GI / Colour-Coated Steel Sheets — Complete Guide
GI (Galvanised Iron) and Colour-Coated steel sheets are thin profiled steel sheets coated with zinc, aluminium-zinc, or organic paint film. They are the most widely used roofing and side-cladding material for industrial sheds, warehouses, godowns, parking, factories, schools, animal sheds and commercial buildings across India.
These sheets are governed by IS 277 (Galvanized Steel Sheets), IS 14246 (Continuously Pre-painted Galvanised Sheets / PPGI), IS 15965 (Galvalume / PPGL) and IS 14164 (Trapezoidal & Tile-Profile Sheets).
What are GI / Colour-Coated Sheets?
The base material is a thin cold-rolled steel coil (0.3–1.2 mm). It is continuously hot-dip coated with:
- Zinc: Produces galvanised steel (GI) with silver-grey finish
- Zinc-Aluminium (55% Al, 43.4% Zn, 1.6% Si): Produces Galvalume / Zincalume
- Aluminium-Zinc + Polymer Paint: Produces Pre-Painted Galvalume (PPGL)
- Zinc + Polymer Paint: Produces Pre-Painted Galvanised (PPGI)
Pre-painted sheets (PPGI / PPGL) are the most common roofing product today — they offer galvanic corrosion protection plus colour, UV resistance and aesthetic appeal in a single material. Modern PPGL with 150–200 GSM AZ coating can last 25–40 years.
Types of Steel Sheets
| Type | Coating | Coating Mass | Use |
|---|
| GI Plain Sheet | Zinc | 120–275 g/m² | Plain sheets, ducting |
| GP Sheet (Galvanised Plain) | Zinc | 120–180 g/m² | Standard sheets |
| Galvalume (GL) | 55%Al-Zn | 150–200 g/m² | Long-life roofing |
| PPGI | Zn + Paint | 120–275 + Paint | Colour roofing |
| PPGL | AZ + Paint | 150–200 + Paint | Premium colour roofing |
| Galvanised Corrugated | Zn / AZ | 120–275 g/m² | Cheaper rural roofing |
| Trapezoidal Profile | PPGI / PPGL | 150 g/m²+ | Industrial sheds |
| Tile-Profile Sheet | PPGL | 150 g/m²+ | Residential (looks like clay tile) |
Common Sheet Profiles
| Profile | Rib Height | Cover Width | Use |
|---|
| Trapezoidal (35 mm) | 35 mm | 1015 mm | Industrial roof |
| Trapezoidal (28 mm) | 28 mm | 1015 mm | General roof |
| Trapezoidal (45 mm) | 45 mm | 1000 mm | Long-span roof |
| Standing Seam | 60–80 mm | 500–700 mm | Premium concealed-fixed roof |
| Corrugated | 18 mm | 800–900 mm | Traditional / utility |
| Tile Profile | Variable | 1080 mm | Residential tile-look |
| Curved / Crimped Sheet | Custom | Variable | Curved/arch roofs |
| Insulated Sandwich Panel | 30–200 mm | 1000 mm | Cold storage, AC building |
Standard Specifications
| Property | Specification |
|---|
| Base Steel Grade | 240 / 280 / 340 / 550 MPa yield |
| Thickness (TCT) | 0.30 / 0.40 / 0.45 / 0.50 / 0.60 / 0.80 / 1.00 mm |
| Zinc Coating (GI) | 120, 180, 200, 275 g/m² (both sides) |
| AZ Coating (Galvalume) | 150, 165, 185, 200 g/m² (both sides) |
| Top Paint System (PPGI/PPGL) | RMP (5µ) + Primer (5µ) + Top Coat (15–25µ) |
| Back Paint System | Primer + 7µ coating |
| Total Paint Thickness | 25 – 35 microns (top) |
| Sheet Width | 1000 / 1015 / 1220 mm coil |
| Length | Up to 12 m (cut to project) |
| Standard Colours | Off-white, sea green, terracotta, blue, grey, custom |
Physical Properties
| Property | Value |
|---|
| Density | 7,850 kg/m³ (steel) |
| Sheet Weight (0.5 mm) | ~4 kg/m² |
| Sheet Weight (0.8 mm) | ~6.5 kg/m² |
| Tensile Strength | 270 – 550 MPa |
| Yield Strength | 240 – 550 MPa |
| Elongation | 10 – 30% |
| Thermal Conductivity | ~50 W/m.K (poor insulator) |
| Coefficient of Expansion | 12 x 10⁻⁶ /°C |
| Melting Point | ~1450°C |
| Fire Resistance | Non-combustible |
| Lifespan (PPGL) | 25 – 40 years |
| Lifespan (GI plain) | 15 – 25 years |
