IS 277 Galvanised Sheet — What It Means for Buyers

IS 277:2003 is the Bureau of Indian Standards specification that governs the zinc coating weight, thickness tolerances, and surface finish of galvanised plain and corrugated steel sheets sold in India. Every GI sheet sold legally in India must comply with IS 277, and BIS-marked sheets carry a CM/L licence number that buyers can verify. Understanding the four zinc coating classes — from Class 1 (lightest) to Class 4 (heaviest) — directly determines how long your roof will last before rusting.

What IS 277 Actually Covers

IS 277 was first published in 1959 and has been revised several times, with the 2003 edition being the currently operative version. The standard covers galvanised steel sheets produced by the hot-dip process — where steel coils or sheets are passed through a molten zinc bath to form a protective coating. It specifies requirements for:

• Zinc coating weight (coating class) on both surfaces combined
• Thickness of the steel base and permissible tolerances
• Mechanical properties — tensile strength and elongation
• Surface finish and freedom from defects
• Dimensions and tolerances for sheet length and width
• Corrugation profile dimensions for GC sheets

The standard does not cover colour-coated sheets (those fall under IS 15965) or galvalume / Zincalume sheets (IS 15961). It specifically applies to hot-dip galvanised plain and corrugated sheets used in roofing, cladding, purlins, and general construction.

Zinc Coating Classes — Class 1 to Class 4

The most important part of IS 277 for buyers is the zinc coating class. Zinc is what stops steel from rusting. More zinc means longer corrosion resistance. IS 277 defines four classes based on total zinc coating weight in grams per square metre (g/m²), measured on both surfaces combined.

For roofing in most inland Indian cities, Class 2 (275 g/m²) is the standard. Tata Shaktee GC sheets, JSW Colouron+, and Jindal Panther all supply Class 2 as their default grade. If your site is within 10–15 km of a coastline or in a high-humidity industrial area, specify Class 3 minimum. Class 4 is used in offshore structures and severe marine environments.

What GSM Means on a GI Sheet

GSM in the context of GI sheets stands for grams per square metre — it refers to the zinc coating weight, not the weight of the steel sheet itself. When a supplier says "120 GSM sheet," they mean the zinc coating on both surfaces together weighs 120 g/m². This is the same as IS 277 Class 1.

A common point of confusion is that GSM on roofing sheets has nothing to do with the GSM used in paper or fabric weight. In roofing, always ask: "Is this GSM the zinc coating weight or the total sheet weight?" A 0.47mm thick GC sheet weighs approximately 4.2 kg for an 8ft length — that is total sheet weight, not GSM.

When comparing quotes from different suppliers, always ask for the IS 277 class or the specific g/m² zinc weight. "120 GSM" and "275 GSM" are not the same product, and a supplier offering a significantly lower price is almost always supplying a lower zinc class.

How to Read a GI Sheet Specification

A properly specified GI sheet entry on a purchase order or quotation should read as follows:

Each element of this specification matters:

• GC sheet — corrugated profile (vs GP = plain/flat)
• IS 277 Class 2 — minimum 275 g/m² zinc coating on both surfaces
• 0.47mm nominal thickness — thickness of the steel base (not including zinc)
• 12ft length — standard commercial length; 8ft, 10ft, 14ft, 16ft also available
• 1000mm width — overall width; effective cover width after side lap is 838mm
• BIS mark — the ISI mark and CM/L licence number confirm BIS certification

Thickness Tolerances Under IS 277

IS 277 specifies permissible thickness tolerances. For a nominal 0.47mm sheet, the actual measured thickness may vary by ±0.05mm on the lower side. This means a sheet labelled 0.47mm can legally be 0.42mm. Some unbranded sheets sold in local markets exploit this tolerance — they are technically within specification but at the minimum end.

BIS Certification — What to Look For

Under the BIS (Certification) Regulations 2018, GI sheets for construction use must carry the ISI mark. This is a mandatory certification, not optional. The ISI mark on a GI sheet confirms that the manufacturer holds a valid CM/L licence number granted by BIS after factory inspection and product testing.

To verify a CM/L licence number, visit the BIS Care portal at bis.gov.in and search for the licence number. Major brands like Tata Steel (Tata Shaktee), JSW Steel, Jindal Steel, and APL Apollo all hold valid licences. If a supplier cannot provide a CM/L number, the sheets may be non-IS and should be refused.

On physical sheets, the ISI mark is typically stamped or embossed at one end of the sheet, often repeating every metre along the length. Colour of the mark varies — it may be ink-stamped in blue or black, or pressed into the galvanised surface.

What to Ask Your Supplier

Before placing an order for GI sheets, ask these specific questions:

• "Is this IS 277 certified? What class — Class 1, 2, 3, or 4?"
• "What is the CM/L licence number for the manufacturer?"
• "What is the nominal thickness — and can you show the mill test certificate?"
• "Is this hot-dip galvanised or electro-galvanised?" (Electro-galvanised has much thinner coating and is not IS 277 compliant for roofing)
• "For coastal sites: Is the zinc coating at least 450 g/m² (Class 3)?"

Reputable suppliers and distributors of brands like Tata Shaktee, JSW Colouron+, or Jindal Panther will answer all of these questions without hesitation. If a supplier is evasive about the IS class or CM/L number, treat that as a red flag.