Welding Fume, LEV and Rooftop Solar: What Fabricators Must Know

Most solar guides written for industry treat the roof as an empty rectangle waiting for panels. On a real fabrication shop it is nothing of the sort. It is already carrying the ductwork and discharge stacks of the one system you are legally obliged to run whenever anyone strikes an arc: local exhaust ventilation for welding fume. Get the two designed together and the array works around the extraction cleanly. Get it wrong and you either block a legally required fume route or find, mid-install, that half your best south-facing pitch is unusable.

This is the detail no generalist solar contractor checks and no competing sector site explains. Here is what a fabricator actually needs to know.

Why welding fume changed everything for fabricators

In February 2019 the HSE issued Safety Bulletin STSU1-2019. It reclassified all welding fume, including fume from mild steel, as a Group 1 human carcinogen following evidence from the International Agency for Research on Cancer. That single reclassification rewrote the control expectations for every welding shop in the country.

The practical consequences are strict and they are enforced:

• General ventilation is no longer enough. Opening the shutter doors or relying on roof ridge vents does not control exposure to a carcinogen. For any indoor welding you now need engineering controls that capture the fume at source.
• Local exhaust ventilation (LEV) is required for all indoor welding, regardless of duration or the material being welded. There is no “it’s only mild steel” exemption and no “it’s only a few minutes” exemption.
• Respiratory protective equipment (RPE) is required on top of LEV wherever the extraction alone does not adequately control exposure. RPE must be face-fit tested to the individual.
• All of this sits under the Control of Substances Hazardous to Health Regulations 2002 (COSHH), so it is a legal duty, not guidance.

You can read the reclassification straight from the source in the HSE Safety Bulletin on mild-steel welding fume. The point for a solar project is simple: LEV is not optional kit you might switch off to make the roof tidy for the installers. It is a running, mandated system, and it lives on your roof.

The 14-month rule that ties LEV to your roof for good

LEV is not fit-and-forget. Under COSHH, every LEV system must have a thorough examination and test (a TExT) carried out by a competent person at least once every 14 months. The report records the system is capturing fume effectively and stays on file.

That 14-month cycle matters for solar in a way that is easy to miss. The examiner needs physical access to the hoods, the ductwork, the fan and the discharge point. If your PV array has been laid across the roof with no thought for the fume stacks, you have quietly made a statutory inspection harder, more expensive, or in the worst case impossible without lifting panels. A well-designed array leaves maintenance walkways to every extraction penetration so the TExT, and any future duct or fan replacement, can happen without touching the PV.

Where the two systems physically collide

A shop-wide LEV system does not vent politely through a wall. The fume is drawn from capture hoods or on-torch extraction, ducted to a fan and filter unit, and discharged, almost always up through the roof via a stack, well clear of any air intake. That is exactly the plane you want to fill with solar panels.

The clash points to plan for are:

Fabrication roof feature	Why it is there	Effect on the PV layout
Fume discharge stack	Vents captured weld fume clear of the building	Keep panels clear of the plume and downwind fouling; maintain access for the stack
LEV ductwork runs	Carries fume from hoods to the fan/filter	Roof-mounted runs shade panels and block rows; route the array around them
Fan and filter unit	Moves and cleans the extracted air	Needs clear service access, not a panel field on top of it
Make-up air inlets	Replaces the air the LEV pulls out	Must not be shaded, blocked or contaminated by array components
Future extraction points	New bays, new hoods as work changes	Leave spare roof zones so tomorrow’s LEV does not mean lifting panels

The design discipline is to survey every existing penetration, ask the shop where extraction is likely to grow, and then lay the array out around all of it, with cable routes and walkways planned in. This is standard practice when the contractor understands fabrication. It is an expensive surprise when they do not.

Keep the extraction running through the works

Here is the operational trap. A rooftop PV install is construction work, and while it is happening you still have to weld, and if you weld you must have working LEV. You cannot shut the extraction down for a fortnight because scaffolders are on the roof.

The sequencing therefore has to protect the fume plant: stacks and duct runs stay live and unobstructed, roof access routes avoid the discharge points, and any temporary works never draw the installers into the fume plume. On sites where hot work continues below, we coordinate the roof programme around the extraction runs and the hot-work zones so cutting and welding carry on safely underneath. None of that is difficult, but it only happens if the extraction was on the drawing from day one rather than discovered on day one.

The upside: extraction is a solar load you should want

So far this reads like a constraint. Flip it round and the LEV becomes one of the best arguments for solar on a fabrication roof.

LEV runs whenever anyone welds, which on a busy shop is most of the working day. A whole-shop extraction system, fans and filters and make-up air heating, is a steady, obligatory daytime electrical load. It is not a spiky peak like the welder itself; it is an anchor load that sits there hour after hour, Monday to Friday, in daylight, which is precisely when the panels are generating.

That is the heart of why fabrication is such a strong solar fit. Your compulsory fume extraction, your rotary-screw compressor, your laser chiller and CNC coolant form a near-constant baseload, and single-shift daytime working means around 70 to 90 percent of everything the array generates is consumed on site at your full 25 to 30p import rate rather than exported cheaply. The law makes you run the extraction; solar lets you run a big slice of it on generation you already own. You size the array to that real load, not to the roof area, which is why we always size a fabrication system from your half-hourly meter data rather than a rule of thumb.

What a fabricator should actually do

If you are weighing rooftop solar and you run any indoor welding, the sequence is:

1. Confirm your LEV is compliant and current. You need working, source-capture LEV for all indoor welding, RPE where it is not enough, and a TExT within the last 14 months. If any of that is lagging, fix it first; it is a live COSHH duty independent of solar.
2. Get the fume plant on the solar survey. Every stack, duct run, fan unit and make-up air inlet, existing and planned, goes on the roof drawing before a single panel is placed.
3. Design the array around the penetrations with walkways. Maintenance access to every extraction point must survive the install, so the next TExT and any duct work can happen without lifting PV.
4. Protect extraction through the construction phase. The programme keeps LEV live and installers clear of the plume, so welding continues legally underneath.
5. Count the extraction as load, not just obstruction. Feed the LEV baseload into the sizing model. It is one of the loads that makes fabrication’s self-consumption so high and the payback so short, and it strengthens rather than weakens the case for a well-sized array.

Do that and welding-fume compliance and your PV design stop fighting each other. The extraction that HSE forces you to run becomes another meter the sun helps to feed.

Where this sits in the wider fabrication picture

Fume extraction is one strand of the compliance and load story that makes welding and engineering shops distinctive. The same coordinate-it-early thinking applies across the sector, from DSEAR-zoned powder-coat booths to EOT crane-rail roof loading. If you want the sector view, our welding and engineering workshops page pulls the electrical, structural and compliance threads together, and there is broader HSE background on the health case in the HSE guidance on welding-fume health risks.

When you are ready to see how it works on your specific roof and your actual welding load, we design the array around your extraction, handle the survey and grid application, and model the numbers from your meter data. Start with a free feasibility study and fixed-price quote and we will tell you honestly whether it stacks up.

This guide is general information on HSE, COSHH and solar design for fabrication, not legal, safety or tax advice. Confirm your specific welding-fume control duties with the HSE guidance and a competent occupational-hygiene adviser, and any tax treatment with your accountant or HMRC.