# Rooftop Solar on a Metal Roof: Check the Roof First

> Before rooftop solar goes on a metal roof, the roof has to be checked as the foundation of the plant: whether the sheet will outlast the array, how panels attach to that sheet profile, whether an engineer confirms the purlins and frame for the added load and wind uplift, whether the roof warranty survives, and how people and cables will cross the roof without causing leaks.

## 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

| Check | Why it matters | Who confirms it |
| --- | --- | --- |
| Remaining life of the roof sheet and coating | The array may need removing to re-sheet later | Roof inspection, sheet manufacturer |
| Sheet profile and how panels attach | Decides whether the sheet is pierced | Roof manufacturer and mounting supplier |
| Purlins, rafters and connections | Added weight and wind uplift go through them | Structural engineer |
| Roof warranty conditions | Fixings and foot traffic can void it | Roof manufacturer, in writing |
| Access, walkways and cable routes | Cleaning and maintenance traffic for years | Designer and solar installer |

## 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](/blog/warehouse-roof-leaks-skylights-and-sheet-laps)
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](/compare/standing-seam-vs-trapezoidal-roof-sheet).

## The purlins carry the array, and the wind

The roof's [purlins and girts](/glossary/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.

## Frequently asked questions

### Can any metal shed roof take solar panels?

Not without checking. The purlins and frame were designed for specific loads, and a solar array adds weight and changes how wind acts on the roof. A structural engineer should check the roof against its original design basis or a survey of what was built. Some roofs are fine as they are, some need local strengthening, and some are better re-sheeted first.

### Is standing seam or trapezoidal sheet better for rooftop solar?

Standing seam roofs can often take clamps that grip the seam without piercing the sheet, which removes a large source of leaks. Trapezoidal sheets usually need fixings through the sheet, each one a penetration that must be sealed and maintained. The clamp must suit the exact seam profile and must not stop the sheet's thermal movement, which the roof manufacturer should confirm.

### What if the roof sheet is already old?

Compare the sheet's remaining life with the life the solar plant is planned for. If the roof will need replacing well before the array is retired, it will have to be removed and reinstalled, at a cost and with a shutdown of generation. Re-sheeting before installation is often the more economical decision, and the roof inspection should tell you which applies.

### Will solar panels void my roof warranty?

They can. Roof sheet and coating warranties often have conditions on fixings, penetrations, foot traffic and dissimilar metals. Ask the roof manufacturer or installer in writing whether the proposed mounting system and fixings are acceptable under the warranty before the array goes up, and keep the reply with the handover records.

### Who decides the wind load on the solar array?

The structural engineer, under IS 875 (Part 3), for the specific building, location, height, terrain and array layout. Edge and corner zones of a roof see higher wind suction, and panels add their own wind effects. The mounting supplier's data and the roof's capacity both have to be checked against that design. No general figure should be assumed.

### Do we need DISCOM approval for rooftop solar on a factory?

Grid-connected rooftop systems connect under the CEA connectivity regulations and the state electricity regulatory commission's metering regulations, administered through the DISCOM. Capacity limits, metering arrangement, inspection and timelines vary by state and by connection, so the process for your site should be confirmed with the DISCOM and your electrical consultant early.

## Sources

- [IS 875 (Part 3):2015](https://standards.bis.gov.in/website/published-standards/department-wise) — Design loads (other than earthquake) for buildings and structures - wind loads
- [IS 801](https://standards.bis.gov.in/website/published-standards/department-wise) — Code of practice for use of cold-formed light gauge steel structural members in general building construction
- [IEC 62446-1](https://webstore.iec.ch/en/) — Photovoltaic systems - grid connected systems - documentation, commissioning tests and inspection
- [CEA (Technical Standards for Connectivity of the Distributed Generation Resources) Regulations, 2013](https://cea.nic.in/regulations/?lang=en) — Connectivity of distributed generation, including rooftop solar, to the distribution system

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Source: https://hagerstone.com/blog/rooftop-solar-on-a-metal-roof-check-the-roof-first