Wind load on a rooftop solar array starts from the basic wind speed for the site in IS 875 (Part 3), which is adjusted to a design wind speed at the height of the array and then converted to a pressure. That pressure sizes the structure, the anchors and the ballast. This note explains the steps; always confirm the figures against the current code and have a structural engineer approve the actual design.
1. Basic wind speed (Vb)
IS 875 (Part 3):2015 gives a basic wind speed for the site, a 3-second gust at 10 m height over open terrain, for a 50-year return period. The code lists values for many towns and cities in an annex, and a map covers the rest. Coastal regions that see cyclones have higher values.
2. Design wind speed (Vz)
The basic speed is multiplied by factors to get the design wind speed at the array:
- k1, the risk coefficient, which is 1 for a 50-year design life.
- k2, which depends on terrain category and on the height above ground. A higher array sees more wind, and a rougher surrounding (a dense city) reduces it.
- k3, for topography such as hills and escarpments.
- k4, an importance factor for the type of structure.
3. Wind pressure (pz)
The pressure is taken as pz = 0.6 × Vz², with Vz in metres per second and pz in newtons per square metre. Divide by 1000 for kN/m². The code then applies further factors, such as a wind directionality factor and a floor on the design pressure, and pressure coefficients for the shape of the array and its position on the roof.
4. Why the roof edge and the tilt matter
Panels near the edge or corner of a roof see higher uplift than panels in the middle, and a steeper tilt catches more wind. Tall parapets can shelter the array. This is why a quick rule of thumb is not enough for large or tall installations.
5. From pressure to steel and anchors
- Rails (purlins) under each row of modules.
- Rafters on each frame, and front and rear columns at their real heights.
- Bracing on tall frames.
- Base plates, anchors, pedestals and clamps, sized for the uplift.
6. What a design tool can and cannot do
A design tool can take the site's basic wind speed, work out the design speed and pressure at the array height, and count the steel for the quotation. That is good for a proposal and a budget. It does not replace the structural engineer's design for the actual roof, which checks the existing building, the connections and the local code.
7. Outside India
Other countries use other codes, for example ASCE 7 in the United States and the Eurocodes in Europe, and the factors differ. A tool for several markets should say plainly when it is showing a typical value instead of the local code.
How RAYVO does this
RAYVO's design studio picks the IS 875 (Part 3) basic wind speed for the nearest listed Indian city, works out the design wind speed and pressure at the array height, and takes the structure steel off member by member for the bill of materials. It marks these as proposal-level figures that a structural engineer confirms. See the .