Can we build fire-safe with wood?

There are a number of building systems within wood construction, and cross-laminated timber (CLT) is a system that is gaining in popularity. Cross-laminated timber panels are built of glued, and cross-laid boards or planks. What is characteristic about walls and floor structures made of CLT is that they are often used as large plane elements. Due to its large cross-sections, CLT has a high load-bearing capacity and rigidity. It can be prefabricated to a large extent and has a low self-weight, which delivers benefits during foundation work, transport and installation. One plane element of CLT would normally consist of 3 to 9 layers, that can be glued together to a thickness of 80 to 300 mm.

Understand the challenges, develop the knowledge

The fact that wood burns is in principle a basic prerequisite for human life but also a challenge for us to tackle when we build with wood. In the past, we have often protected wooden constructions with different types of cladding, however since the focus is now on sustainability and climate impact, there is a demand for and an increased exposure to wooden surfaces. As a result of this development along with the new construction systems, we need new knowledge regarding everything from the spread of fire in wooden constructions to detailed design so that fire safety can be satisfactory. Briab has recently completed a development project, financed by the Development Fund of the Swedish Construction Industry, SBUF, along with fire safety consultants and other industry players deeply engrossed in the challenges of building largely with wood. Our work has mainly implied improving the methods used to ensure fire safety in tall wooden buildings and, in this way, facilitate designing fire safety in them. To be successful with the verification, a good understanding of the behavior of fire in wood buildings is required, as well as specific knowledge of the challenges of building tall wooden buildings with respect to the technical property requirements for fire safety.

Delamination provides new fuel

Wooden constructions have long been used as load-bearing elements and the knowledge of how traditional wooden elements behave in the event of fire is sound. There are established standards and models for dimensioning. The starting point is that it is possible to establish a burning speed and thickness of the protective carbon layer that is formed when wood burns. However, for building elements made of CLT, it is different, as above all, the choice of glue determines how well the product performs in a fire. Examples of the existing glue are MF, MUF, EU, and PR and PRF, which have different advantages and disadvantages. At present, PU glue is one of the most common glues, which is astonishing, as this glue type has the worst qualities with respect to fire. The glue loses its gluing capacity at relatively low temperatures which leads to delamination when a lamina (layer) loosens. The problem with delamination is that the wooden board loses its carbon layer which otherwise works as decelerating for penetration. Instead of the fire extinguishing itself when the fuel has run out, the fire regains intensity each time a layer loosens.

Risk of an unexpected fire development

With incombustible materials in the construction, we have a very good picture of the fire development: from ignition, then flashover to a fully developed fire that finally extinguishes. This unexpected development underlies the entire fire protection design, our choice of solutions and whether our ways of testing the solutions comply with the society’s requirements. With a combustible construction, there is a risk of having an unexpected fire development, which our traditional fire protection solutions cannot manage. A long-term and more intensive fire development, where a larger part of the fuel is combusted outside the room, will affect façade solutions and repeat ignitions in the fire room and create an impact on the construction that the current verification methods cannot manage.

New requirement specifications

Our knowledge of how the wooden construction contributes to fire development, and its behavior during a fire, means that certain cautionary measures should be adopted when using a combustible structure of CLT. A fire that self-extinguishes when loose furniture is burned is an important condition for the fire development to be predictable. This can be achieved by either ensuring that CLT does not delaminate or by using cladding that protects the wooden construction until the fuel runs out.

Since our knowledge is limited and there is no standardization, my recommendation is that building elements of CLT be protected with non-combustible cladding. However, cladding on its own is not a universal solution, but it brings a challenge to ensure that it protects the underlying combustible structure throughout the fire development. A standard protection time for a plasterboard is 10-20 minutes, depending on the type and installation. This does not provide sufficient protection, but we need to ensure that a fallout does not occur before the fire has burned out.

Sustainability and safety

In conclusion, I see that we face several challenges in the shift towards building with renewable materials. Only when we, through research, testing, standardization and international cooperation, have created a building system with CLT where fires self-extinguish, will we be able to find solutions that do not include cladding. In the meanwhile, we need to make sure that we place the right requirements on the cladding so that it gives the wooden structures the protection they need.

Download the entire SBUF report here