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Building with wood is a rising trend. In Germany, the proportion of approved residential buildings made with timber construction has risen from 12.2% of all approved residential buildings in 2003 to 22% in 2023.1 In the U.S., more than 90% of residential buildings constructed each year were built using the timber frame construction method.2 This trend is expected to continue in conjunction with increasing politically driven pressure to reduce residential construction’s ecological footprint. In addition, timber construction has a higher prefabrication rate, which significantly reduces construction time, as well as good thermal insulation properties, high strength, and load-bearing capacity with a comparatively low dead weight.
However, fires in residential buildings occur frequently. The increase in timber construction of residential and commercial buildings is likely to result in more frequent fires with considerable damage. Legislators are attempting to counteract this through legal requirements such as Germany’s model timber construction guidelines.3 Particular attention is being paid to structural fire protection in timber construction, an indisputably fundamental aspect to consider when evaluating timber construction.
However, the experience of property insurers has repeatedly shown that structural fire protection is not sufficient to prevent significant fire damage in timber buildings. This article presents the current risk situation from a property insurance perspective. It also suggests potential additional preventive protective measures to minimize the probability of major fires in timber buildings and their devastating consequences.
Timber construction is a construction method in which wood is used as the primary building material to construct structures such as residential buildings, bridges, or high-rise buildings. The timber components are used as load-bearing and non-load-bearing components. Wood is used in a wide variety of forms, from solid wood to boards, fiberboard, or chipboard to decorative panels.
Timber Panel Construction
It consists of using prefabricated wall and ceiling elements. The load-bearing structure of a house is formed by a timber-supporting frame to transfer the vertical loads and stiffen the building and is usually clad with wood-based panels. This construction method, which was initially used mainly in the U.S. and Canada, is now also increasingly found in Central Europe.
Timber Frame Construction
The basic element of timber frame construction is a supporting structure (skeleton) made of wood. It consists of vertical supports and horizontal beams, which are connected to each other at nodal points and thus form a wide-span structural grid. In contrast to timber panel construction, the space-enclosing walls in timber frame construction are completely separate from the supporting structure. The position of the walls can therefore be freely selected and changed at any time without having to take structural aspects into consideration. An early form of timber frame construction is the half-timbered structure.
Solid Timber Construction
Depending on the manufacturer, slats, boards, planks, or panel materials are used to produce board stack, wood dowel, and glulam elements. The use of large-format timber components is typical. In solid timber construction, the outer walls are combined with additional insulation layers and subsequently plastered or clad with a timber façade.
Fire damage to large residential and apartment buildings made of wood in the U.S. shows that, although buildings constructed of wood collapse less quickly, they have a high fire load and therefore tend to be a major or total loss if the fire cannot be detected and extinguished early,4 or if no other preventive fire protection measures, such as sprinkler systems, are installed.5
Fires are not usually caused by wood construction but result from interior fires. Typical causes of fire in residential buildings are:6
In addition, the fire load in residential buildings has increased significantly in recent decades due to interior furnishings made from flammable materials, including plastics, textiles, mattresses, and upholstered furniture.7
If an incipient fire is not detected early and successfully fought, there is a risk that the fire will spread to the building structure and lead to a full fire.8
An analysis of available fire statistics shows that the building construction and the building materials and components used have a significant influence on the course of the fire and the extent of the damage. Loss reports show combustible building materials and components, in particular wood as well as plastic materials, are conspicuous and contribute to significant insurance claims.
Reliable damage statistics that focus on timber buildings are largely not publicly available. However, an indication of the incidence of damage and its consequences can be gleaned from property insurance quotes found online for which wooden buildings generally result in a higher insurance premium. That is, buildings made of wood or with wooden construction components are assessed more critically than conventional solid buildings, such as those with masonry walls, for example. Compared to a building with masonry walls, the premium factor for timber buildings is 2.4. The same applies to water damage insurance, where we can see a difference of up to a factor of 1.2.
A study comparing Canadian timber frame buildings with concrete buildings confirms that timber frame buildings have a higher fire risk and greater water damage problems. These higher risk factors are reflected in higher insurance rates for such buildings both during construction and over the life of the buildings.9
The differences in the premium factors are due, as far as the data indicates, to the fact that damage to wooden structures following fire or water damage is more complex and therefore more costly to repair.
