explain briefly why new corrugated iron sheets used for roofing are very shiny but after sometime they appear dull
Answers
Many people choose metal roofing because it defends against corrosion and degradation for decades (typically 60+ years) which makes it the last roof they ever have to purchase. Manufacturers and contractors are able to effectively help you choose a metal material that will best stand up to corrosion in the environment your home or building is in.
Most metal roofing has protective paint systems or coatings specifically engineered to resist corrosion, but there are special circumstances that could occur, including:
Underside corrosion – If the metal is improperly coated on the underside of the panel, corrosion could take place. This occurs when water molecules get trapped underneath between the panel and the insulation or underlayment, which can cause the metal to weaken if not coated. Also, underside corrosion could occur if you use a granulated underlayment or install right over a shingle roof because it could scratch the protective backside coating and cause the metal roof system to fail from the inside out.
Saltwater corrosion – Homes or buildings close to the coast often don’t qualify for substrate or paint warranties because of saltwater’s effect on metal. There are types of metals that work better in coastal environments, including aluminum and zinc, but there is no guarantee it will last as long as a roof where there is no threat of saltwater. Make sure to read and analyze the warranty or product specs thoroughly to determine if you’re in a location that could be subject to premature corrosion.
Dissimilar metals – When certain metals are put together, there could be a negative interaction that causes early corrosion that would not occur otherwise. We will talk more about dissimilar metals in the next common problem.
Cut edges – Cut edges on a steel roof will experience edge creep, which is minimal rusting occurring at the cut edge. Many details in a standing seam metal roof have folds or hemmed edges that conceal the cut edge of the metal. A contractor can minimize the appearance of edge creep by using lap flashing, hemming exposed edges when possible, or using a paint pen on visible cut edges.
Corrosion will happen at some point in the lifetime of the roof. Whether it happens early on, or in 60 years, is dependent on being educated about the different metal types, paint systems, and the environments these work best in.