

Supplier

Bayerische Landesanstalt für Wald- und Forstwirtschaft
Bundesministerium für Ernährung und Landwirtschaft (BMEL)
Fachagentur Nachwachsende Rohstoffe e.V. (FNR)
Fraunhofer-Institut für Holzforschung Wilhelm-Klauditz-Institut WKI

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