Red Rust on Metal Roofs

Red rust is the familiar reddish-brown iron oxide that forms when steel corrodes. On metal roofs it signals that the steel base metal itself is now exposed to moisture and oxygen. Most steel roofing is protected by a metallic coating, usually zinc (galvanised) or an aluminium-zinc alloy, often with paint on top. That coating corrodes slowly and sacrificially to protect the steel; red rust appears where the coating has worn away, been scratched through, or been consumed.

Red rust is not a concern on aluminium, copper or zinc roofing, which do not contain iron, though those metals have their own corrosion behaviour.

Where it starts

• Cut edges and drilled holes, where the coating is thinnest or absent.
• Scratches from installation, foot traffic or debris.
• Areas that stay wet: panel laps, low spots, behind debris, under snow guards or in valleys.
• Fastener heads on exposed-fastener systems, especially with incompatible or uncoated screws.
• Swarf (metal filings) left on the roof after cutting, which rusts and stains the finish.
• Contact with incompatible materials such as copper runoff, treated timber or wet cementitious products.
• Aggressive environments: coastal salt spray, industrial pollution, or animal housing.

Why it matters

Light surface rust at a scratch is mainly cosmetic but spreads under paint over time. Advanced corrosion thins the steel, enlarges fastener holes and eventually perforates the panel, causing leaks and loss of strength.

Treatment and prevention

Early rust is generally cleaned back to sound metal, treated with a compatible primer and recoated. Rusted fasteners can be replaced with oversized or properly coated screws. Widespread rust-through usually means panel replacement or a full roof restoration system assessed by a specialist. Prevention includes sweeping off filings after cutting, using fasteners and flashings compatible with the panel coating, keeping roofs free of debris, and choosing coatings and metals suited to the local corrosion environment.