Solar Structure Design and Wind Load (IS 875 Part 3)
Rooftop solar structures and wind in India: IS 875 (Part 3) wind speeds, design pressure, uplift per module, anchors, ballast, tin sheds and coastal sites.
Short answer: rooftop structures in India should be designed for the site's basic wind speed (Vb) from IS 875 (Part 3):2015, which ranges from 33 m/s (Bengaluru) to 50 m/s and above on the east coast. Design wind pressure is pz = 0.6 × Vz², where Vz adjusts Vb for design life, terrain and height, topography and cyclone importance. At 39 m/s, uplift on a single 2.6 m² module can be close to 300 kgf, so anchors, ballast and connections, not the module weight, govern the design.
Step 1: find the basic wind speed
| City | Vb (m/s) |
|---|---|
| Bengaluru | 33 |
| Pune, Ahmedabad, Bhopal, Indore, Kochi, Thiruvananthapuram | 39 |
| Mumbai, Nagpur, Surat, Hyderabad | 44 |
| Delhi, Lucknow | 47 |
| Chennai, Kolkata, Visakhapatnam, Vijayawada | 50 |
From an IS 875 (Part 3) city table (infralens.in, accessed Oct 2026). Always confirm Vb from Annex A or the wind map of the standard for the exact site.
Step 2: design wind speed
Vz = Vb × k1 × k2 × k3 × k4
| Factor | Meaning | Notes for rooftop solar |
|---|---|---|
| k1 | Risk coefficient (design life) | 1.0 for 50 years; Table 1 gives lower values for shorter lives (about 0.9 for 25 years). Many designers keep 1.0 for rooftop solar. |
| k2 | Terrain roughness and height | Higher roofs and open terrain raise it; dense city centres lower it. Use the height of the array above ground. |
| k3 | Topography | 1.0 on flat ground; higher on hills, ridges and cliffs. |
| k4 | Cyclonic importance | Applies in cyclone-prone regions (east coast, Gujarat coast); higher for industrial and post-cyclone-important structures (check clause). |
Step 3: design pressure and force
- pz = 0.6 × Vz² (N/m²)
- Design pressure pd applies area, directionality and combination factors to pz, with a floor of 0.7 × pz in the standard.
- Force on a module ≈ Cp,net × pd × A, where Cp,net is the net pressure coefficient for the panel's tilt, position (edge, corner or interior) and wind direction, from the standard's tables for free-standing roofs and canopies or from wind-tunnel data.
Illustrative uplift per module
Module area 2.58 m² (2.28 × 1.13 m); Cp,net taken as 1.2; pd taken equal to pz for simplicity.
| Site | Vz (m/s) | pz (kN/m²) | Uplift per module |
|---|---|---|---|
| Bengaluru, Vb 33 | 33 | 0.65 | about 2.0 kN (205 kgf) |
| Pune, Vb 39 | 39 | 0.91 | about 2.8 kN (290 kgf) |
| Mumbai, Vb 44 | 44 | 1.16 | about 3.6 kN (365 kgf) |
| Chennai, Vb 50, k4 1.15 | 57.5 | 1.98 | about 6.1 kN (625 kgf) |
Simplified to show the scale. k1 = k2 = k3 = 1.0 assumed; a real design uses the exact factors and coefficients. The module weighs about 28–32 kg. Wind uplift is roughly ten to twenty times that, so every bolt, clamp, purlin and anchor is in the load path.
Step 4: tilt and height
- Tilt: higher tilt raises wind load, rear-leg height and row spacing. Studio caps flat-roof tilt at 20°, because above that the extra yield is under about 1–2% and the wind penalty is larger.
- Height: an elevated structure (front leg about 1 m, common for homes in Maharashtra and Karnataka) adds lever arm. Use bracing and check leg buckling.
- Edges and corners: roof edges and corners see higher suction. Keep setbacks from parapets (Studio default 0.6 m) and strengthen edge rows.
Step 5: anchoring
RCC slabs
- Most Indian house slabs are 100–125 mm thick. An M12 wedge anchor needs about 80 mm embedment and generous edge distance; in thin slabs or near parapets it can crack the slab and lose uplift capacity.
- Chemical anchors (M12 stud with vinyl-ester or epoxy capsule) are safer on thin slabs. Use mechanical anchors only where the slab is about 150 mm or thicker.
- Seal around base plates with PU sealant; check the waterproofing.
- Never anchor into a parapet or chajja without checking it.
Ballasted (non-penetrating) systems
- Used on freshly waterproofed or weak slabs.
- Check sliding (friction between ballast and roof, often reduced when wet), overturning and uplift, plus the slab's capacity for the added dead load.
- Ballast weight is large at high wind speeds; get a stamped design.
Tin and metal sheds
- Fix mini-rails or rails on the trapezoidal crests with self-drilling screws and EPDM washers.
- Where the sheet is thin (under about 0.45 mm), screws must reach the purlins, not just the sheet.
- Replace rusted sheets before mounting; check purlin spacing and capacity.
Step 6: materials
- Hot-dip galvanised steel (around 80 microns) for elevated structures; pre-galvanised GI for low-rise, inland use.
- Coastal (within about 10 km of the sea): heavier HDG sections, SS304 fasteners, and isolation between aluminium and steel to prevent galvanic corrosion.
- Aluminium rails and clamps on sheds; check clamp zones from the module datasheet.
When to get a stamped design
- Vb 47 m/s or more, coastal and cyclone-prone sites
- Buildings above about four storeys, or arrays near roof edges
- Ballasted systems
- Large commercial roofs and older buildings
Keep the design calculation, material test certificates and galvanising thickness reports on file. Inspectors and insurers increasingly ask for them.
Talk to Sunlectric
We stock elevated HDG structures (including heavier high-wind sections), low-rise GI, ballast systems, tin-shed mini-rails, clamps, chemical anchors and sealant at our Pune hub. WhatsApp +91 89567 89307 (wa.me/918956789307) with the site city, building height and roof type. Sunlectric Studio picks the structure by wind zone, and the Academy structures module covers IS 875 in detail.
Sources
- IS 875 (Part 3):2015 Design loads (other than earthquake) for buildings and structures, wind loads (check current edition and amendments)
- City basic wind speeds, k-factors and pz formula: infralens.in IS 875-3 handbook and walkthrough (accessed 20 Oct 2026)
- Anchor, tilt-cap, setback and coastal rules: Sunlectric Studio bom_rules and engineering review, Oct 2026
Questions people ask
What wind speed should a rooftop solar structure be designed for?
The basic wind speed (Vb) for the site from IS 875 (Part 3):2015, adjusted for design life, terrain and height, topography and, on cyclone coasts, importance. Vb ranges from 33 m/s (for example Bengaluru) to 50–55 m/s on the east coast.
How much uplift does wind put on one solar module?
At 39 m/s basic wind speed, design pressure is roughly 0.9 kN/m². With a net pressure coefficient around 1.2 on a 2.6 m² module, uplift is about 2.8 kN, roughly 290 kgf, about ten times the module's weight. Real values depend on tilt, height and position on the roof.
Are mechanical anchors safe on house slabs?
Many Indian house slabs are only 100–125 mm thick. M12 expansion anchors need deep embedment and edge distance, and can crack thin slabs. Chemical anchors are safer there; use mechanical anchors only on slabs of about 150 mm or more.
Do I need a stamped structural design?
For high-wind zones, coastal and cyclone-prone sites, tall buildings, ballasted systems and large commercial roofs, yes. For standard small residential jobs in normal zones, use a tested standard design and keep its calculation on file.