Wood screws for photovoltaic mounting systems

Wood screws are an essential connecting element in the assembly of photovoltaic systems and provide the critical connection between roof hooks and the supporting roof structure. Due to their safety-relevant function, they must meet the highest quality requirements and have special building authority approvals.

The wood screws used for photovoltaic mounting systems are typically made of high-quality, hardened steel and have an anti-corrosion coating, usually in the form of galvanization or comparable high-quality surface treatments. The corrosion protection must correspond to at least class C3 according to DIN EN ISO 12944 in order to guarantee durability over the entire service life of the photovoltaic system.

The dimensions of the screws depend on the expected loads and the nature of the wood. Diameters of 6 to 8 millimeters are used as standard, whereby the length must be selected so that a screw-in depth of at least 80 millimeters is achieved in the load-bearing wood. The total length of the screw is therefore the result of the screw-in depth plus the thickness of any intermediate layers such as thermal insulation or formwork.

In terms of construction, these special screws are characterized by several characteristic features:

• An asymmetrical thread with optimized screwing behavior
• A lower head with milling ribs for better guidance
• A special tip geometry to reduce the splitting effect
• A reinforced screw head for safe force transmission when screwing in
• An optimized thread-core ratio for maximum pull-out strength

The installation of the screws requires particular care. Depending on the type of wood and diameter, pre-drilling is essential to avoid cracks in the wood. The drill diameter must be around 70% of the screw diameter. The screw-in depth must be precisely maintained, with modern screws often having screw-in markings.

The permissible loads on the screw connections are determined through extensive tests and documented in the approvals. Both static and dynamic loads are taken into account. The alignment of the screw to the grain of the wood plays a decisive role in the load-bearing capacity. Transverse loads must be absorbed by appropriate design measures such as double screwing or reinforcement plates.

Quality assurance begins during production with strict controls of mechanical properties such as tensile strength, yield strength and elongation at break. Each batch is tested and documented accordingly. When installed, the screws must be checked regularly for tightness and signs of corrosion as part of the maintenance of the photovoltaic system.

Special requirements apply to use in conjunction with chemically treated wood or in regions with an increased risk of corrosion, for example near the coast. Specially coated screws or screws made of high-alloy steel are used here, which offer extended corrosion protection.

An aspect that is often underestimated is the correct storage of the screws before installation. They must be stored dry and protected from the effects of the weather in order not to impair the functionality of the anti-corrosive coating. Damaged or already corroded screws must not be used.