Screws, as important fasteners for PV mounting systems, play a key role in different application scenarios. Their materials, specifications and usage need to be selected according to the specific installation environment and needs.

This article focuses on analyzing the selection of screws for metal roofs and wood roofs.

I. Metal roofing: precision matching of self-tapping screws

Metal roofs are usually made of galvanized steel or aluminum alloy panels with smooth and hard surfaces, making self-tapping screws a central choice. These screws need to meet the following characteristics:

Anti-corrosion treatment: zinc-plated or stainless steel is preferred to avoid galvanic corrosion caused by contact between the metal and the screw.

Sealing gasket: The head of the screw should be equipped with an EPDM rubber gasket to prevent rainwater from penetrating through the screw hole.

Length Matching: The length of the screws should be at least 25mm through the metal plate and embedded in the structure below, e.g. 0.5mm thick steel plate needs to be matched with 30-35mm screws.

II. Wooden roofs: mechanical adaptation of anticorrosive screws

Wooden roofs are based on timber and the screws need to balance the need for strength of fixing with the need for protection against moisture and insects:

Coating protection: Hot-dipped galvanized screws or copper screws are recommended to prevent metal corrosion from acids in the wood.

Thread design: Deep threaded screws enhance the bite of the timber and are suitable for load-bearing beam connections.

Necessity of pre-drilled holes: To prevent the wood from cracking, pre-drilled pilot holes with a diameter slightly smaller than the screws are required before installation.

III. Scenario suggestions:

Coastal/high humidity areas: 316 stainless steel screws for metal roofs, copper screws for mullion roofs.

Large-span structures: Increase screw density (20-25 screws per square meter) for metal roofs, use reinforced threaded screws for mullion roofs.

IV. Maintenance points: screws “health checks”

Annual inspection:

Metal roofs: Check for deterioration of gaskets and looseness of screws (retighten with a torque wrench).

Mullion roofs: Look for cracks or mold in the wood around the screws.

Cleaning Strategy:

Metal screws can be wiped with a soft cloth soaked in a neutral cleaner; timber beam screws need to avoid the impact of a high-pressure water gun .

Replacement Criteria:

Screws need to be replaced immediately when the head is more than 30% corroded or the threads are worn resulting in a loss of biting force.