Steel Roofing
Two quotes that both say "steel roofing" can describe roofs that will not last the same number of decades, and the difference is almost never visible in a photograph. Steel is the metal most metal roofs are made from, and how long yours lasts is decided by three numbers that rarely appear on a proposal unless someone asks: the gauge of the sheet, the metallic coating applied at the mill, and the paint system over it. Ask for all three in writing. It takes one sentence and it is the most useful sentence in the whole negotiation.
The layer under the paint is doing the real work
Bare steel corrodes, so roofing steel always carries a metallic coating applied before it is ever painted. Traditional galvanising uses zinc, and it is specified by weight: G90 means about nine-tenths of an ounce of zinc per square foot of sheet, counting both sides. Zinc sacrifices itself slowly to protect the steel beneath, which makes it forgiving at scratches and cut edges — the zinc nearby corrodes preferentially and keeps the exposed steel intact. Aluminium-zinc coatings, sold under the Galvalume name and specified as AZ50 or AZ55, deliver considerably better flat-surface corrosion resistance and are the common choice today, with the honest caveat that they are less self-healing at raw edges than plain zinc. Neither one is universally right. Near salt air the coating question gets serious enough that aluminium sheet, which does not rely on a sacrificial layer at all, is often the better specification than any grade of steel.
Paint is a performance component, not a colour choice
Over the metallic coating goes the paint system, and the gap between the two common families is large. Silicone-modified polyester finishes cost less and chalk and fade sooner, particularly on south-facing slopes and at altitude where ultraviolet exposure is more intense. Fluoropolymer finishes — the resin usually named as PVDF — hold colour and gloss far longer and carry much longer finish warranties as a result. On a roof expected to last thirty or forty years, the paint is the layer that visibly gives up first, so the finish decides when the roof starts looking old regardless of how sound the steel underneath still is. Dark colours fade more visibly than light ones, which is worth knowing before choosing from a chip card. The Metal Construction Association publishes the technical material behind coating and finish specification for anyone who wants to read past the brochure.
Where corrosion actually begins
Almost never in the middle of a panel. Rust on a steel roof starts at a cut edge, a drilled hole, a deep scratch, or a point where something dissimilar was allowed to touch the sheet — copper, certain treated lumber, or steel filings left behind by cutting a panel with an abrasive blade, which is why panels are cut with shears or a nibbler instead. Trapped moisture is the other trigger: debris packed behind a chimney, a valley that never dries out, panels stacked wet on site before installation, or a roof assembly with no ventilation beneath it. All of that is controllable at installation and touch-up time and nearly uncontrollable afterwards, which is the practical reason field cutting and edge treatment belong in the scope of work rather than in the cleanup. Steel's other quiet virtue is worth stating: it is made with high recycled content and is fully recyclable at the end of its service, which is more than most roofing materials can claim.
