Why Metal Sheets Cut with an Angle Grinder Can Rust Faster Than Those Cut with Electric Shears

Why Metal Sheets Cut with an Angle Grinder Can Rust Faster Than Those Cut with Electric Shears

During the installation of a metal roof, it is common for the material to be cut and adjusted to fit the roof geometry.

The cutting method can directly influence the durability of the edges and the long-term performance of the material.

The most commonly used cutting methods are:

• electric metal shears;
• manual shears or a guillotine;
• an angle grinder.

Although all these methods produce a seemingly similar cut, their impact on the protective layers of the metal sheet is significantly different.

What Happens When Metal Roofing Sheets Are Cut with an Angle Grinder

An angle grinder cuts metal through friction and high temperature.

During the cutting process, the following occur:

• extremely high temperatures at the cutting edge;
• incandescent metal sparks;
• molten metal particles.

These elevated temperatures can affect the protective layers of the metal sheet in several ways.

1. Thermal Damage to the Protective Layers

Roofing sheets consist of several layers:

• paint coating;
• primer;
• passivation layer;
• zinc coating;
• steel core.

The heat generated by an angle grinder can damage these layers, especially in the area adjacent to the cut.

Zinc has a relatively low melting point (approximately 419°C), and near the cutting edge it can be affected or consumed much faster.

2. Deposition of Incandescent Particles on the Surface

During cutting, the sparks produced by the angle grinder are actually extremely hot metal particles.

These particles may land on the sheet surface and:

• locally perforate the paint layer;
• penetrate the protective coatings;
• reach the steel core.

Once embedded, these particles begin to oxidize.

This oxidation accelerates localized corrosion and may cause seemingly spontaneous rust spots to appear on the roof surface.

For this reason, such stains are often mistaken for a material defect, although they are actually the result of the cutting process.

Why Mechanical Cutting Is Preferred

With electric shears or a guillotine, cutting is performed mechanically, without generating high temperatures.

The advantages of this method include:

• the paint coating is not burned;
• the zinc layer is not thermally affected;
• the cut edge remains protected by redistributed zinc.

Because zinc is a ductile metal, it can partially cover the cut edge, helping to maintain corrosion protection.

Warranty Implications

For these reasons, most roofing sheet manufacturers explicitly recommend avoiding the use of angle grinders.

Using an angle grinder to cut roofing sheets may result in the loss or invalidation of the product warranty, since thermal damage to the protective layers can no longer be controlled.

Therefore, the recommended cutting methods are:

• electric metal shears;
• manual shears;
• guillotine shears.

Following these recommendations helps preserve the integrity of the protective layers and ensures the long-term durability of the roof.

Conclusion

Although angle grinders are widely used on construction sites, using them to cut roofing sheets can seriously damage the material's protective layers.

The high temperatures and incandescent particles generated during cutting can damage both the zinc coating and the paint layer, accelerating the onset of corrosion.

For this reason, mechanical cutting remains the recommended method for the proper installation of metal roofing systems.