Following a sharp increase in the construction of wooden buildings, the number of substantial damages has also risen, highlighting the complex risks associated with this material.
In this article, we focus on two types of wooden structures: cross-laminated timber or CLT construction, and modular house construction. When planning a wood-based building project, understanding the risks linked to these types of construction is crucial.
During the 1800s, the number of wooden house fires increased in the Nordic countries due to urbanisation. For example, there were some 400 major city fires in Sweden during that time period. One of the most devasting events was the Gävle fire in 1869, which destroyed the entire town and left 8,000 residents homeless. These fires led to stricter legislation, limiting wooden structures to only a few stories.
During the 1990s and 2020s, building legislation has changed, and continues to change, in several Nordic countries. Today, it has become possible to build multi-storey buildings with load-bearing wooden frames again. This shift was driven by function-based building codes, which require buildings to meet functional requirements regardless of the material used. In many cases, these legislative changes often lacked sufficient impact assessment regarding wood buildings and fire protection. After the revision, the expectation was that traditional structures with only a few floors would remain the norm, as newer techniques of modular construction and CLT were not yet in use.
The Nordic countries have set ambitious climate goals, with the aim to significantly reduce greenhouse gas emissions by 2050 compared to 1990 levels. Finland, for example, is committed to becoming carbon neutral by 2035 and Denmark by 2050. This presents a major challenge for the construction industry, which must lower its emissions to meet the Paris Agreement standards.
It is generally known that building with wood can reduce climate impact compared to using traditional, non-combustible materials for larger structures. However, the regulatory framework has not yet adapted to newer construction technologies and there are challenges— especially when it comes to fire protection.
Wooden structures are becoming more common due to the growing awareness of their sustainability benefits. If Insurance is committed to supporting environmentally responsible construction, but it is important to assess each project from a risk perspective. Current building regulations have not fully accounted for CLT and modular construction technologies, making early involvement from If’s risk engineers essential for managing risks effectively.
Cross-laminated timber panels are made up of planed timber that has been finger-jointed and glued together to form slats. The slats are then laid crosswise in layers, which gives a solid element, known as CLT. This is a construction that provides a strong and rigid building element with high dimensional stability. The elements are adapted in the factory to the dimensions desired by the customer.
Generally speaking, CLT structures are considered to be more positive than modular structures from a risk point of view, as the spread of fire is highly limited due to solid construction. However, it remains a fact that utilising wood will result in a combustible building, which needs to be taken into account in the risk assessment stages of the building project, and especially during the construction phase, where this can play a greater role.
Additionally, the potential for water damage from firefighting efforts also needs to be considered. Other risks, such as moisture, pests, and mould infestation can increase with this type of material, though there is insufficient data to determine the exact extent. Companies are advised to involve risk engineers early in the process to ensure these issues are properly addressed.
Today, there is no harmonised standard for CLT in Europe. In contrast, the US has established a common standard for the CLT industry. An updated version of Eurocode 5, which will introduce fire resistance calculations for CLT walls and floors, is currently under consultation and is expected to become the approved standard by 2025.
According to Lina Sundgren, Risk Engineer at If, “There are two main fire risks with CLT structures: delamination and reduced bearing capacity during the cooling phase. Essentially, CLT elements can delaminate in the event of a fire due to the release of the adhesive when heated by the flames. This leads to the loss of the protective carbon layer and new fuel is continuously added to the fire, which leads to longer fire processes even if the other fire load in a room has been burned.”
Lina Sundgren continues, “There are a variety of adhesive types on the market that are approved for use in load- bearing structures. Currently, there are no temperature requirements linked to fire performance, rather existing standards only specify a temperature requirement of 70°C, which the glued structural components must be able to withstand over two weeks exposure while carrying a certain load. In the United States, the standard is based on fire tests in which CLT elements are placed in a furnace and exposed to different temperatures, and in this way the impact on delamination is analysed.”
She explains, “One measure to protect the load-bearing wooden structure in a fire is to install fire-retardant cladding, such as fire-rated plaster. The American standard stipulates the maximum amount of exposed CLT and requires the installation of sprinklers in these buildings.”
Reduced load-bearing capacity after a fire is another risk with CLT structures. While fire tests are conducted over a specific time period to check the structure’s load-bearing capacity, the cooling phase also impacts this capacity.
Research has shown that even after the fire is extinguished, load-bearing elements like R60 columns can lose their strength 10–15 minutes into the cooling phase, with full failure occurring within 120–240 minutes.
One measure to protect the load-bearing wooden structure in a fire is to install fire-retardant cladding, such as fire-rated plaster. The American standard stipulates the maximum amount of exposed CLT and requires the installation of sprinklers in these buildings.
Modular houses are manufactured indoors in a factory setting, which offers several advantages such as standardised work methods, better working environment, and cost efficiency. Once ready, prefabricated modules are delivered and assembled on the construction site, often completed within a few weeks. This rapid installation helps ensure compartment boundaries are established early, adding a layer of safety.
Modular constructions, however, come with their own set of risks that need to be properly managed.
Modular house constructions are particularly complex, as minor damage can lead to extensive consequences. Stacked modules create gaps where smoke, water and fire can spread between sections. By contrast, in non-combustible material structures, a fire typically remains confined to the individual apartment.
A further complexity is that the restoration of modular buildings is much more difficult as the modules are part of the load-bearing structure, stacked on top of each other. This drives up the costs and time required for restoration. The assessment is that a maximum of two storeys is most suitable for modular house constructions. On the other hand, while CLT construction presents fewer challenges, there remains a risk of collapse in multi-storey buildings if the glue fails, presenting an unacceptable risk of structural integrity.
Extensive research is being conducted worldwide, especially on CLT structures from a fire safety perspective. Swedish company Södra, a producer of solid wood structures, published Träsäker—fire protection guidance for cross-laminated timber—in December 2023. This guide outlines the measures needed beyond current building legislation requirements.
Guidelines like these provide a useful reference for proposing different solutions that offer effective and reliable fire protection for wooden structures. There are also numerous ongoing research projects, including Fire Protection Wooden Structures during Construction (BIV), Delamination of CLT (BIV), and Fire-Impregnated Wooden Facades (DBI and RISE).
Lina Sundgren, Risk Engineer