Structural assessment of a tin-shed roof for rooftop solar in Bangladesh

By Vvon Engineering Team | Published 2026-02-07 | Updated 2026-09-13

What a survey of a corrugated shed roof has to measure before anyone quotes a kilowatt figure: purlin section and spacing, real sheet thickness, corrosion, existing services load, and who signs the report.

Solar EPC & street lighting RFQ checklist (TXT)

For a proposed array on a corrugated factory shed, establish the structure beneath the sheet, existing loads and the complete load path before settling the capacity. Long-term operation also requires inspection and maintenance.

This is the part of a rooftop project most often skipped, because it is the part that reduces the number of kilowatts a contractor can sell. A survey done properly sometimes comes back saying the roof takes 220 KWp, not 350. That answer is worth more than the difference.

The load path is the whole question

The design must trace loads through fixings, purlins, rafters or trusses, columns and foundations. Which component governs depends on the measured structure; do not assume the sheet or purlin is always the weakest link.

There are also two quite different loads to consider. The gravity load of module plus rail plus clamp plus structure is modest and spread out. The uplift load is not. A shed roof in a nor'wester sees suction, and an array raised above the sheet gives the wind something to get under. At the roof edges, and especially at the corners, the suction coefficients are the highest anywhere on the building. That is why a sensible layout keeps a setback from the eaves and the gable, thins the modules at the corners, and puts more fixings there than in the middle of the field. The design must address the higher uplift demands that may occur at exposed edges and corners.

The design wind speed comes from the zone value in the Bangladesh National Building Code for that location, adjusted for terrain and exposure. A coastal site near Cox's Bazar and a sheltered site inside a built-up area of Narayanganj are not the same problem. A mounting supplier's claim that their system is rated to a certain speed refers to their own aluminium, not to your shed.

Purlins: measure them, do not read them off a drawing

Purlin spacing sets the rail span, and the rail span decides which mounting system is possible at all. Cold-formed C and Z purlins on newer sheds sit at spacings of roughly a metre to a metre and a half. Older sheds framed with MS angle or pipe purlins can be far wider and far less predictable, and some have been extended by a previous owner with whatever section was in the yard that week.

Measure and record at several bays, not one:

Spacing centre to centre, across the full slope, because it is rarely uniform near the ridge and the eaves.
Section depth, flange width and lip, so the section can be identified rather than guessed.
Actual steel thickness, taken with a vernier or an ultrasonic gauge, never assumed from the section name. Cold-formed sections are rolled in several thicknesses to the same nominal depth and the capacity difference between them is large.
Span between rafters, and whether the purlin is single span, continuous, or lapped over the support. A lapped continuous purlin behaves very differently from a simply supported one.
Sag rods and bracing, present or absent, and their condition. A Z purlin without sag rods twists readily under an eccentric load, and a rail bracket bolted to one flange is exactly that.

The reason for measuring rather than reading a drawing is that the shed as built and the shed as drawn diverge. Purlins get cut for a duct. A run gets replaced after a fire. Somebody adds a mezzanine and hangs a hoist rail off the frame. The drawing, where it exists at all, records intent.

Use the exact module installation manual for permitted clamping positions and load conditions. Coordinate those limits with the mounting and roof layout. Non-compliant clamping can affect safety and warranty coverage; obtain the manufacturer's terms rather than asserting automatic cancellation.

The sheet, its real thickness and its rust

Corrugated galvanised sheet sold here is described by a gauge number that often flatters the actual base metal thickness. The number that matters is the steel under the zinc, measured at a cut edge with a micrometre or with a coating-compensating gauge. It matters because the pull-out capacity of a self-drilling screw or a J-bolt is a direct function of that thickness, and because thin sheet distorts around a loaded fixing long before it tears.

Map corrosion using a safe survey plan. Treat the roof as potentially fragile until a competent person has assessed access; use suitable remote or protected inspection methods.

Around existing fixings, where the coating was broken and the washer has perished. Rust rings and perforation here mean the sheet will not hold a new fixing either.
At the sheet overlaps, where capillary action holds water for days after rain.
Along the eaves and the gutter line, where debris dams the flow and keeps the metal wet through the monsoon.
Around any process vent or exhaust, where the discharge is warm, damp or chemically aggressive. Above a dye house or a chemical store the sheet ages far faster than the rest of the roof.
The underside, viewed from inside with a torch. Condensation attacks the unpainted face, and a sheet that looks acceptable from above can be lace underneath.

