Where did the rust come from on the new sheet metal roof?
I have visited many roofs to inspect the reddish-brown rust marks that have appeared within a few weeks of installation. The customer believes that the roofing sheet has always been ‘almost completely rusted through’ and ‘needs to be replaced urgently by the supplier’. It is difficult for those with less experience of corrosion to understand that if the sheet metal starts to rust from the top, a substantial amount of white rust must first appear, as zinc corrodes before steel. It is only much later that the steel itself rusts, meaning red rust appears on the roof. So, what is the problem then?
In the past, when installation wasn’t carried out very professionally, the cause was usually a hot metal spray from an angle grinder – in other words, grinding dust. When the profile was cut on the roof or on the ground, a spray of grinding dust would fly onto the painted surface. Metal particles often burn their way into the paint, but sometimes even loose dust can become embedded in the surface. This phenomenon occurs whenever a painted product is within reach of the metal spray. At best – or at worst – the distance can be as far as a couple of tens of metres from the angle grinder cutting site. This adhesion problem is worse with thick coatings than with thin ones.
Steel that has become embedded in the painted surface will rust as soon as the first rains fall. A small speck of rust often leaves behind a much larger streak resembling the colours of the rainbow, which requires scrubbing with a brush to remove. This is particularly troublesome on matt-finish products (gloss level below 10). The stain comes off more easily from glossier surfaces (gloss level 30–50). Unsightly residue and rust do not damage the paint, but the dent left by a hot metal weld weakens the durability of the paint coating.
Nowadays, as installation has become more professional, rust marks on roofs are mainly caused by metal shavings produced when securing screws. Professionals avoid cutting metal on the roof and use better tools, but fastening screws on the roof can still produce loose metal shavings. The size and shape of these metal particles depend on the way the screw cuts into the metal. Generally, self-tapping screws produce flake-like drilling waste, whilst pointed-tip screws produce thread-like drilling waste. Drilling waste should be cleared away immediately after installation. When left loose on the roof, drilling debris causes only the aforementioned aesthetic problems to the painted surface; however, if it gets stuck under a shoe or on the sole, it can damage the paint finish.
The most dangerous factors for the roof’s lifespan – that is, its durability – are scratch-like coating damage, extending down to the metal surface, caused during screw fastening. These most commonly occur when a screw is fitted at a slightly angled position and the screw begins to ‘scrape’ across the painted surface. Another cause of damage is a poor fit between the screw and the fixing sleeve. In this case, the screw comes loose from the mounting socket at the start of the fastening process, and the mounting socket in turn scrapes the painted surface.
If left unrepaired, these scratch marks will cause red rust to form on the surface within 5–8 years. The reason for this rapid rusting is the anode/cathode surface area ratio phenomenon, whereby corrosion accelerates in the scratches. At worst, unrepaired scratches can lead to the roof rusting through after a couple of decades. When a metal roof is constructed using the best materials and the highest level of expertise, it can last for up to 100 years at its best.