In wood construction, it’s often the smallest details that make the biggest difference: A washer, barely larger than a coin, distributes the force of a screw or bolt over an area many times the cross-sectional area of the fastener, protects the timber structure from localized indentations, and ensures the durability of a joint for decades. Washers in timber construction are far more than just metal inserts: They are load-bearing elements that distribute forces, protective components against corrosion and moisture, and standard-compliant structural components whose selection and sizing directly influence the load-bearing capacity and serviceability of a timber structure.
- What washers do in timber construction and why they are indispensable
- What types of washers exist and how they differ
- Which standards and design rules apply to washers in timber construction
- Which materials and surface protection measures are suitable for various applications
- How to correctly size and install washers
- What typical mistakes occur during selection and installation, and how to avoid them
- How washers interact with other fasteners in timber construction
- What role washers play in wood-concrete composite structures, steel-wood connections, and structural timber construction
What washers do in timber construction: basic principle and function
Washers in timber construction fulfill a function that stems from the specific mechanical properties of wood. Wood is an anisotropic, fibrous natural material with significantly varying strength values depending on the direction of loading. Across the grain—that is, perpendicular to the wood’s longitudinal axis—the compressive strength of wood is significantly lower than parallel to the grain. This is precisely where the washer comes into play: It increases the contact area of a fastener—whether a screw, bolt, or threaded rod—thereby reducing the local surface pressure on the wood to below the critical threshold.
Without a washer, the entire preload or compressive force of a bolt would be concentrated on the tiny surface area of the screw or nut head. In softwood species such as spruce or pine, as well as in glued laminated timber and cross-laminated timber, this leads to the head indenting plastically into the wood surface, resulting in a loss of preload and, over time, loosening of the connection. The washer distributes this force over a defined, standard-specified minimum area and keeps the surface pressure within the permissible range. This mechanism is at the core of every washer’s function in timber construction, regardless of construction type, fastener type, or wood species.
In addition to force distribution, washers perform other functions in timber construction. They protect the wood surface from direct contact with metal and the associated galvanic corrosion processes; they prevent sharp screw heads from cutting into the wood structure; and, in the case of steel bolt connections, they allow the nut to be tightened without damaging the wood. In exterior applications and in environments with high wood moisture content, the corrosion protection provided by the washer itself is particularly important, as corrosion products can discolor the wood, acidify it, and cause long-term damage.
Types of Washers in Timber Construction: Standard Parts and Special Shapes
The variety of washers used in timber construction can be categorized by geometry, material, and intended use. The most common basic shape is the round washer with a centered hole, as standardized in DIN 125 (Flat Washers). This standard distinguishes between Series A, with a smaller outer diameter, and Series B, with a larger outer diameter. In timber construction, Series B or even larger custom sizes are used almost exclusively because the larger contact surface compensates for the low shear strength of wood.
For timber connections with bolts according to DIN EN 1995-1-1 (Eurocode 5, the applicable European design standard for timber structures), specially dimensioned washers are required. Eurocode 5 requires washers for bolted connections in timber construction to have a minimum outer diameter that depends on the bolt diameter. As a guideline, the outer diameter of the washer should be at least three times the bolt diameter, and the washer thickness should be at least one-third of the bolt diameter. These specifications are not recommendations but design requirements: Only if these minimum dimensions are met may the load-bearing capacity values specified in the Eurocode be applied to the connection.
In addition to round washers, square and rectangular washers are used in timber construction, particularly for connections between timber members and steel or concrete structures. Their rectangular shape allows for more targeted force transmission in specific grain directions and is advantageous in tight installation situations. Special shapes such as wedge washers (also called inclined washers or compensating washers) are used when the bearing surface is not perpendicular to the bolt axis, such as with sloped rafters or angled connections. The wedge washer compensates for the angle and ensures that the nut rests flat against the surface and that the preload force is transmitted evenly.
Large-hole washers and special formats for timber engineering
In timber engineering—that is, in structural systems for industrial buildings, wooden bridges, towers, and other special structures—large-hole washers are frequently used. These washers have a significantly larger outer diameter than standard parts conforming to DIN 125 and are designed to withstand the high forces that occur in such structures. They are often custom-made or fabricated from semi-finished steel plates and must be explicitly verified in the structural analysis.