If the sheet has fewer years left in it than the array, say so and price the reroof into the project. Nobody wants to demount and remount a twenty five year asset in year four to change a roof covering. Compare reroofing before installation with its later removal and reinstatement costs. Replacing it afterwards costs the reroof, plus the demount, plus the remount, plus the lost generation, and it is a conversation nobody enjoys.

What is already hanging from the roof

The frame was designed for the sheet, the wind, and whatever allowance the original engineer made for services. Twenty years of operations use that allowance up quietly. A survey should list and locate every one of these, with an estimate of weight:

Exhaust fans, ridge ventilators and turbine ventilators.
Dust extraction ducting, which in a jute or textile mill is large, long, and full of settled fibre.
Cable trays, compressed air lines, steam lines and sprinkler pipework, all heavier full than empty.
Crane or hoist rails, and whether the crane is suspended from the roof frame or runs on its own gantry columns.
Water tanks, and any tank sitting on the roof structure rather than on its own legs to the ground.
False ceilings, insulation retrofits, and any second skin added for temperature control.
Signage, especially large gable boards, which also collect wind.
Accumulated dust and fibre on the members themselves. In a jute mill that is not a rounding error.

Include column connections, base plates and foundations in the assessment. Where hidden corrosion is suspected, the structural engineer should specify a safe investigation method; do not break out material around a loaded base informally.

Why the signature has to be a structural engineer's

Three different people can hold an opinion about your roof, and only one of them should be putting a stamp on it.

The mounting structure supplier can certify their own product. Their calculation covers the rail, the clamp and the bracket under a stated wind pressure. It says nothing about your purlins.

The solar salesman can tell you what fits geometrically. That is a layout exercise, and their commercial interest points in one direction: more modules. This is not a character judgement, it is simply how the incentive works, and it is exactly why the person who signs off the structure should not be the person whose commission depends on the answer.

The structural engineer takes the measured survey, models the existing frame under the added dead load and the revised wind case, and states in writing what the roof can carry, where, and with what strengthening if any. That document is what your insurer will ask for after a storm, what a lender's technical adviser will want during due diligence, and what protects you if a fixing lets go. A serious EPC contractor arranges it and prices it into the design rather than offering it as an extra you can decline to save money.

What the survey report should contain

If you are writing this into a tender, specify the deliverable rather than the activity. A survey report that earns its fee contains:

A measured grid of columns, rafters and trusses with member sizes, so the frame can be modelled without a second site visit.
Purlin type, section, measured thickness, span, spacing, continuity and bracing, recorded bay by bay.
Sheet profile, roof pitch, measured base metal thickness at several locations, and a corrosion map keyed to photographs.
Existing fixing type and condition, with any fixing test specified and supervised by the responsible engineer.
A schedule of existing services loads with locations marked on the roof plan.
Roof orientation and true azimuth taken on site, not off a satellite image.
A shading survey: water tanks, stair headroom, lift machine rooms, chimneys, adjacent taller buildings and trees, each with a measured height and distance.
Access and safety notes: how equipment reaches the roof, where a person can walk, where the fragile areas are, and where fall arrest anchor points can go.
Photographs with a scale object in shot, every one of them located on the plan.
The engineer's statement of permissible loading, with strengthening options and their consequences set out separately from the conclusion.

When the answer is no

If the proposed layout exceeds verified capacity, compare the following engineered alternatives. Their cost order depends on the site.

Reduce and redistribute. Fewer modules, concentrated over the bays with the shortest purlin spans and kept clear of the corners.
Supplement the purlins. Add intermediate runs, or sister the existing ones over the loaded length. Straightforward where access from below is clear, awkward over a running production line.
Bypass the purlins entirely. An independent steel frame landing on the rafters or the columns, so the array load goes straight into members that were designed to carry load. On an old shed with tired purlins this is often the honest answer, and it costs what it costs.
Move part of the array. A ground mount on spare land, or a car park canopy, which also buys you shaded parking and a much easier maintenance life.
Reroof first. If the sheet is finished, do it now, not later.

Keep the signed structural assessment and approved mounting layout with the project records, including assumptions, restrictions and inspection requirements.

Sources and project checks

Consult the relevant edition and the documents for the exact equipment and site: HSE: planning work around fragile roofs; JinkoSolar: model datasheets, certificates, installation manuals and warranties; IEC 62548-1: PV array design requirements; IEA PVPS: operational and economic impacts of extreme weather.

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