When exposed wood burns, a layer of charcoal forms on the surface, which leads to slower burning of the remaining cross-section due to its low thermal conductivity.10 The strength and stiffness properties of the remaining cross-section initially remain largely unchanged. With increasing fire duration, however, the residual cross-section decreases by approximately 0.7 mm/min for softwood, according to DIN EN 1995‑1‑2:2010‑12.11 Failure of the timber construction is to be expected at approximately 50% of the load-bearing cross-section of the relevant timber components; in the case of a 10 cm-thick timber beam, for example, this would be reached after around 20 minutes assuming burning from three sides.12
To restore or repair the affected timber component, the layer of charcoal must be removed. Depending on the depth of the charcoal layer, this can lead to a reduction in the cross-section, which in the worst case may require the timber component in question to be replaced. Even if the fire can be extinguished before the building is damaged to such an extent that it is in danger of collapsing, the installed timber materials may already be damaged to an extent that they need to be replaced for visual or structural reasons. It should also be noted that odor-intensive substances are released in the fire room by the products of the fire, which settle in the wood-based materials and require costly removal.
In addition, the fire department usually extinguishes the fire with water, which leads to further damage to the wood materials. This is because the extinguishing water penetrates the wood and must be laboriously removed from the wood after a fire, as otherwise it can lead to rot, mold growth, and ultimately a total loss if the restoration measures exceed the value of the building.
In general, wood-based building materials and components are sensitive to moisture and humidity. Apart from the consequences of a possible fire, damage caused by water leaks, from water-bearing pipes for example, or moisture is also problematic. In addition, leakage or moisture damage is only detected with a considerable delay, usually only when the wood has already been saturated with water or moisture. As a result, time-consuming and costly drying and restoration measures are required. If the moisture cannot be completely removed from the installed timber elements, this can lead to rot and mold growth, which may lead to a complete replacement of the affected timber areas or even the total loss of the building. Experience shows that the damage to a timber construction building after water damage is significantly higher than in a conventional building.
The fire resistance of wooden components is often either increased by covering the wooden components with non-combustible materials, or the flammability and combustibility of wood is influenced by treating it with chemical substances. Some of the impregnating agents used can have negative effects on the human organism.
Just like steel and concrete buildings, timber buildings require insulation to be energy efficient. To comply with energy standards, these buildings are often insulated with combustible materials (e.g., insulating materials made of polystyrene, polyurethane, or increasingly with natural insulating materials such as dried natural fibers, straw, or grain). If a fire breaks out in the combustible insulation of wooden walls and ceilings, a fire department will have problems successfully fighting such a fire; since the insulation cannot be reached by the extinguishing agents from outside. In such cases, the fire department often must limit itself to preventing the fire from spreading to neighboring buildings, with the result that the affected building burns down completely.
In Europe, there are increasing legal and official regulations for the fire protection of timber buildings. A closer look at these regulations reveals that they are essentially limited to structural fire protection (e.g., cladding of timber components to improve fire resistance and protect against charring) and monitoring buildings with automatic fire alarm systems. Preventive, technical active fire protection measures are only recommended or prescribed in isolated cases. A 2021 overview of the various fire protection requirements for timber construction buildings13 outlines requirements set for the resistance of load-bearing elements in residential buildings depending on the maximum number of stories or maximum height of a building. This shows that the preventive installation of sprinklers makes the fire protection requirements for timber buildings easier.
Damage experience in the U.S., Canada, and other countries shows that the installation of sprinklers can increase personal safety and significantly reduce damage to the building.14 For this reason, the installation of a sprinkler system should be considered from the outset when planning timber buildings. In particular, sprinklers should be installed in areas of increased risk in timber buildings. Examples of such higher risk areas are:
In addition to limiting potential fire damage, the installation of sprinkler systems can also significantly reduce the insurance premium that may be required for a timber building.
When planning fire protection in buildings, two different phases of a fire scenario must be considered with regard to the building materials, components, and structures used:
In the event of an incipient fire, the contents of the building (e.g., furniture) are decisive for both the start of the fire and the development of the fire. However, this is not usually regulated in the building regulations.
In a fully developed fire (i.e., after a flashover in a room), the fire resistance duration of the load-bearing and separating construction structures is a critical factor impacting the fire to the affected room or area and the stability of the building. In this respect, it is important and correct that building regulations for timber buildings pay particular attention to the fire resistance duration of load-bearing timber components and room- or area-enclosing timber walls. Nevertheless, the obvious should be noted: timber contributes to fire occurrence in a full fire. It is therefore advisable to take additional preventive technical fire protection measures to reduce the consequential fire damage to a timber building.
