The mild steel substrate is continuously hot dipped in a formulation of Aluminium (55%), Zinc (43.5%) and Silicon (1.5%). The process of applying an Aluminium-Zinc alloy in a hot dip delivers superior oxidation-resistance therefore extending the service lifespan of a steel roof by up to four times that of galvanised steel.
Offering some defence against corrosion-causing elements, galvanised metal is a sheet of steel which has been coated with a thin layer of Zinc oxide (99.8% Zinc). Galvanising offers almost twice the service life of the steel substrate.
There is a visible difference between the two coatings as an Aluminium-Zinc alloy metal appears to almost have a matte finish while galvanised steel has a unique shiny spangle appearance.
Galvanised metal corrodes linearly, and eventually, the Zinc coating is completely depleted, whereas an Aluminium-Zinc alloy experiences some corrosion, but on a smaller and less significant scale. An Aluminium-Zinc alloy metal is less prone to rusting than normal galvanised steel, and less likely to completely break down, even after a few decades.
Solar reflective index compared
The solar reflective index (SRI) is a measure of the constructed surface’s ability to reflect solar heat, as shown by a small temperature rise. It is defined so that a standard black surface (reflectance 0.05, emittance 0.90) is 0 and a standard white surface (reflectance 0.80, emittance 0.90) is 100. The solar reflective index incorporates both solar reflectance and thermal emissivity in a single value.
A roofing surface with high solar reflectance and high thermal emittance will reflect solar heat and release absorbed heat readily. High thermal emittance material radiates thermal heat back into the atmosphere more readily than one with a low thermal emittance. The table below shows that the SRI value of a roof with an Al-Zn coating is almost double that of one with a galvanised coating.