“Corrugated galvanized iron” is now nothing to do with iron, it is made from mild steel. The sheet metal is crimped into a corrugated shape to make it stronger. Originally the grooves were made by pressing the metal, now the effect is achieved by grooved rolls. Although mild steel is cheap it corrodes fairly quickly, and the metal is galvanized to delay this process.
The mild steel is dipped in molten zinc. The advantages of corrugated metal are that it is strong, inexpensive and light and these properties make it cheap to transport.
Iron and steel are not naturally durable. They tend to corrode in the presence of water and oxygen and revert to reddish iron oxide or rust.
Rust once present is porous and it allows further corrosion to attack the corrugated metal roofing. When zinc corrodes it provides a stable non porous crust. This does help to stop the rust from spreading. If a galvanized sheet of corrugated metal roofing is scratched the steel and zinc are exposed when it rains electrical current flows through the water and both metals. This process later causes the zinc to corrode rather than the steel.
When corrugated metal roofing is placed on very steep pitches of roof it lasts a lot longer. As the dirt and water both run off, the flatter verandah roofs should be kept clear of any twigs or debris and the roof will last longer.
Another aspect in maintaining a corrugated iron roof is that it should be checked often to see that it is still battened down, when the screws or nails become loose, the action of the wind can drive the sheets up further from the structure underneath because of the large surface area of the corrugated metal sheets.
Smaller sheets of metal can be bought and they are effective for repairs, it is placed beneath a rusted joint and is secured to the adjacent piece. If larger areas need to be replaced the sheets should be matched in appearance to the older metal. This makes it a green option as second hand sheets can be ideal. In cases where a corrugated roof has failed it can be insulated with new metal sheet which add insulation to the existing roof.
Also new panels are available for insulation which does reduce the noise of living under a metal roof. These retrofit systems are available in standard widths of 10″ and their standard length is 100″. The insulation can be applied to the roof in several ways, it can be laid directly into the grooves, by being loose laid or it can be set into adhesive.
Ironically “rusty Roof” has become trendy it is new steel which has been designed and treated to look a couple of hundred years old. The old galvanized sheet metal rusted in streaks as the corrosion attacked the zinc and steel in different ways. The oxidized dark red look is achieved faster now as a result of new technologies. As a roofing material it has been criticized as not being very durable and it is now designed with a heavier gage. However it is regarded as a green product as it has been made with 30 percent recycled materials. Ironically what was a design fault has become desirable.
If a corrugated roof is being replaced with another covering it is worthwhile reinforcing your roof, as the old corrugated metal roofing material was very light. There is a chance that the roof will sag in a few years without this reinforcement. Once it has bowed like this it can be shored up and kept in the same sagging position, it is almost impossible to repair it fully. Also poor ventilation may cause condensation which can lead to deterioration of roof framing and covering materials. In hot sub-tropical areas there can also be a severe heat build up inside the house when corrugated roofs are applied.