Fire safety in wood construction

Wood construction is on the rise and will play a key role in supporting the restructuring of the Finnish forest industry and ensuring its international competitiveness. Large-scale industrial wood construction will therefore increase significantly in the coming years. However, large buildings made from wood may cast doubt on the safety of combustible materials. So, what about the fire safety of a wooden house? 

Forests are our country’s most significant natural resource, and Finland is among the best growing areas for wood that ends up as building material in the world. In the short summer of Finland, the best growing season for trees hardly lasts a quarter of the year. Trees grow slowly and become hard and tough without too much internal stresses and cracks. Therefore, Finnish wood is used as a high-quality material for many purposes.

Wood is a renewable, organic and environmentally friendly material. It can be simultaneously both a heat-insulating and a load-bearing structure. Wood breathes and it is environmentally friendly, light but strong, it is easy to work with – and it’s also beautiful. Wood construction consumes relatively little energy, buildings generate little waste and the material is easy to reuse and recycle. Wood construction is also competitive in price, which may help with the continued increases in construction costs.

Wooden building and carbon footprint

In terms of the carbon footprint of construction industry, wood construction is an excellent choice with significant impact on the greenhouse effect. Compared with other building materials, wood arguably has the smallest carbon footprint. That is a significant advantage, because our climate goals require a reduction in the carbon footprint of construction and, in particular, of building materials. The goal of the Finnish Ministry of the Environment is to regulate the carbon footprint of a building during its life cycle by legislation by the mid-2020s.

Each building has a load of carbon bound to the building materials in manufacturing. For example, the production of cement used in the manufacture of concrete is estimated to account for 5-8% of global greenhouse gas emissions. In Finland, the problem is nationally quite small, but even though Finland’s two cement plants account for a fairly low percentage of our country’s carbon emissions (about 1.6%), globally cement production produces more climate emissions than air traffic.

Wood, on the other hand, sequesters carbon dioxide as it grows, and every tree felled for building material will act as a long-term carbon storage for decades to come. A new forest growing to replace felled trees binds even more carbon dioxide. A one cubic meter of wood can bind about a ton of carbon dioxide.

Naturally, the production of wood products also consumes energy, but the amount of carbon locked up in a wooden building is huge compared with the emissions generated during manufacturing. In fact, wood is currently the only known building material that can be used to achieve a carbon neutral building. Wood can be used to produce energy-efficient buildings that save nature and that can be truly carbon-neutral in the future: a true carbon-neutral building should have a zero carbon footprint throughout its entire life cycle.

So wood construction is not a rising trend for nothing! Of course, wood has its downsides too. Wooden buildings must be carefully designed and built, since the mechanical wear resistance of wood is worse than, for example, with concrete or steel. Wood is also sensitive to moisture, absorbs water, shrinks as it dries, is prone to molding and decaying, and… well, it might also burn.

Fire safety in wood construction

Although we Finns live from the forest, as an old saying here goes, wood is still considered by many to be a somewhat strange and dubious building material and its fire safety makes us wonder.

Properly designed and constructed, a wooden building is as fireproof as buildings made of other materials. For example, the same 60-minute fire resistance requirement is imposed on the structures of wooden apartment buildings as for concrete apartment buildings. In any case, the frame material of a building is of little importance for personal safety in the event of a fire – the most dangerous are the toxic flue gases produced by the burning of furniture.

Fire safety is based on charring

When a fire ignites, heat begins to be applied to the surface of the wood. Heat may come from further away as radiation, be transported in structures by transport, or flames may lick the surface of the wood. When the surface temperature of the wood rises to a hundred degrees, the water contained in it begins to evaporate. As the water evaporates and the temperature continues to rise, the surface structure begins to decompose, and eventually the wood ignites. The ignition temperature is affected by how long the wood is exposed to heat, but most often the wood ignites at 250-300 degrees Celsius. If the surface temperature rises to just over three hundred degrees, it will ignite even from a small flame or spark.

Everyone who has watched burning wood knows that wood charts when it burns. The carbon layer formed on the surface of the wood acts as insulation that slows down the heating of the interior of the wood and protects the wood under the carbon layer from fire stress. It insulates so well, that at an inch depth below the carbon layer the temperature is well below the boiling point of water. Therefore, the strength properties of the solid wood structure protected by the carbon layer remain unchanged in a fire and the wood retains its load-bearing capacity.

Thus, wood burns, but it is still a fireproof material, as its load resistance and collapse can be predicted due to uniform carbonization, and on this basis sufficient structural thicknesses can be calculated for structures that meet fire resistance requirements.

