Rooftop Solar on a Metal Roof: Check the Roof First

The roof is the foundation of the solar plant

A large factory or warehouse roof looks like the easiest place to put solar: flat enough, unshaded, and otherwise unused. The commercial case is usually made on generation and tariff. The roof underneath tends to be assumed.

That assumption is where projects go wrong. A solar array is a second structure fixed to the first, for as long as the plant runs. If the sheet will corrode out before the array is retired, if the purlins were never designed for the added load, or if every fixing becomes a leak, the cost lands on the building owner, not the solar installer: wet stock, a voided roof warranty, a strengthening job under a live array, or removing and refitting the whole system to re-sheet.

For a CEO or CFO, the decision is simple to state: check the roof before the solar contract is signed, not after the panels arrive.

Five checks before the array goes up

Will the sheet outlast the array?

A solar plant is typically planned to run for decades, while a metal roof sheet's life depends on its base metal, coating, environment and how it has been maintained. A roof that has been in service for some years, in a coastal or industrial atmosphere, or with visible coating loss at laps and fasteners, may not have that much life left.

If the roof will need re-sheeting well before the array is retired, the array has to be dismantled, stored and reinstalled, and the plant stops generating while that happens. Re-sheeting first is often cheaper than re-sheeting later around an array. A proper inspection of the whole roof, including laps, fasteners, gutters and skylights, is the starting point. Our article on why warehouse roofs leak at laps and skylights shows where to look.

Standing seam clamps or through-fastened fixings

How the panels attach depends on the sheet.

• Standing seam roofs can often take clamps that grip the raised seam, so the sheet is not pierced at all. The clamp must match the exact seam profile, and the mounting must not lock the sheets together in a way that stops their thermal movement on the concealed clips. The roof manufacturer should confirm both.
• Trapezoidal through-fastened roofs usually need brackets or short rails fixed through the sheet, either into the sheet crest or down into the purlin. Every fixing is a penetration that relies on a washer or seal for its life. Whether to fix into the sheet or the purlin is a structural decision, not an installer's preference.

The wider trade-off between the two roof types is set out in our standing seam versus trapezoidal roof sheet comparison.

The purlins carry the array, and the wind

The roof's purlins and girts were designed for a particular set of loads: the roof's own weight, maintenance load, any collateral load allowed for services, and wind. A solar array adds dead weight, adds point loads at every bracket, and changes how wind acts on the roof. Panels catch wind, and suction at roof edges and corners pulls through the clamps into the sheet and the purlin.

A structural engineer should check the purlins, their connections, the rafters and the bracing for the actual array layout. That needs the original design basis and drawings, or where those are missing, a survey of the members as built. Wind loading is assessed under IS 875 (Part 3) for the specific site and building, and cold-formed purlins are typically designed under IS 801. This article gives no load or wind values; they are the engineer's to set. Some roofs pass as they are, some need local strengthening, and some arrays need re-planning to keep off the weakest zones.

Warranty, walkways and leaks at penetrations

The roof warranty. Sheet and coating warranties often carry conditions about fixings, penetrations, foot traffic and contact with dissimilar metals. Ask the manufacturer in writing whether the proposed mounting is acceptable under the warranty.

Maintenance access. Panels on dusty industrial sites need regular cleaning and inspection, so people will walk the roof for the life of the plant. Planned walkways, safe access points and fall protection stop that traffic damaging the sheet. Skylight sheets cannot take a person's weight, so arrays and routes should keep clear of them, and the array should not shade them or block ridge and turbo ventilators.

Cable penetrations. DC and AC cables have to come off the roof. Routing them through a few properly flashed entry points, rather than many sealant-filled holes, removes the most common new leak a solar project creates.

Electrical safety and records. The installer's design, commissioning tests and handover documentation for a grid-connected system are addressed in IEC 62446-1. The project's electrical consultant confirms what applies.

Do the roof works first, and record them

Where the checks find work to do, the order matters. Leak repairs, replacement of aged fasteners and washers, re-sheeting of corroded areas and any purlin strengthening are far easier on an empty roof than around a live array. Once panels are in place, a leak under them is hard to find and harder to reach, and the plant has to be partly switched off for the repair.

Record what was done. The structural check, the agreed fixing method, the roof manufacturer's written position on the warranty, the array layout and the cable entry details all belong in the building's handover file. The next person to work on that roof, whether a roofer, a solar maintenance contractor or an engineer assessing another added load, will need them. A design-and-build contractor should be able to coordinate the roof works and the solar installer's requirements as one package rather than two separate jobs that meet on the roof.

The DISCOM framework

Grid-connected rooftop systems connect under the CEA (Technical Standards for Connectivity of the Distributed Generation Resources) Regulations, 2013, with net metering, gross metering and capacity limits set by each state's electricity regulatory commission and administered by the DISCOM. Capacity permitted, the metering arrangement, inspections and timelines vary by state and by connection. Confirm the route for your site with the DISCOM and your electrical consultant before the system is sized, because it can change the size that makes sense.

Common mistakes

• Signing the solar contract before inspecting the roof. The sheet's life and the purlin capacity then become variations.
• Assuming the PEB was designed with spare capacity. It may have been designed close to its original loads.
• Through-fixing a trapezoidal roof without a sealing method agreed. Hundreds of new leak paths.
• Clamps that do not match the seam. Or that lock standing seam sheets together.
• No walkway plan. Cleaners walk on the sheet and through skylights.
• Cable entries sealed with mastic. The first monsoon finds them.

What to ask before rooftop solar on a metal roof

• How much life is left in the sheet and coating, and has the whole roof been inspected?
• Does the clamp or bracket suit this sheet profile, and has the roof manufacturer agreed it?
• Has a structural engineer checked the purlins and frame for this array layout and wind?
• Will the roof warranty survive the installation, in writing?
• Where are the walkways, access points and fall protection?
• Where do cables leave the roof, and how are those entries flashed?
• What has the DISCOM said about capacity and metering?

Standards referenced

Wind loads under IS 875 (Part 3):2015; cold-formed steel members under IS 801; documentation, commissioning tests and inspection of grid-connected PV systems under IEC 62446-1; grid connectivity of distributed generation under the CEA (Technical Standards for Connectivity of the Distributed Generation Resources) Regulations, 2013. The structural adequacy of a specific roof, the wind design of the array, the fixing method and the grid connection must be confirmed by the project's structural engineer, electrical consultant, roof manufacturer and the DISCOM.