Galvanized Steel Roofing

Galvanized steel is carbon steel sheet that has been coated with zinc. For roofing, the coating is almost always applied by continuous hot-dip galvanizing: the steel strip passes through a bath of molten zinc, and air knives wipe the surface to control how much zinc stays on. A small amount of aluminium in the bath improves adhesion and makes the coating easier to form.

Zinc protects steel in two ways. It acts as a barrier, and because zinc is more electrochemically active than iron it corrodes first, shielding small scratches and cut edges. That protection lasts only while zinc remains; once the coating is consumed, the steel beneath begins to rust.

History

Zinc galvanizing of iron was developed in France in 1837, and by the 1850s galvanized corrugated iron was being used on post offices, train sheds and factories. Early sheets were hand-dipped and carried heavy coatings. Continuous galvanizing lines, adopted from the 1930s, gave thinner, more uniform and more formable coatings.

Coating weights

In North America, ASTM A653 designates coatings by total zinc on both sides. G90 means a minimum average of 0.90 oz/ft2, equal to about 275 g/m2 and labelled Z275 in metric; G60 is about 183 g/m2. G90 adds roughly 42 micrometres (1.6 mils) of total coating thickness. For zinc coatings, corrosion life rises roughly in line with coating weight, so G90 lasts about half as long again as G60 in the same environment.

Use today

Plain galvanized sheet is still used for agricultural, industrial and low-cost roofs, often painted. For many roofing products it has been largely displaced by aluminium-zinc coatings such as Galvalume, which generally last longer in most atmospheres. Galvanized steel corrodes faster in marine or acidic environments, and regular painting extends its life.