However, it must be remembered that wood is indeed a combustible material, and although it retains its load-bearing capacity as a massive structure in a fire, it must not compromise the fire and personal safety of the building. For example, burning wood surfaces must not impede the safe exit from the building. Therefore, the structures of wooden apartment buildings have protective covering, the premises are equipped with automatic fire extinguishing systems and all apartments have at least two exits.

What kind of wooden buildings can be built in Finland?

The construction of more than two-storey wood framed residential and workplace buildings became possible in Finland as early as 1997. In the case of traditional design based on tables, the limit of storeys at that time became four storeys. With performance-based fire safety design, there has been no limit for storeys since 1997.

The 2011 reform of the fire regulations already made it possible to have eight-storey residential and workplace buildings. As a result, areas of wooden apartment buildings were zoned and construction projects were launched during the last decade clearly more than before. In 2016, fire regulations allow the use of wood also on visible surfaces in the apartment, such as floors, walls and ceilings.

The new regulation on fire safety in buildings, which entered into force at the beginning of 2018, has sought to clarify the application and predictability of construction. The new fire safety regulations allow the fire-safe use of solid wood in the construction of houses over eight storeys. Even more wood may be left visible on the interior surfaces, as long as the load-bearing and partitioning building components can withstand the required fire resistance time. The new regulation allowed for more growth in wood construction than before, but also brought with it more stringent requirements such as mandatory sprinkler systems.

Fire engineering design of a wooden building

In traditional concrete and steel construction, load-bearing structures are usually non-combustible, but the fire design of wooden buildings requires a different approach. The load-bearing capacity of a wooden structure can be measured on three different principles, either a non-fire-retardant structure protection, which takes into account the change in effective cross-section of the charring structure during the duration of the fire, a fire-protected structure where the wood is not discharged as required for charring the fire during the duration, or a combination of these, where the fire protection protects the wood from charring at the beginning of the fire and at the end of the fire when the protection has already ended the surface is allowed to char.

The fire safety of a wooden building can be designed either based on traditional fire classes and numerical values in tables, or performance-based fire safety design. The tables are based on the main use of the building and on predefined fire load groups, as well as on the classification of the building components. In the case of wood framed buildings over two storeys, table design can only be used if the building functions mainly as a residential building or workplace.

The regulations on fire safety in buildings consist of defined fire classes, which describe requirements for structures, size, height and fire compartmentation, as well as the restrictions on the size and number of people in the building depending on the type of use. A wooden building can be implemented in all fire classes.

Fire load, which refers to the amount of thermal energy released from a combustible material, sets requirements for load-bearing and partitioning components as well as firewall materials.

Building supplies are also classified based on fire properties. The classification tests, among other things, the flammability of the material, the spread of fire and the production of smoke and burning droplets. There are seven different categories for supplies, which affect the choice of materials for surfaces and protective covering according to the surface requirements determined by the use of the premises.

Performance-based fire safety design

Based on assumed fire development performance-based fire safety design is used when the traditional table design is not sufficient or suitable for the project. It is also used as an alternative method, for example, to ensure the fire safety of demanding projects or to seek possible cost savings. Typical applications for performance-based fire safety design include the use of load-bearing timber structures without protective covering and the design of wood framed buildings with more than two storeys for purposes other than residential or work premises. Performance-based fire safety design must always be used in the fire safety planning of wood framed buildings with more than eight storeys.

More on this topic

More detailed information on the fire classes of wooden buildings and building materials, laws, regulations and building regulations related to wooden construction, as well as the fire engineering design of wooden buildings can be found in a comprehensive book Fire technology of a wooden apartment building by KK-Palokonsultti Oy’s experts Esko Mikkola and Satu Holopainen. The book can be copied free of charge.

Fire technology of a wooden apartment building

An expert study of the fire engineering of a wooden apartment building was carried out by Karelia University of Applied Sciences in collaboration with KK-Palokonsultti Oy. The aim of the publication is to help the reader understand the technical properties of wooden structures as well as the regulations affecting fire protection, and thus avoid typical design and construction errors. The publication has been produced as part of the projects Puurakentamisen osaamisen siirto (“transfer of the know-how of timber construction”) and Teollisen rakentamisen ratkaisut (“solutions of industrial construction”).

(Mikkola E., Holopainen S., (2017). Fire technology of a wooden apartment building. Karelia University of Applied Sciences Publications C, Reports: 46, Karelia University of Applied Sciences. URN: ISBN: 978-952-275-246-8 )