Galvanized Roof Rust

Left untreated white rust corrodes the zinc and the metal underneath begins to rust exhibiting the more traditional brownish red color.

Galvanized roof rust.

Galvanized metal is coated with a layer of zinc to protect the iron or steel underneath. The fiber acts a bit like a tire plug in the old nail holes and prevents leaks and it sticks well. The roofs i have done have lasted a minimum of 20 years now. Unfortunately over time this coating can fail leading to minor rust outbreaks.

If the item is exposed to continually wet conditions the zinc develops a film known as white rust. Rust corrodes metal and if left untreated it can cause. But galvanized metal which exhibits a shiny adherent but rust colored stain is not truly rusting. It is not simply a cosmetic problem however.

Galvanized metal roofs are coated with a layer of zinc that inhibits rust. If you are preparing to paint a rusted galvanized metal roof pay special attention to surface preparation or you will have a recurrence of rust and or flaking. Rather this is just a phenomenon that often occurs in galvanized materials due to intermetallic alloying of iron and zinc i understand and is not indicative of the end of the useful life of the galvanizing. I have used both regular paint and roof coat on old metal roofing with pretty good results.