The future of CLT

Building with wood is an opportunity for the construction industry to reduce its negative environmental impact. Interest in building with cross-laminated timber, or CLT as it is called, is growing even among builders who have traditionally used other materials. Developed in the mid-1990s in Central Europe, CLT is a wooden board glued together from sawn, strength graded boards (usually spruce). The structure of CLT boards with crosswise layers of boards means that the strength and stiffness of the board is largely equal in both directions in the plane of the board. In load-bearing structures, this property is not always the most optimal in terms of utilisation of the material. This means that relatively large amounts of wood raw material are required when building with CLT. In today's CLT, high quality timber is often used in all layers of boards. However, using the same type of timber in all layers makes the production process easier to handle than using multiple wood qualities, species or changing the direction of the layers of boards in the panel.

Since the players who manufacture and build with CLT use high quality timber, they are more affected by the availability and increased prices of wood raw material compared with manufacturers and suppliers of other timber construction systems. The effect of price increases is reduced competitiveness in relation to other timber construction systems available on the market. For Sweden, global warming may mean longer periods of drought or longer periods of abundant rainfall, which will change the conditions for forestry and the choice of tree species. In such a future scenario, it becomes more interesting to look at optimised solutions to change the design of today's CLT products.

Building with CLT
Building with CLT is similar to conventional construction with prefabricated concrete elements. Customised building elements are manufactured in the factory and adapted for assembly on the construction site based on design drawings. The size of the CLT panels is limited by production capacity and full-sized panels are cut to exact dimensions and customised with holes for doors, windows and wiring according to the customer's wishes. These customised board elements are then assembled at the construction site to form the load-bearing frame of the house and are then completed with insulation and façade systems on the outside and often with plasterboard on the inside. CLT panels are stable and self-supporting and with few types of structural elements the load-bearing frame of a house can be built. The relatively low density of the wood material compared to concrete means that more building elements, a larger volume, can be lifted and transported in the same transport. This contributes to better shipping and lifting logistics in the construction phase, which in turn contributes to better overall project economics. Good dimensional stability together with good workability makes it possible to manufacture elements with exact dimensions and small tolerances, another advantage over conventional construction with concrete. Building with CLT is also perceived as flexible because it is easy to use simple tools to make adjustments and drill holes etc. both in the factory and on the construction site. The production-related disadvantages of CLT are that wood as a material is sensitive to moisture and there is therefore a risk of microorganism growth. Measures to manage moisture throughout the construction process are a factor that can make construction more expensive than conventional construction with other materials such as concrete. As a rule, a CLT frame is supplemented with surface layers on the construction site, and the work of completing the facades of tall CLT buildings tends to be resource-intensive and inefficient, as scaffolding is needed and insulation and facade materials are installed at various stages. The work can be protracted before a complete climate shell is in place.

Looking to the future - resource efficiency
At RISE, a future exploration has been carried out together with researchers and actors from the value chain for construction with CLT. Proposals for how a future resource-efficient CLT product could be designed, how production can be carried out and how it can be designed were discussed. A summary of the proposals is presented below.

Raw material for CLT
There is potential for improvement to become more efficient in the use of raw materials by developing the logistics from the forest to the manufacture of CLT panels. In the forest, this may involve investigating which stands in the forest are suitable for use in the manufacture of CLT, selecting the timber already in the forest and optimising for more or fewer varieties of tree species. With regard to the CLT panel as a final product, start optimising the use of the timber against current orders for CLT panels. In order to reduce the costs of producing panels, the availability of raw material and the use of lower quality timber need to be investigated. Specifically, it would be possible to investigate the advantages and disadvantages and the possibility of using timber that is currently used for paper and pulp and firewood in the manufacture of CLT, but also whether changes to the cant rules could enable the use of timber with cant in the centre layers of CLT panels. In order to create incentives and evidence to increase the use of CLT, environmental impact analyses need to be developed to assess how the use of an increased proportion of lower quality wood in CLT panels, which are long-lived products, can increase the overall effect on carbon sequestration compared to using lower quality wood for short-lived products, such as packaging material or energy.

CLT as a material
With regard to the properties of CLT panels, a development is desired to adapt the structure of the panel to the final panel's use as a structural element in the building. The possibility of concentrating materials with good load-bearing capacity in the parts where loads are to be transferred and fixings made. Reinforcement in those zones possibly also with other materials such as LVL, which is stronger than ordinary timber. However, the parts of the CLT panel that are not to carry loads or where windows and doorways are to be located, inferior and/or smaller amounts of material can be used.

Building with CLT
In a desired future situation with regard to building with CLT, it would be desirable to see a wide range of skilled contractors who know about and build with CLT. Economic evidence that it is profitable is needed for more people to start building.
In order to increase resource efficiency with regard to the use of wood raw materials, greater knowledge is needed about where CLT is most useful in a building structure so that CLT panels are used in the most efficient way. This could mean, for example, combining different building systems with CLT to create hybrid structures using both other wooden building systems and other materials such as concrete and steel, such as a combination with concrete floors on a load-bearing CLT frame. Development of frames where the load-bearing capacity of the CLT panel in both directions would be utilised to a greater extent. One such advanced example is column decks in CLT that would provide new design opportunities for architects who want open spaces. Ongoing development that would need to be done is the development of load-bearing joints between CLT panels and column tops and foot connections that can handle high loads.

In a desired future situation, assembly and carcass completion on the construction site also needs to be more efficient. There is a need for a greater degree of customisation, processing and prefabrication in the factory for future assembly on the construction site. This requires the development of joints and connections to ensure high precision and speed of assembly. Frame completion with windows, doors, insulation and facade materials needs to be rationalised, to reduce the time between erection of the frame and a tight house. Carcass completion inside the house also needs to be rationalised, for example, apartment separating floors currently require many different work steps to be completed. In addition, maintenance of the built environment needs to be done cost-effectively and with sustainable materials. To future-proof construction, circularity and reuse - being able to dismantle and reassemble buildings - must be possible. RISE's offer in the field of CLT includes testing and/or calculation for verification of product properties, product certification (CE marking or type approval) and inspection.

Further reading
This article is based on the report The future of CLT from a project conducted at RISE and financed by RISE, Swedish Wood and TMF Trä- och möbelföretagen.

This is an article from our magazine Trävärden, it is available in full here! (Link)