Disc springs and spring washers, which are used as variations of washers in other engineering fields, play a minor role in timber construction. The reason lies in the creep behavior of wood: Wood settles under sustained loads, particularly when wood moisture content fluctuates, and thereby loses preload in connections. Spring elements could theoretically compensate for this loss of preload, but they are not regulated by standards in timber construction and are rarely encountered in practice. Instead, experts recommend regularly retightening bolted connections in the first few years after construction, especially in structures made of green wood.
Materials and Corrosion Protection: Which Washer for Which Application
The choice of material for washers in timber construction depends on the service class of the structural member, as defined by Eurocode 5. Service Class 1 covers indoor areas with normal ambient humidity; Service Class 2 covers covered outdoor areas or indoor areas with occasionally elevated humidity; and Service Class 3 applies to components directly exposed to the weather. The higher the service class, the more stringent the requirements for corrosion protection of metal fasteners, including washers.
For Service Class 1, washers made of non-galvanized steel in accordance with DIN EN 10083 or similar standards are sufficient, provided there are no special requirements. In practice, however, hot-dip galvanized washers are often used even in interior applications because they offer better protection against construction moisture and during the construction phase. Hot-dip galvanizing in accordance with DIN EN ISO 1461 produces a zinc coating at least 45 micrometers thick on the surface and provides lasting protection in service classes 2 and 3. For more aggressive environments—such as in swimming pools, near seawater, or when in contact with certain wood preservatives and pressure-treated wood—stainless steel discs in accordance with DIN EN ISO 3506 are required.
Pressure-treated wood treated with copper-based preservatives poses a particular challenge. The copper contained in the wood can cause galvanic corrosion on steel components that come into contact with the wood. For such applications, the approvals issued by wood preservative manufacturers and the technical rules of the German Institute for Building Technology (DIBt) generally specify stainless steel of material group A4 (austenitic steel with an elevated molybdenum content). The use of galvanized washers with pressure-treated wood in outdoor applications is a common mistake that can lead to premature corrosion and failure of the connection.
Normative Basis: Eurocode 5, DIN Standards, and General Building Authority Approvals
The design of wood connections using washers is regulated in Germany by Eurocode 5 (DIN EN 1995-1-1), which is supplemented by the corresponding National Annex (DIN EN 1995-1-1/NA). Eurocode 5 contains explicit requirements for washers used in bolted connections, dowel connections, and other pin-type fasteners. It specifies minimum dimensions and defines the conditions under which the tabulated load-bearing capacity values apply. If these minimum dimensions are not met, the standard values may not be applied, and the designer must either provide experimental evidence or make more conservative assumptions.
For standardized washers, DIN 125 (flat washers) and DIN 126 (washers for screws with a wide seating flange) apply; for special applications, DIN 440 applies, which describes washers for wood connections with larger outer diameters. DIN 440 is particularly relevant in timber construction because it defines washers with outer diameters that meet the requirements of the Eurocode for bolted connections. Professionals in timber construction should be familiar with this standard and explicitly reference it in tenders and during construction to avoid confusion with smaller standard washers.
In addition to these standards, general building authority approvals (abZ) and European Technical Assessments (ETA) play an important role when special fasteners or system solutions are used. Many manufacturers of timber construction fasteners offer system solutions in which washers are approved as an integral part of the fastening system. In these cases, only the washers specified by the manufacturer may be used, as the approval is issued for the entire system. Substituting standard washers—even if they appear geometrically similar—can invalidate the approval, and the contractor bears legal liability.
Installation and Assembly: Common Mistakes and How to Avoid Them
Proper installation of washers in timber construction begins with the correct selection. A common mistake is the use of washers conforming to DIN 125 Series A, which are designed for metal fasteners and do not achieve the required surface pressure in timber construction due to their insufficient outer diameter. Anyone creating bolted connections in timber construction must consistently use washers conforming to DIN 440 or the special sizes specified by the structural engineer. This mistake is frequently encountered on construction sites because standard washers conforming to DIN 125 are available at any hardware store, whereas washers specifically designed for timber construction must be procured separately.
