Cut edge corrosion is one of the most common reasons an otherwise sound industrial roof starts to look tired. It begins at the exposed edge of a sheet, works its way back under the coating, and eventually lifts the paint film away from the steel.

Why edges are vulnerable

A coated steel sheet is protected across its faces, but wherever it has been cut the raw steel is exposed at the edge. On a roof, the most exposed cut edges are at the eaves and at end laps, and those are also the places that stay wet longest.

Water sitting in a lap, held there by capillary action, keeps the edge permanently damp. Add airborne salts or agricultural chemicals and the process accelerates considerably.

What it looks like

Early signs are fine rust staining along the edge of the sheet and slight blistering or lifting of the coating just back from the edge. Left alone, the coating peels progressively and the steel section is reduced. It is nearly always visible at the laps before anywhere else.

Reducing the risk when you specify

• Choose the coating for the exposure. We generally recommend PVC plastisol for its resistance and cost-effectiveness. Polyester is a lighter-duty finish better suited to domestic garages, workshops and garden rooms. Plain galvanised, available from 0.4mm to 0.7mm, carries no guarantee and is chosen where appearance and longevity matter less.
• Minimise site cutting. Every cut made on site is a new unprotected edge, usually made with a tool that heats the steel and damages the coating either side of the line. We cut every sheet to your exact length before it leaves us, which is the simplest way to avoid this.
• Get the laps right. Correct end laps, sealed where the pitch is shallow, keep water moving rather than standing. Our guide to overlapping roof sheets gives the dimensions.
• Think about the environment. Coastal and agricultural buildings are harsher than most, and the specification should reflect that.

Existing corrosion

Where corrosion is caught early and confined to the edges, specialist cut edge treatment systems can arrest it and extend the life of the roof, and that is normally far cheaper than re-sheeting. Where the coating has failed over large areas or the steel section is visibly reduced, replacement is the honest answer.

If you are re-sheeting, send us your sizes and we will cut to length so there is as little site cutting as possible.