The Base Metal Thickness (BMT) measures the steel substrate prior to any metallic coating or paint being added to the substrate. The Total Coated Thickness (TCT) measures the steel substrate as well as the metallic coating, resin coating and/ or paint film. If a Tata BlueScope Steel AZ150 coated product is labeled as 0.42mm BMT means that it will have TCT of 0.47mm. If the BMT is measured at 0.48mm means that the product will have a TCT of 0.53mm. For design calculations or structural design one should use BMT since the actual strength of the material is from its base steel. The metallic coating layer provides the corrosion resistance against natural weathering.
The Solar Reflectance Index (SRI) is a numerical expression of a constructed surface’s ability to reflect solar heat. SRI values are derived from a calculation in ASTM E1980 which uses a roof’s solar reflectance, thermal emittance and medium wind coefficient to generate a unit-less value from 0 to 100. A standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100. The higher SRI values indicate a roof whose surface temperature is lower, thereby reducing heat gain in the living space beneath the roof. Green building rating tools such as Leadership in Energy and Environment Design (LEED) and Malaysia’s Green Building Index (GBI) encourage using building materials with high SRI values to reduce the intensity and impact of the Urban Heat Island (UHI) effect. SRI Values required by LEED & GBI
Roof Slope SRI
Low-Slope _ 2/12 78 or greater
Steep-Slope > 2/12 29 or greater
A solar reflectance technology is incorporated into COLORBOND™ steel to lower surface temperature by absorbing less heat from the sun. Thermatech™ optimizes the thermal performance of every colour in the standard COLORBOND™ steel palette, without changing their appearance. Thermatech™ provides greater thermal comfort (Cool Roof) all-year round, whilst using less energy for air-conditioning and helps to mitigate the urban heat island effect.