Top Indian Manufacturers
- JSW Steel — JSW Colouron+, JSW Pragati
- Tata BlueScope — ZINCALUME, COLORBOND
- Essar Steel
- SAIL
- JSPL (Jindal Steel)
- Bhushan Steel
- Uttam Galva Steels
- NSL Steel
- Pennar Industries
- Asian Paints (PPGI coil coatings)
Installation — Step by Step
- Structure: Erect MS angle / channel / tubular purlins at 1.2–1.5 m c/c spacing
- Roof Pitch: Minimum 5° (1 in 12), preferred 10–15° for water shedding
- Sheet Alignment: Start from gable end opposite to prevailing wind
- Overlap: 150–200 mm end lap, 1 rib side lap minimum
- Fasteners: Self-drilling screws (5.5 x 50 mm) with EPDM washers on crest
- Fastener Spacing: Every 2nd rib at end laps, every 3rd rib in body, max 600 mm
- Sealing: Apply silicone sealant at all laps before final fastening
- Ridge: Ridge cap with closure profiles & foam fillers
- Eaves: Eaves trim and gutter with downpipes
- Curing: Wipe shavings / metal swarf with magnet within 1 hr of installation (prevents rust spots)
- Insulation: Air-bubble / glass-wool / PUF roll laid between sheets & purlins (optional)
- Skylights: FRP / polycarbonate skylight inserts at 1 per 30 m²
Tests on GI / Colour-Coated Sheets
- Coating Mass Test (Dipped / Quench) — IS 6745
- Coating Thickness (Eddy Current) — IS 13871
- Adhesion Test (T-bend) — IS 14246
- Cross-cut Adhesion Test
- Salt Spray Test — ASTM B117 (500–1000 hours)
- Pencil Hardness Test — IS 101
- Reverse Impact Test
- Gloss Reading (60° gloss meter)
- UV / QUV Accelerated Weathering Test
- Solvent Resistance (MEK rub test)
- Tensile, Yield, Elongation — IS 1608
- Tile / Sheet Profile Dimension Check
| Parameter | GI Sheet | RCC Slab | Mangalore Tile |
|---|
| Material | Coated steel | Concrete | Fired clay |
| Weight | 4 – 8 kg/m² | 275 kg/m² | 60 kg/m² |
| Construction Speed | 1–3 days | 21–28 days | 10–15 days |
| Span (max) | Up to 30 m (with truss) | 6–8 m | 4–6 m |
| Insulation | Poor (need PUF/glass-wool) | Moderate | Excellent |
| Lifespan | 15–40 years | 50–75 years | 40–60 years |
| Cost (Rs/sqft) | 120 – 250 | 180 – 280 | 100 – 180 |
| Best for | Sheds, warehouses, factories | Multi-storey, terrace use | Residential, heritage |
| Re-painting needed | Every 8–15 yr (GI) | Every 5–7 yr (waterproof) | Not needed |
Uses
- Industrial roofing (factories, warehouses, godowns)
- Commercial sheds, parking, gas stations, fuel stations
- Cold storage, food processing plants (with insulation)
- Pre-Engineered Buildings (PEB) — entire roof + walls
- Schools, hospitals, community halls
- Poultry, cattle, animal sheds
- Greenhouses (tile profile)
- Tile-profile residential roofs
- Side cladding for industrial buildings
- Curved sheds (auditoriums, sports halls)
- Temporary structures, mobile cabins
- Bus stops, metro stations
- Solar mounting structures
- Awnings, canopies, porches
Advantages
- Lightweight — reduces structural cost
- Fast installation (1–3 days for 1000 sqft)
- Long span possible (up to 12 m without joints)
- Many colour and profile options
- Galvanic + paint corrosion protection
- UV-resistant SDP / SMP / PVDF paint systems
- Recyclable steel — eco-friendly end of life
- Non-combustible / fire-resistant
- Easy to extend / modify
- Works as both roof and side cladding
- Available with built-in insulation (sandwich panels)
- Available in tile-profile that looks like clay tiles
- 15–40 year lifespan (depends on coating)
- Low maintenance — periodic cleaning & touch-up paint
- Termite-proof, rot-proof
Disadvantages
- Poor thermal insulation — gets very hot in summer (60–70°C)
- Noisy in heavy rain (without insulation)
- Corrosion at scratched / drilled / cut edges if not sealed
