Crane Rails and Rooftop Solar on Structural-Steel Shops

Structural-steel fabrication shops make some of the best rooftop-solar sites in the country. They tend to occupy large clear-span sheds, the roofs are big and cheap per kilowatt, and the daytime load from drilling lines, saws, shot-blast plant and heavy welding lands neatly under the generation curve. But they also carry a constraint no other fabrication trade does to the same degree: the overhead electric travelling (EOT) crane and its rails hang their weight off the same steel frame you want to load with panels. Get the roof-load budget wrong and the array is either refused at survey or, worse, signed off on optimistic numbers. This guide explains how crane rails, gantry loads and roof trusses shape the panel layout and the residual roof-load budget on a heavy-lifting fabrication shed.

Why crane loads change the whole calculation

On a portal-frame shed with no crane, the roof structure carries its own dead weight, wind and snow, plus a modest allowance for services. Adding a solar array is usually a question of whether the frame has enough spare capacity for roughly 15 to 25 kg/m2 of framed PV dead load plus the wind uplift that comes with it.

A structural-steel shop is different. The EOT crane rails are typically carried on gantry beams bracketed to the columns or the crane legs, and that crane, its rails, the gantry beams and the dynamic loads from lifting and travelling are all reactions the frame is already resolving. Every kilonewton the crane system imposes is capacity that is no longer available for anything you bolt to the roof. In practice this means the crane loads are deducted from the roof’s residual capacity first, and only what is genuinely left over forms the budget a PV array can draw on.

That is the single point competitors miss when they treat a fabrication shed like any other warehouse: on a heavy-lifting bay, the residual roof-load budget after the crane can be materially smaller than the raw roof area suggests.

What actually eats the budget

Several things compete for the same finite frame capacity:

• Crane dead load - the rails, gantry beams, end carriages and the crane bridge itself, permanently present whether the crane is lifting or parked.
• Crane dynamic and impact loads - the vertical and lateral surge from lifting, braking and travelling, which codes amplify above the static figure.
• Existing services - LEV weld-fume ductwork and discharge stacks, compressed-air lines, lighting rafts and any mezzanine hung from the roof steel.
• Wind uplift on the new array - panels are aerofoils; a framed array adds suction as well as weight, and the uplift case often governs the fixing design more than the gravity case.
• Snow and maintenance access - the code-mandated imposed loads that must still be carried with the array in place.

Only once all of that is accounted for do you know the residual budget for the PV dead load itself.

The residual roof-load budget, in numbers

A framed rooftop PV array on an industrial roof typically adds around 15 to 25 kg/m2 as a distributed dead load, before wind. To put that against the other numbers a structural engineer weighs on a steel shop:

Load component	Typical order of magnitude	Notes
Framed PV array dead load	~15 to 25 kg/m2	Panels, rails, ballast or fixings, distributed over the covered area
Wind uplift on the array	Site and height dependent	Often governs the fixing, not the frame; suction, not weight
EOT crane system	Deducted first, per crane rating	Rails, gantry beams, bridge, plus dynamic amplification
Existing LEV and services	Site specific	Ductwork, stacks, air lines, lighting

These PV figures are consistent with the sector guidance we work to; the crane figure is entirely site-specific and comes from the crane rating and the original frame design. The order of the sum matters: crane and services come off the top, then you see what is left for the array.

Ballast versus penetrating fixings

How you fix the array interacts directly with the roof-load budget, and on a crane shop the trade-off is sharper than usual.

Penetrating (rail-fixed) systems bolt through the roof sheet into the purlins. They add the least dead weight because they rely on the frame for hold-down, and they suit trapezoidal and profiled metal roofs, which is what most structural-steel sheds have. The catch is that every penetration is a potential water path and each fixing transfers uplift straight into the purlin, so the purlin and its connections have to be checked, not just the main frame.

Ballasted systems sit on the roof and resist uplift with weight, typically on single-ply membrane or flat roofs. They avoid penetrations, but ballast is exactly the thing you have least room for once the crane load is deducted. On a heavy-lifting bay a ballasted approach can be a non-starter simply because the residual budget will not carry the kilograms.

On most structural-steel shops the answer is a penetrating rail-fixed system engineered to the purlin capacity, with the layout thinned or zoned away from the areas of the frame most heavily loaded by the crane. It is a structural decision as much as a solar one, which is why it cannot be settled from a satellite photo.

Layout follows the frame, not the roof plan

Because the crane loads are not uniform across the shed, the residual capacity is not uniform either. The bays around the heaviest-lifting runs and the frames nearest the gantry brackets have less to give than the quieter end of the shop. A competent design responds by shaping the array to the structure:

• Concentrating panels over the bays with the most residual capacity, and thinning or omitting them over the heaviest crane runs.
• Keeping the array clear of the LEV fume-discharge stacks and maintenance walkways so the two roof systems never clash. We cover that coordination in detail in our guide on designing solar around weld-fume extraction.
• Leaving access and firefighter walkways per the RC62 rooftop-PV fire-safety code, which insurers increasingly make a condition of cover.

The result is often an array that covers less than the full roof but is properly grounded in what the frame can actually carry, which is a far better outcome than a full-roof design that fails at structural sign-off.

Structural engineer sign-off is non-negotiable

For all these reasons, a chartered structural engineer’s assessment of the existing frame is essential on any structural-steel shop, and it is the item we commission first, alongside the G99 grid application. The engineer works from the original frame design or a measured survey, adds up the crane, services and code-imposed loads, and reports the true residual budget the array can use. Only then is the PV layout finalised. This is standard structural practice; the framework for steel-building assessment is set out by the industry body at Steel Construction Info (BCSA/SCI), and the CDM 2015 regime makes you, as the building owner, the Client with a duty to provide pre-construction information such as the crane rating and any fragile-roof or asbestos risk.

Doing the structural work up front also protects the commercial case. A structural-steel plant typically supports a 150 to 500 kW array with a payback around four to four and a half years, so the numbers are strong; the survey simply makes sure the size you cost is the size the roof will carry. You can see how that flows into the wider cost picture and the capital allowances and business-rates relief that make on-site solar attractive for a fabricator, and request a modelled figure through our quote form once the residual budget is known.

The takeaway for a structural-steel fabricator

A crane shop is a genuinely excellent solar site with one honest caveat: the crane comes first in the load sum. Deduct the EOT rail and gantry loads, account for the 15 to 25 kg/m2 array plus wind uplift, choose fixings that respect the residual budget, shape the layout to the frame, and get a structural engineer to sign the whole thing off before a single panel is ordered. Handled in that order, the big clear-span roof that makes a structural-steel shed a good place to fabricate steel makes it an even better place to generate power.