Structural timber is used for creating load-bearing and structural elements such as beams, posts, floors, rafters, or building frames. It is one of the oldest building materials, which remains indispensable to this day in many construction projects, both traditional and modern.
Structural timber – what are its characteristics?
Structural timber is a naturally sourced material that combines the durability and strength of a construction-grade building material. Of course, it is processed timber that has been treated in a way that enables it to meet specific standards and requirements necessary in construction for forming load-bearing and structural components.
It is characterized by excellent mechanical strength, allowing it to carry significant loads, as well as resistance to weather conditions, especially when appropriate protection methods such as impregnation or drying are applied. Structural timber is also valued for its insulating properties, which contribute to improving the energy efficiency of buildings.
In construction, structural timber with a strength class of C24, as defined by European technical standards such as PN-EN 338, is most commonly used. In practice, this means that a C24-class wooden element meets the requirement of bending strength of 24 MPa per 1 mm². Translated into more illustrative terms, we can say that 1 cm² of structural timber can withstand a load of 240 kilograms.
In order for structural timber to be certified, it must undergo a series of thermal processing and quality testing procedures. First and foremost, for the material to undergo the certification process, it must be dried, after which the timber’s moisture content reaches between 15% and 18%. Next, the material is subjected to four-sided planing and edge chamfering, which results in its sides and edges being smooth, free of splinters and sharp elements. After such processing and quality control, a certified grader with the appropriate qualifications classifies the product as load-bearing, thereby making the timber a structural material that receives the CE certificate. The certification process is strictly described by relevant standards, and the final product bears a stamp on each batch of material.
What tree species is structural timber made from?
Structural timber used in construction can come from various tree species. The choice of a specific species depends on the required physical and mechanical properties as well as local availability. However, the most commonly used structural timber comes from pine and spruce.
Pine is one of the most commonly used species of structural timber. Its popularity is due to its wide availability, high bending strength, and relatively low cost. Pine is lightweight yet offers good resistance to both bending and compression.
Spruce, like pine, is widely used in wooden structures. It is characterized by its lightness and ease of processing. It is chosen for its insulating properties and attractive appearance.
In wooden construction, products made from other tree species can also be found. However, these are most often used for finishing and decorative purposes rather than as structural material. Such species include:
Oak is a tree synonymous with strength, durability, and resilience. It is a hard and dense wood, most commonly used in the construction of self-supporting staircases. Its hardness and density also make it resistant to mechanical damage.
Fir is valued for its simplicity and uniform structure. It also has good insulating properties and is often used in roof structures and the production of boards and panels.
Larch, on the other hand, stands out for its high resistance to weather conditions and decay, which makes it frequently used in outdoor structures such as terraces, cladding boards, façades, or garden constructions.
Douglas fir is a hard and very durable type of wood. It is chosen for structures with high strength requirements.
Maple, beech, and ash are less common types of wood. These are deciduous species known for their exceptional aesthetic qualities. They are hard and durable, and sometimes used in specialized structural applications. Undoubtedly, they are more expensive than their coniferous counterparts. However, they feature a low number of knots, contributing to their minimalist and elegant appearance. These types of wood are most often used in places where the structure, grain pattern, and visual effect of the material are of the highest importance.
Planed or unplaned structural timber?
When choosing timber, one may also come across the terms planed structural timber and unplaned timber. What do they mean?
Unplaned timber refers to a type of wood that, after cutting and processing, does not undergo any additional surface treatment. This kind of raw material is not permitted for the construction of structural buildings. It remains in its natural state, with visible marks of machine processing. As a result, the surface is rough and contains visible splinters. Undoubtedly, it is a cheaper material (the technological process is less demanding). However, due to its uneven surface, it is most commonly used in constructions where the aesthetics and geometry of the material are of little importance (e.g. formwork boards for foundations, boards for ceiling formwork).
It should be noted that unplaned timber is not classified as structural timber. This means that the raw material has not undergone sufficient thermal processing and does not meet the moisture content range of 15–18%. Consequently, such timber is prone to drying out, twisting, geometric deformation, and cracking.
Planed timber, as the name suggests, undergoes an additional treatment process in which its surface is smoothed. In the vast majority of cases, if timber is planed, it is also properly dried and classified as a structural material. The planing process not only improves the appearance of the timber but can also affect its dimensions and usability. It is worth noting that planing allows for precise dimensioning of timber, which is important in projects requiring high accuracy. Additionally, the smooth surface limits moisture absorption and makes the timber more difficult to ignite, as fire tends to slide along the surface without catching easily. As a result, planing significantly increases the lifespan and durability of the wood.
Types of Structural Timber – C24, KVH, BSH, LVL
Another term you may encounter when choosing structural timber is its industry abbreviation, which usually relates to the timber’s cross-section. This may refer to Scandinavian, Canadian, or German (KVH) profiles, or laminated timber with a much more varied cross-section. So, what distinguishes these materials?