The flammability of wood is one of the main reasons why many building regulations severely restrict the use of wood as a building material. In addition to structural fire protection, preventive fire protection systems and technology play an important role in fighting or extinguishing potential fires and minimizing the consequences of damage.
In Germany automatic fire alarm systems in particular are seen as an effective fire protection measure among possible active preventive fire protection measures, while sprinklers are seen as unnecessary, too expensive, or not possible to install by the building owner.
Automatic fire alarm systems:
Fire alarm systems are often required in official fire protection concepts as a compensatory measure for deficits or deviations in structural fire protection. This is almost always a matter of personal protection. From the point of view of property protection, they are only helpful to the extent that they call in firefighters as quickly as possible, ideally through a direct connection to a body providing assistance, such as the local fire department. Depending on the progress of a fire, the fire department may be able to successfully fight the fire by means of an interior attack thanks to the early alarm from a fire alarm system. However, this presupposes that the fire department is on site within a very short time to start fighting the fire. Fires are often so far advanced after a short time, especially if the building contains flammable building materials and components, that an interior attack is no longer possible and the fire department must concentrate on protecting the neighboring rooms and buildings. It is crucial that the neighboring sections or neighboring buildings are separated by fire-resistant structural measures (e.g., fire walls) in accordance with the model building regulations15 VdS 2234 “Firewalls and complex partition walls – leaflet for arrangement and execution”.16
To ensure their effectiveness, automatic fire alarm systems should be planned and installed by specialists in accordance with the relevant technical regulations, notably VdS 2095 “Planning and installation of fire alarm systems (BMA)”17 or a comparable guideline, and their effectiveness should be checked regularly after installation.
In contrast to fire alarm systems, initial firefighting systems installed in buildings are capable of actively fighting an incipient fire within the first few minutes, possibly containing it until or even extinguishing it before the fire department arrives on site. Automatic sprinkler systems are particularly widespread and have proven their worth, especially in wooden buildings. While sprinkler systems are standard in private homes in many European countries and the U.S.,18 you will search in vain for them in German residential buildings.
Sprinkler systems are automatic water spray extinguishing systems. Their task is to actively fight a fire in its incipient stage and to keep it limited to the source of the outbreak, thus supporting the fire department in ultimately extinguishing the attack. In addition to the sprinklers, important components of such a system are a pressurized pipe network that runs through all parts of the building to be protected, as well as a separate, secured water supply. Since the sprinkler system only triggers above the source of the fire, it only extinguished locally, which can reduce the fire spread ahead of the fire fighters’ arrival.
A sprinkler system is used to fight an incipient fire in its early stages and to limit it to one room, thereby facilitating the safe evacuation of people. At the same time, when there is a direct connection, the fire department is alerted.
Architects, planners, and operators often do not consider or repeatedly reject the installation of a sprinkler system in the run‑up to planning. The same arguments are used again and again: the water damage caused is far too high and the installation of a sprinkler system is not compatible with the design of the interior. But what is often overlooked is that in over 90% of all fires, the fire department uses water to fight fires. The amount of water used in fighting a fire is many times greater than that sprayed by a sprinkler system. As a result, the larger quantities of water used in firefighting by the fire department cause significantly more damage than would a sprinkler system. In fact, experience shows that water consumption is around 90% higher than when fighting a fire using an automatic sprinkler system:19 The amount of water used in the course of fighting fires when both sprinklers and the fire department are involved is considerably lower than for fires fought by fire departments in structures lacking sprinkler systems. This is also confirmed by the claims experiences of property insurers; overall, the fire damage and thus the financial loss expenditure to be compensated are generally significantly lower in buildings protected by sprinkler systems.
While sprinkler systems are used frequently in commercial and industrial fire protection, in Germany they are rarely considered as an alternative fire protection measure in the residential sector. Yet it is precisely there, especially in buildings constructed from flammable materials that they are an excellent way of preventing or reducing major consequential fire damage. There are specialized sprinkler systems that take into account the special characteristics and requirements of residential areas and buildings. These sprinkler systems are used in:
Special technical guidelines for use in residential and care areas have been developed for planning and installation.20 Among the advantages these sprinkler systems offer is that low volumes of water are required to cover large protected areas with extinguishing water.
