Cyclone-Proofing Rooftop Solar on the Godavari Delta

An array that leaves your roof in a cyclone is worse than no array at all. On the coast, mounting design is not a detail — it is the difference between a system that lasts twenty-five years and one that becomes a liability.

The number that governs everything

Under IS 875 (Part 3), India is divided into wind zones. Coastal Andhra Pradesh sits in the highest mainland zone: a basic wind speed of 50 metres per second, or 180 kilometres per hour. This is verified for Vijayawada and Visakhapatnam, and Rajahmundry sits on the delta between them.

Designing to 47 m/s instead of 50 saves very little money. It creates a great deal of risk. There is no sensible argument for the lower figure here.

The trap in the code — and how to spot it

The wind calculation includes an importance factor, k4, which applies within roughly 60 km of the coast. Its values are:

• 1.30 for post-cyclone-critical structures — hospitals, shelters, communication towers
• 1.15 for industrial structures
• 1.00 for everything else, including ordinary houses

Rajahmundry is inside the coastal belt. But the belt determines where the factor applies, not that it exceeds 1.0. A home is neither industrial nor post-cyclone-critical.

If a vendor justifies a price premium by citing k4 of 1.15 or 1.30 on a residential job, they are misapplying the code. The genuine engineering margin comes from designing to 50 m/s, not from inflating the importance factor.

Uplift, not weight, is the enemy

People imagine wind pushing panels down. It does the opposite. Wind flowing over a tilted array generates lift, exactly as it does over an aircraft wing.

At coastal design speeds, a single two-square-metre module can experience uplift in the order of 370 kilograms-force. Multiply across an array and the loads on your fixings become serious.

This is why ballast-only mounting deserves scepticism on a coastal terrace. Non-penetrative systems — which sit on weight alone — are easier to sell, because homeowners worry about drilling into a waterproofed roof. But resisting that uplift with dead weight alone requires a great deal of it, and then you must verify your roof can carry it.

In a cyclone belt, a mechanically anchored and properly flashed system is generally the sounder engineering. The easy sell and the right answer diverge here, and you should know that when someone offers you the easy one.

Questions worth asking your installer

• What basic wind speed have you designed to?
• Can I see the IS 875:2015 calculation for my roof?
• Is the structure anchored, ballasted, or both?
• How are roof penetrations flashed and waterproofed?

A "180 kmph rated" claim on a brochure is not an answer. That figure is the basic wind speed — the starting input, not the design output after factors and pressure coefficients are applied. Ask for the calculation.

One thing to be aware of

There is no mandatory third-party wind-tunnel or structural certification for rooftop mounting structures in India. The regulatory bar is lower than most people assume, which shifts the burden of diligence onto you and your installer. Treat every "cyclone-rated" claim as marketing until someone shows you stamped calculations.