IS 875 Wind Loads for Solar: Parts and Calculation

IS 875 is the Indian loading code that structural engineers, EPCs and mounting-structure suppliers use to set the loads a solar array must resist. This page explains the five parts of IS 875, the Part 3 wind method, and how a rooftop solar wind calculation should be documented for review.

What is IS 875 and how is it used in solar design?

IS 875 supplies design loads for Indian buildings and structures, apart from earthquake loads. For solar work, it helps define dead, imposed, wind, snow, and special actions. Part 3 supplies the wind method. It does not size steel, certify an existing roof, or approve a mounting system by itself.

The Bureau of Indian Standards catalogue identifies IS 875 Part 3:2015 as the third revision for wind loads. The standard is one part of the structural basis. IS 800 covers general steel construction. IS 456 applies to concrete, while earthquake actions sit outside IS 875.

This separation prevents a common review error. A note saying “designed to IS 875” does not show the load path. A useful calculation identifies each input, derives pressure, assigns area, applies coefficients, and checks every affected component.

What are the five parts of IS 875?

The five parts separate different actions so an engineer can select the relevant edition and combination. The BIS catalogue listed the editions below on 26 September 2026. Project documents should cite the exact part and year. A reference to IS 875 alone is incomplete.

The catalogue entries for Part 1:2026, Part 2:1987, Part 4:2021, and Part 5:1987 provide the titles observed during review. Confirm amendments and contract requirements before issuing calculations.

How does an IS 875 wind load calculation work?

IS 875 Part 3 starts with the mapped basic wind speed, Vb. It adjusts that speed for risk, terrain and height, topography, and cyclonic importance. The resulting design wind speed, Vz, produces the pressure pz. The pressure still needs project specific coefficients before it becomes a component load.

The core sequence is:

Vz = Vb x k1 x k2 x k3 x k4
pz = 0.6 x Vz^2

India’s mapped Vb values are 33, 39, 44, 47, 50, and 55 metres per second. The correct value follows the project coordinates. It should not be selected from a broad state label because a state can cross mapped zones.

What does a worked rooftop wind pressure calculation look like?

Consider an illustrative rooftop site with Vb equal to 39 metres per second. Assume k1, k2, k3, and k4 each equal 1.00 only for this arithmetic example. The design wind speed is 39 metres per second. The resulting velocity pressure is 912.6 newtons per square metre, or 0.913 kilonewtons per square metre.

This is a pressure stage, not a finished panel uplift value. The engineer must still select applicable design pressure modifiers and external or internal pressure coefficients. The next steps also need panel geometry, tributary area, direction, roof zone, and load combinations.

Changing any assumed factor changes the answer. If k2 were 1.10 while the other factors stayed at 1.00, Vz would become 42.9 metres per second. The pressure would become 1.104 kilonewtons per square metre. That is about 21 percent higher because pressure varies with speed squared.

Do not copy the example factors into a project. Use the standard tables, the real site, and the applicable roof geometry. A site survey and desktop assessment should capture the inputs that imagery cannot verify.

Why is pressure not the same as panel uplift?

Pressure becomes force only after the engineer applies the relevant coefficient and loaded area. Rooftop arrays also interact with the host building. Openings affect internal pressure, while roof edges, corners, parapets, tilt, gaps, and panel position can affect external action.

A complete load path follows this order:

1. Wind acts on the module or array surface.
2. Clamps transfer force into rails or purlins.
3. Rails transfer reactions into brackets, fasteners, or ballast.
4. Attachments transfer reactions into rafters, purlins, slabs, or frames.
5. The existing building transfers those actions into its lateral and foundation systems.

A mounting member can pass while an anchor, weld, roof sheet, or existing purlin fails. That is why a STAAD.Pro model cannot replace connection checks or a roof capacity review. The model is only as reliable as its loads, restraints, sections, and connection assumptions.

What should a rooftop solar calculation package show?

A reviewable package should let another engineer reproduce the wind load from the source inputs. It should also connect calculated reactions to drawings. A pressure value without attachment spacing or roof member data cannot establish the installed load path.

Use this document checklist:

For steel framing, carry the actions into IS 800 member and connection checks. A STAAD.Pro structural report should disclose inputs and utilization checks. It should not present software output as approval evidence on its own.

How do rooftop and ground mount checks differ?

Both systems need site wind inputs, but their load paths differ. A rooftop array depends on the host building and attachment details. A ground mount table depends on exposed frame geometry, bracing, posts, soil response, and foundation resistance.

Ground mount foundation work should link structural reactions to the pile foundation design. Rooftop work should identify whether the original building design has reserve capacity. Neither conclusion follows from wind pressure alone.

Which mistakes cause an IS 875 calculation to fail review?

Most review failures come from missing traceability, not difficult arithmetic. The calculation must show where each factor came from and how pressure reached the installed components.

Common problems include:

• selecting Vb from a city name without showing the mapped site.
• using k2 without recording terrain category, height, and structure class.
• setting k3 to 1.00 without checking local topography.
• omitting k4 from a Part 3:2015 calculation.
• applying one pressure to every roof location without a geometry basis.
• ignoring internal pressure and building openings.
• checking rails but omitting clamps, bolts, welds, and anchors.
• using drawings that do not match the calculation revision.
• treating CEIG terminology as proof of structural acceptance.

An engineer should also confirm whether the contract, owner, lender, or authority asks for another standard or a stricter project criterion. IS 875 is an Indian load standard. It is not interchangeable with ASCE 7-22 or another national code.

What should an EPC send for a structural review?

Send the project coordinates, roof or site survey, structure drawings, module and mounting data, material grades, attachment details, and geotechnical information where relevant. Mark every revision. State the required code editions and the authority or reviewer who will receive the package.

Heaven Designs prepares solar structural calculations and coordinated drawings within the agreed engineering scope. The civil and structural engineering service covers the wider load path. The STAAD.Pro reporting service supports model based analysis where the structure warrants it.

The responsible project engineer must still confirm the governing code, input data, professional signatory needs, and reviewer requirements. A calculation cannot correct missing survey data or an installation that differs from its drawings.

Source and review note

This entry was reviewed on 26 September 2026 against the BIS catalogue entries for all five IS 875 parts. Part 3:2015 is the cited wind standard. Confirm later amendments, contract criteria, and local authority requirements before issuing a project calculation.

For a code-to-code comparison, see ASCE 7-22 vs IS 875 wind load comparison.