- Coastal areas need special premium coatings (or aluminium roof)
- Lower aesthetics for residential premium buildings
- Sheet can deflect under foot traffic (use catwalks)
- Thermal expansion — needs slip joints in long sheets
- Cut edges and screw holes are vulnerable spots
- Roof penetrations (skylights, vents) are leak-prone
- Galvalume reacts adversely with green/wet timber and lead/copper
- Looks dated after 10–15 years (paint fade)
- Wind uplift in cyclone zones — needs proper fastening
Quantity Estimation
| Item | Per m² of roof |
|---|
| GI / PPGI / PPGL Sheet (0.5 mm) | ~1.05 m² (with 5% overlap) |
| Self-Drilling Screws | 6–8 nos |
| EPDM Washers | 6–8 nos |
| Ridge Cap | 1 m / m run of ridge |
| Sealant (silicone) | 0.05 kg |
| Insulation (optional) | 1.05 m² |
Sheet Area Required = Plan Area × (1 / cos θ) × 1.05 (lap wastage)
Cost (Approximate, Indian Market)
| Type / Thickness | Rate |
|---|
| GI Plain Sheet (0.5 mm) | Rs 80 – 110 / kg or Rs 50 – 75 / sqft |
| PPGI Sheet (0.5 mm, 150 GSM) | Rs 90 – 130 / kg or Rs 60 – 90 / sqft |
| PPGL Sheet (0.5 mm, 150 GSM, AZ) | Rs 105 – 145 / kg or Rs 70 – 100 / sqft |
| Sandwich Panel (50 mm PUF) | Rs 300 – 500 / sqft |
| Tile Profile Sheet (PPGL, 0.5 mm) | Rs 130 – 220 / sqft |
| Self-drilling screws (per kg) | Rs 250 – 350 |
| Complete roof (incl. truss + sheet + labour) | Rs 120 – 250 / sqft |
| Installation labour only | Rs 25 – 50 / sqft |
Best Practices
- Choose PPGL over PPGI for > 15-year applications
- Use minimum 150 GSM AZ coating for coastal / industrial areas
- Use minimum 0.5 mm TCT for roofing (residential / commercial)
- Use 0.8–1.0 mm TCT for cyclone-prone / high-load areas
- Maintain minimum 5° pitch (1 in 12) — flatter sheets pond water
- Always use rubber-washer self-drilling screws (never nails)
- Apply touch-up paint on cut edges and screw heads immediately
- Sweep metal swarf off roof within 1 hour to prevent rust spotting
- Provide expansion / slip joints for sheets longer than 12 m
- Use insulation (rock wool / PUF / air bubble) for habitable buildings
- Install at least 1 skylight per 30 m² for natural light
- Use roof safety harness; never walk between purlins
- Clean roof annually with soft brush & water
- Repaint every 8–12 years if needed
Applicable Standards
| Standard | Description |
|---|
| IS 277 | Galvanized Steel Sheets (Plain & Corrugated) |
| IS 14246 | Continuously Pre-painted Galvanised Steel Sheets (PPGI) |
| IS 15965 | Hot Dip 55% Aluminium-Zinc alloy metallic coated steel (Galvalume / PPGL) |
| IS 14164 | Profiled / Tile-Profile Steel Sheets — Code of Practice |
| IS 513 | Cold-rolled low-carbon steel sheets and strips |
| IS 1079 | Hot-rolled carbon steel sheets and strips |
| IS 875 (Part 3) | Code of practice for design loads — Wind |
| IS 6745 | Method for determination of mass of zinc coating |
| IS 800 | General construction in steel — Code of practice |
| ASTM A653 / A792 | Steel Sheet, Zinc-Coated / Aluminum-Zinc Alloy-Coated |
Conclusion
GI and Colour-Coated steel sheets are the most economical, fastest and most flexible roofing system for any building larger than a small house. From village godowns to large factories and PEB warehouses, modern PPGL sheets offer 25–40 year service life, fast erection (1–3 days), wide colour choice and excellent fire resistance.
For residential roofing, prefer tile-profile PPGL or Mangalore tiles for aesthetics, or RCC slab for terrace use. For daylight skylights, combine with polycarbonate sheets; for premium residential roofing, look at bitumen shingles or solar roof tiles.