A sprinkler system for residential areas consists of a water supply and one or more sprinkler groups. Each group includes a device for sounding the alarm when a sprinkler is triggered (an alarm valve station) and a pipe network onto which sprinklers are installed. The sprinklers are installed at specified points under roofs, ceilings, or walls. Sprinklers are used that have been specially developed for use in residential areas. The sprinklers open at set temperatures to spray water onto the areas affected by the fire. The flow of water through the alarm valve station triggers a fire alarm. For example, in accordance with the Germany VdS 2896 guidelines,21 the minimum water application is usually between 2.25 mm/minute and 5 mm/minute, depending on the fire load and building situation, with a minimum operating time of between 10 and 30 minutes. Generally, only the sprinkler closest to the fire opens and sprays water directly onto the fire, while the rest of the building remains dry and safe. It has been reported that in about 85% of house/flat fires, only one or two sprinklers were activated.22 The possible protected area per sprinkler is between 12 m² and 21 m², depending on the type of building. The water supply required for the sprinkler system is usually provided via the public water network. However, other water supplies such as water tanks, intermediate tanks, or compressed air water tanks can also be considered as water sources. A pump may also be required if the water pressure is not sufficient to supply the sprinklers.
In order to ensure the highest possible effectiveness, sprinkler systems in Germany should be planned and installed using recognized components in accordance with VdS 2896. Another standard is “NFPA 13D, Standard for the Installation of Sprinkler Systems in One- and Two-Family Dwellings and Manufactured Homes”, which serves as a basis in Anglo-American countries in particular.23
All areas of a building or at least one usage unit must be provided with sprinkler protection. Appropriate components with a fire resistance duration of at least 30 minutes must be provided between sprinklered and unsprinklered usage units, buildings, or parts of buildings in order to prevent the fire from spreading to neighboring areas. In addition, the sprinkler system should be inspected at regular intervals after installation by a competent inspection body to check its condition and any defects found should be rectified as soon as possible.
By installing a sprinkler system, it may be possible to dispense with some structural fire protection requirements such as fire-retardant walls within groups of rooms, fire-resistant glazing, or self-closing doors within groups of rooms. The possibility of omitting structural fire protection elements when sprinkler systems are present clearly shows that sprinkler systems offer a major advantage with regard to the upgrading and implementation of structural fire protection.
Regarding the commonly expressed fear of leaks from sprinkler systems, practice shows that leaks are very rare and no more likely than leaks in other plumbing systems in a home. Sprinklers for residential use are calibrated to activate when they detect a significant temperature change, typically at 57° to 79° Celsius. Sprinklers will not activate from smoke, cooking fumes, steam, or the sound of a smoke detector.24
It should be noted that nowadays sprinkler systems for residential buildings/rooms can be integrated very unobtrusively and do not disrupt the aesthetics of a room. They can be installed flush in walls or ceilings and can be concealed behind decorative covers.
Statistics from insurers show that buildings constructed using wood have a significantly higher claims requirement than buildings of conventional solid construction materials. Therefore, it is crucial for a residential building or commercial insurer, as in industrial property insurance, to know the type of building construction to be insured. In the case of wooden buildings, this relates not only to the fire risk to be insured, but also to the risk of water damage.
It is advisable to record in detail the type of building construction before concluding an insurance contract. Also to be considered is whether the building has comprehensive technical fire prevention measures.
Although an installed automatic fire alarm system, which automatically alerts firefighters in the event of an incipient fire, is a good first fire protection measure, an installed sprinkler system has a significantly greater influence on the probability of preventing a major fire. Only with the right planning and installation can it automatically fight an incipient fire from the outset, offering a measure of fire intervention until firefighters arrive. Sprinkler systems not only offer increased level of personal safety by ensuring that people present can get outside unhindered and safely, but they also provide significant protection against a major or total loss of the building.
Buildings made of wood or wood-based materials are increasingly being constructed in both the private and commercial sectors. They are intended to replace conventional building materials as they offer ecological and cost advantages over conventional building materials such as steel and concrete.
Because wood is combustible and normally flammable, buildings made of wood pose particular challenges for fire departments and insurers. Wood begins to decompose at temperatures as low as 200° to 350° Celsius, depending on the type of wood. Exposed wood contributes to the fire at an early stage and provides sufficient fuel in a fully developed fire. With wooden buildings, the associated risks must also be considered, and attempts should be made to reduce these risks through defensive damage prevention and mitigation measures, for example by installing sprinkler systems. The future challenge will be to reduce the potential risks through measures that go beyond structural and organizational fire protection.
All endnotes last accessed 15 January 2025.
All endnotes last accessed 15 January 2025.