Another critical point is the position of the washer relative to the wood grain. On end-grain surfaces—that is, on cut surfaces perpendicular to the grain direction—the compressive strength of the wood is higher than on side surfaces, but the tendency to split and crack is also increased. Washers used on end grain surfaces should be dimensioned with particular care, and the bolt spacing from the edge of the end grain must be maintained. For side surfaces parallel to the grain, the transverse compressive strength is the determining factor, and the washer must ensure that the permissible surface pressure is not exceeded.
The tightening torque when tightening nuts is another aspect that is often neglected in practice. Excessive tightening torque causes the washer to be pressed into the wood surface, resulting in a loss of preload as the wood continues to creep beneath the washer. Too low a tightening torque leaves the joint loose from the start. For bolted connections in timber construction, experts recommend tightening with a torque wrench according to the manufacturer’s specifications and retightening after the wood’s initial drying phase, typically after six to twelve months. This retightening is particularly essential for structures made of green wood or for components that continue to dry out after installation.
Washers in Wood-Frame and Panel Construction
In wood-frame and panel construction, washers are used less frequently than in solid wood construction because the joints here are predominantly made with nails, staples, and screws without preload. However, where bolts or threaded rods are used for bracing or anchoring wall panels, the same requirements apply as in solid wood construction. Special attention should be given to the anchorage points of wood-frame walls at the foundation or ceiling slab, where tensile forces from wind loads are transferred into the wood frame via threaded rods with washers.
Washers in Special Connection Situations: Steel-Wood, Wood-Concrete, and Cross-Laminated Timber
Steel-to-wood connections place special demands on washers because they involve two materials with very different stiffnesses and coefficients of thermal expansion. When steel plates are used as inner or outer flanges in bolted connections, the washer is positioned on the wood side of the connection and must transfer the forces from the bolt into the wood. The washer must be dimensioned such that neither the wood beneath the washer nor the steel plate fails due to hole wall failure. Design is performed in accordance with Eurocode 5 in combination with Eurocode 3 for the steel components.
In wood-concrete composite floors—a common construction method in both existing and new buildings—shear connectors are installed in the wood to ensure the composite action between the wooden beams and the concrete slab. Washers play a role here in anchoring screw connectors and securing formwork elements. Since concrete is alkaline and wood in the composite area may be exposed to permanently elevated moisture levels, corrosion-resistant materials are absolutely essential for all metal parts, including washers.
Cross-laminated timber (CLT) has gained significant importance in recent decades as a material for multi-story timber buildings. The cross-lamination of the layers results in a panel-shaped component with improved transverse compressive strength compared to solid wood, but the requirements for washers remain the same. For point loads from columns or bolted connections in CLT panels, the surface pressure under the washer must be verified based on the actual transverse compressive strength of the CLT product, as specified by the manufacturer in the relevant ETA. General values from solid wood tables must not be applied uncritically in this context.
Washers in Wood Construction: A Small Component with a Big Responsibility
Washers in timber construction are a prime example of how the quality of a structure does not depend solely on the large, visible components. An incorrectly selected or missing washer can permanently weaken a structurally correctly dimensioned connection because the wood compresses under the support that is too small, the preload is lost, and the connection loses its stiffness. This process is slow, often invisible, and is frequently not noticed until deformations or cracks occur.
The regulatory framework established by Eurocode 5 and the associated product standards provides a reliable basis for selection and design. Those who understand these rules and apply them consistently can effectively manage the key risks. The challenge lies in practice: on the construction site, where washers are grabbed from the nearest available stock under time pressure, and in the bid specifications, where unspecific requirements such as “washer according to standard” without specifying the standard or the size invite errors. Precise planning, clear tender specifications, and careful supervision of execution are the three pillars upon which the quality of timber connections using washers rests.
Anyone who views washers in timber construction as mere accessories fundamentally underestimates their function. They are structurally relevant components governed by standards, whose material, geometry, and installation position must be planned just as carefully as the fasteners themselves. Knowing which washer is suitable for which bolt diameter, wood species, and service class is a core competency in timber construction and pays off in the form of durable, low-maintenance structures.











