How do I find my city's wind zone per IS 875?

Use the wind zone map in IS 875 Part 3 published by the Bureau of Indian Standards. Major cities: Delhi and Hyderabad are Zone 2; Mumbai, Pune, Bengaluru, Chennai, and Kolkata are Zone 3; coastal Odisha and Andhra Pradesh are Zone 4; coastal Tamil Nadu and Andaman Islands are Zone 5.

Is this calculator suitable for snow load regions?

This calculator is based on wind loading per IS 875 Part 3 only. For hilly regions above 1000 m elevation where snow loads apply per IS 875 Part 4, spacing should be reduced by 20–30% depending on design snow depth. Consult a structural engineer for sites in Himachal Pradesh, Uttarakhand, Jammu and Kashmir, and similar locations.

Purlin Spacing Calculator for Roof Sheets

Find the correct purlin spacing for your sheet profile, wind zone, and thickness — based on IS 875 Part 3 guidelines.

Sheet Profile Most common: GC Corrugated for residential and small sheds

Wind Zone (IS 875 Part 3) Check IS 875 Part 3 wind zone map for your city

Sheet Thickness (mm) 0.47 mm is standard for industrial sheds; 0.63 mm for heavy-duty

Roof Length (metres) Total length of roof along the purlin direction (ridge to eave direction)

How to Use This Calculator

Select sheet profile — GC Corrugated for standard roofing, Trapezoidal for longer spans, Hi-Rib for heavy industrial roofs.
Select wind zone — Use the IS 875 Part 3 wind zone map to find your city's zone. Higher zones require closer purlin spacing.
Select sheet thickness — Thicker sheets allow greater spacing. 0.47 mm is the most common for industrial sheds.
Enter roof length — This is the dimension running parallel to the purlins (ridge-to-eave direction) and is used to calculate the total number of purlins needed.
Click Calculate — Results show recommended spacing, maximum safe spacing, and total purlin count.

IS 875 Wind Zone Reference — Major Indian Cities

Wind Zone	Basic Wind Speed	Major Cities	Spacing Factor
Zone 1	< 33 m/s	Parts of Rajasthan, Madhya Pradesh	1.00 (no reduction)
Zone 2	33–39 m/s	Delhi, Jaipur, Agra, Lucknow, Kanpur, Nagpur, Hyderabad	0.85
Zone 3	39–44 m/s	Mumbai, Pune, Ahmedabad, Surat, Bengaluru, Kolkata, Chennai	0.75
Zone 4	44–50 m/s	Coastal Odisha, coastal Andhra Pradesh, Assam	0.65
Zone 5	> 50 m/s	Coastal Tamil Nadu, Andaman & Nicobar, parts of Odisha	0.55

Source: IS 875 Part 3 (Wind Loads on Buildings). Verify your city's zone with a structural engineer for critical structures.

Frequently Asked Questions

What is purlin spacing and why does it matter?

Purlin spacing is the centre-to-centre distance between adjacent purlins — the horizontal structural members that support your roof sheets. Correct spacing prevents sheet deflection, cracking at screw holes, and uplift failure during high winds. Too wide a spacing causes the sheet to flex excessively under load; too narrow wastes steel and increases cost.

What purlin spacing should I use for GC corrugated sheets in Surat?

Surat falls in Wind Zone 3 (basic wind speed 44 m/s). For standard GC corrugated sheets at 0.47 mm thickness, this calculator recommends 900 mm spacing. The absolute maximum is 1125 mm. For coastal and cyclone-prone locations, always err towards the recommended spacing rather than the maximum.

Does sheet thickness affect purlin spacing?

Yes. Thicker sheets have higher bending stiffness and can span greater distances between purlins without deflection. A 0.63 mm sheet can safely use about 15% greater spacing than a 0.47 mm sheet of the same profile. Thinner sheets (0.35 mm) require purlins closer together to avoid visible sagging between supports.

How do I find my city's wind zone?

Use the wind zone map in IS 875 Part 3 (Bureau of Indian Standards). Most states publish simplified zone maps — your structural engineer or local municipal body can confirm the correct zone for your district. Coastal locations are often one zone higher than nearby inland areas due to sea-facing wind exposure.

Is the purlin spacing from this calculator sufficient for snow loads?

This calculator is based on wind loading per IS 875 Part 3. For hilly regions above 1000 m elevation where snow loads apply (IS 875 Part 4), spacing should be reduced further — typically by 20–30% depending on snow depth. Consult a structural engineer for sites in Himachal Pradesh, Uttarakhand, Jammu & Kashmir, and similar locations.