Galvalume Sheet — Coating, Grades and Service Life
By Dhruv Agarwal · · 2 min read
What it is
Cold-rolled steel sheet coated with an aluminium-zinc alloy, nominally 55% aluminium, 43.5% zinc and 1.5% silicon. Supplied bare, or pre-painted as PPGL for a coloured finish.
It is the default substrate for industrial roofing and wall cladding across Indian PEB construction.
Specification at a glance
| Property | Typical specification |
|---|---|
| Standard | IS 15961 or ASTM A792 |
| Coating mass | AZ70 / AZ100 / AZ150 and above |
| Base steel yield | 345 MPa and 550 MPa grades common |
| Thickness | 0.45mm — 0.80mm TCT for roofing |
| Finish | Bare Galvalume, or pre-painted PPGL |
| Paint system | Polyester, SMP or PVDF by exposure |
TCT means total coated thickness — always confirm whether a quoted thickness is TCT or base metal thickness, because the difference is real money.
Why coating mass is the specification that matters
Two sheets can both be "Galvalume" and differ in service life by many years. The variable is coating mass:
| Coating | Indicative use |
|---|---|
| AZ70 | Sheltered, low-exposure, short-life structures |
| AZ100 | General inland industrial |
| AZ150 | Standard for durable industrial roofing |
| AZ200 and above | Coastal, chemically aggressive, long-design-life |
What drives the cost
Base metal thickness · coating mass · steel grade · bare versus pre-painted · paint resin class, with PVDF above SMP above polyester · profile and rib depth · sheet length and transport.
Where projects go wrong
- Coating mass unspecified. The most common and most expensive omission.
- Thickness quoted as TCT and read as base metal. A 0.50mm TCT sheet is thinner in steel than a 0.50mm base metal sheet.
- Cut edges and swarf left on the roof. Drilling swarf rusts in place and stains and pits the sheet. It must be swept at the end of every shift.
- Contact with wet concrete, copper or lead. Each drives accelerated corrosion; isolate the interfaces.
- Fastener spacing uniform across the roof. Edges, corners and ridges see higher suction under IS 875 Part 3 and need closer spacing.
Standards referenced
Aluminium-zinc coated sheet under IS 15961 and ASTM A792. Wind loading on roof and wall cladding to IS 875 (Part 3).
Standards referenced
- IS 15961 — Steel sheet, aluminium-zinc alloy metallic coated
- ASTM A792 — Steel sheet, 55% aluminium-zinc alloy-coated
- IS 875 (Part 3) — Wind loads — roof and wall cladding