- Canadian Profile – this is a timber frame construction system characterized by the use of wall, floor, and roof structures built with timber posts measuring 38 mm (1.5 inches) thick. This technology uses the smallest possible cross-section of structural timber, which is why it is referred to as a lightweight frame system.
- Scandinavian Profile – structural timber with a post thickness of 45 mm and standard post widths increasing in 25 mm increments. These are the following dimensions: 45×45 / 45×70 / 45×95 / 45×120 / 45×145 / 45×170 / 45×195 / 45×220 / 45×245 mm. Scandinavian profile timber typically comes in the standard strength class C24 and is most commonly used in Scandinavian countries such as Norway, Sweden, Denmark, and Finland.
- German Profile (KVH) – this abbreviation comes from the German term Konstruktionsvollholz. As the name suggests, KVH timber is most commonly used in Germany. Due to its robust nature, it is colloquially referred to as “heavy German frame.” KVH timber is also a structural material, planed on all four sides, with a moisture content of 15–18%, and has a bending strength class of C24 (meaning it withstands a bending force of 24 MPa per 1 mm²). However, it is characterized by the largest available cross-section on the market, with a post thickness of 60 mm and widths increasing in 20 mm increments. Additionally, KVH timber is finger-jointed along its length, which allows it to be sold in lengths up to 13 linear meters (while standard solid timber with Canadian or Scandinavian cross-sections is available in lengths up to 6 meters). KVH timber is available in the following cross-sections: 60×100 / 60×120 / 60×140 / 60×160 / 60×180 / 60×200 / 60×220 / 60×240 / 60×260 mm.
In addition to the three solid timber profiles described above, the market also offers laminated materials – BSH / CLT / LVL – which are characterized by much larger cross-sections, as they are laminated in layers. As a result, it is possible to produce beams with very large dimensions. Thanks to the lamination process, these products exhibit better geometric stability, improved visual properties (e.g. BSH-SI, CLT), and are less prone to delamination and cracking over time. Consequently, these materials are often used as exposed structural elements, left uncovered to emphasize their decorative qualities. Massive constructions made from such timber are typically used for high-load-bearing structural components with wide spans – such as girders, ridge beams, floor beams, posts, and the like.
Applications of Structural Timber
Structural timber is a fundamental material used in the construction of timber frame houses, which are gaining popularity due to their quick assembly, energy efficiency, and adaptability to the individual needs of the user. In this type of construction, timber elements form the structural framework of the house, which supports the entire building and whose cavities are filled with insulating materials. Structural timber is used for the production of posts, beams, rafters, and other load-bearing elements.
Roof trusses are another example of structural timber applications. In this case, timber forms the roof framework, ensuring its stability and resistance to loads such as the weight of the roofing, snow, or wind. Due to its resistance to bending and compression, structural timber is an ideal material for rafters, battens, and other structural roof components. It is worth noting that regardless of the technology used to build the walls of a structure, the final topping-out element — the roof — is, in the vast majority of cases, made from structural timber.
Structural timber is also used in the production of roof trusses, which are prefabricated structural elements enabling the quick and efficient construction of roofs with complex geometries. These are typically produced in factories and delivered to the construction site ready for assembly. The use of structural timber in the production of trusses guarantees a lightweight structure while maintaining high strength and durability.
Structural timber also plays an important role in landscape architecture, where it is used for building pergolas, gazebos, terraces, pathways, fences, and other small architectural elements. Its natural aesthetics, ease of processing, and resistance to weather conditions (especially when properly treated with protective agents) make it a popular choice for creating cozy and functional outdoor spaces.
Prices of Structural Timber
The prices of structural timber can vary significantly depending on several factors, such as the wood species, its quality, dimensions, as well as regional availability and transportation costs. Generally, structural timber prices are determined by its durability, resistance to external factors, and aesthetic qualities.
The cheapest types of structural timber are usually those most readily available locally. In many regions, these are softwoods such as pine or spruce, which grow quickly and are easy to process. Pine and spruce timber is often chosen for standard constructions not exceeding four storeys and designed to bear loads within the normal wind and snow load requirements in Poland. These types are most commonly used in residential construction, particularly in the building of single-family homes.
Laminated timber, which is characterized by the highest bending resistance and the ability to carry large loads — and therefore most often used in massive structures — is at the same time more expensive than standard solid wood. Laminated timber is also most commonly made from pine or spruce, but thanks to the layered gluing process, it features better geometric stability and enhanced visual characteristics (e.g., BSH-SI, CLT), and is therefore less prone to cracking. As a result, it is a visible building material that is not covered by other cladding materials, which increases its price but also gives it a decorative function.
Hardwoods, such as oak, beech, or ash, are usually more expensive than softwoods due to their hardness, durability, and longer growth time, which leads to higher production costs. Moreover, exotic wood, which must be imported, is typically significantly more expensive than local wood species — not only because of transportation costs but also due to its limited availability and highly desirable aesthetic or strength properties. Exotic timber is generally used in smaller quantities and applied in decorative building elements and interior finishes, such as stairs, benches, terraces, and facades.
