What is the difference between BMT and TCT?

BMT (Base Metal Thickness) is the thickness of the steel core. TCT (Total Coated Thickness) includes the steel, metallic coating, and paint layers. Structural strength depends on BMT (Clause 5.1).

What does AZ150 mean?

It denotes a zinc-aluminium alloy coating with a minimum total mass of 150 grams per square meter on both surfaces, as specified in Table 3.

What is the common high-strength grade for roofing sheets?

Grade 550, which has a minimum yield strength of 550 MPa, is commonly used for high-strength roofing and cladding profiles (Table 2).

What is the T-bend test?

It's a test to check the flexibility and adhesion of the paint coating by bending a sample. The 'T' value indicates the number of sheet thicknesses used as the bending radius (Clause 6.2).

IS 15965

: 2012

Coated steel sheets for roofing and cladding

CurrentFrequently UsedSpecificationBIMMaterials Science · Roofing Materials

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This standard specifies the requirements for steel sheets intended for roofing, cladding, and related applications. It covers both metallic coated (like galvanized or zinc-aluminium alloy) and pre-painted/organic coated steel sheets, detailing material properties, dimensional tolerances, coating characteristics, and performance test methods.

Specifies requirements for pre-coated and pre-painted metallic coated steel sheets/coils for roofing and cladding.

Quick Reference Values

Minimum Yield Strength for High Strength Grade550 MPa

Standard Metallic Coating Mass (Zinc-Aluminium)150 g/m² (AZ150)

Standard Metallic Coating Mass (Zinc)275 g/m² (Z275)

Nominal Top Coat Paint Thickness (Exterior)20 microns

T-Bend Test requirement for flexibility≤ 6T

Practical Notes

Differentiate between Base Metal Thickness (BMT) and Total Coated Thickness (TCT). Structural calculations must use BMT.

Ensure the specified coating mass (e.g., AZ150) is verified, as lower coating mass drastically reduces corrosion resistance and service life.

Avoid using heat-generating abrasive cutters for cutting sheets on-site as it damages the protective coating, leading to premature rusting at the edges.