Purlin Spacing for GI Sheet Roof India — IS 875 Guide

For standard GC corrugated GI sheets (0.47mm thickness, Class 2 zinc), the recommended purlin spacing is 1200mm to 1400mm centre-to-centre for most inland Indian wind zones. According to IS 875 Part 3 (Wind Loads on Buildings and Structures), sites in higher wind zones — particularly the coastal districts of Odisha, West Bengal, Tamil Nadu, and Andhra Pradesh — require purlin spacing reduced to 900mm or less. Thicker sheets (0.50mm and above) allow wider spacing; thinner sheets (0.35mm) require closer purlins.

What Is Purlin Spacing?

Purlins are the horizontal structural members that run across the slope of a roof, spanning between rafters or portal frames. They provide the direct support on which roofing sheets are fixed. Purlin spacing refers to the centre-to-centre distance between adjacent purlins measured along the slope of the roof.

The term is sometimes confused with rafter spacing (the distance between the main structural frames) and bay spacing (the distance between portal frame bays). Purlin spacing is always the smaller dimension — a typical shed might have portal frames at 6m bay spacing, with purlins at 1.2m spacing fitting 5 purlins per bay.

Getting purlin spacing right is critical for two reasons: too wide and the sheets deflect excessively between supports, causing oil-canning (waviness), fastener pull-through under suction loads, and eventual leakage. Too narrow and you are adding unnecessary steel weight and cost.

IS 875 Part 3 — Wind Zones in India

IS 875 Part 3:2015 divides India into six wind speed zones based on design basic wind speed (V_b in m/s):

For most industrial sheds and agricultural warehouses in central India (Zones I and II), a purlin spacing of 1200–1400mm is standard practice. Zone III and above require engineering review and typically a reduced spacing.

Recommended Purlin Spacing by Sheet Thickness and Wind Zone

The following table gives recommended maximum purlin spacing (centre-to-centre) for standard GC corrugated GI sheets at various thicknesses. Values are based on IS 875 Part 3 wind load requirements and assume normal weight sheeting, no heavy concentrated loads (equipment), and standard 150mm end laps.

Important: These values are guides for preliminary design. Final purlin spacing for any structure must be confirmed by a structural engineer using the specific site wind speed, terrain category, building height, and topography factor as per IS 875 Part 3. For buildings over 10m height or in high-value industrial applications, always get a structural drawing.

Effect of Roof Profile on Purlin Spacing

GC corrugated sheets are the most common profile for agricultural and light industrial sheds in India. However, other profiles have different spanning capabilities:

• GC corrugated (standard): Corrugation depth approximately 17–18mm. Moderate spanning ability. Values in table above apply.
• Trapezoidal / Hi-Rib: Corrugation depth 35–45mm. Significantly higher spanning ability — purlins can be spaced 1800–2400mm even at 0.47mm thickness in Zone II. JSW Colouron+ Hi-Rib and Tata Shaktee TP profiles fall in this category.
• Standing seam: Concealed fastener, deeper profile — professional installation required; spacing typically 1500–2000mm depending on manufacturer specs.

When using a trapezoidal or deep rib profile, always refer to the manufacturer's span tables rather than generic GC values. JSW and Tata both publish span tables for their specific profile dimensions.

Purlin Sections Commonly Used in India

The most common purlin sections for light industrial and agricultural sheds in India are:

• MC (Mild Steel Channel / C-purlin): 75×40mm, 100×50mm. Widely available, simple connection to rafters. Used in spans up to approximately 5m between rafters.
• Z-purlin: 75mm, 100mm, 150mm. Better efficiency than C-purlin for the same weight. Nested for transport. APL Apollo and other roll-formers supply Z purlins in galvanised or painted finish.
• Angle iron (L-section): 50×50×5mm or 65×65×6mm. Older style, less efficient but simple to connect and readily available at any steel merchant.

For spans up to 1400mm, 75×40mm MC or 75mm Z purlin at 2–3mm thickness is adequate for most Zone I–II applications. For spans of 1500–1800mm or higher wind zones, use 100mm sections or increase thickness to 2.5–3mm.

What Happens If Purlin Spacing Is Wrong?

Over-spacing purlins (too far apart) is a common construction defect in India, especially in unengineered rural sheds. The consequences include:

• Excessive sheet deflection: GI sheets sag between supports, creating flat low points where water pools rather than draining. This dramatically accelerates corrosion at the retention point.
• Oil-canning: The visual waviness that develops in panels spanning too far. Not a structural failure, but it indicates the sheet is operating beyond its design limit.
• Fastener pull-through: Under wind suction loads, the sheet is pulled upward. If purlins are too far apart, the upward bending moment at the fastener hole exceeds the sheet's resistance and the fastener tears through the metal. This is the primary mode of roof failure during cyclones.
• Progressive collapse: In severe wind events, fastener pull-through on one sheet exposes adjacent sheets to the full wind load, leading to cascading failure across the roof.

Under-spacing purlins (too close together) wastes steel and increases structure weight and cost, but causes no structural problem. When in doubt, go closer rather than wider.