A sandwich panel is two steel sheets glued to a thick core. When something presses down on it, the panel bends. How much it can carry before bending too far depends on four things.
Why panels bend instead of break
Press down on a sandwich panel and it does not snap. It sags.
The top steel sheet gets squeezed. The bottom steel sheet gets stretched. The core in the middle holds them together and stops them from sliding past each other. If the core is strong enough to keep the two sheets working as a pair, the panel stays stiff. If not, the sheets start moving independently and the panel sags fast.
So the limit is not "when does it break." The limit is "how much sag is acceptable." For roof panels, the standard allows a sag of L/200 - meaning a 4-meter panel can sag 20mm. For wall panels, it is L/150. Past that, the panel is considered to have failed its job, even if it is still in one piece.
The four things that change the answer
Panel thickness
A 100mm panel carries much more than a 50mm panel of the same core. The thicker the core, the further apart the two steel sheets sit, and the harder it is to bend them.
Core material
A dense rockwool core (100–120 kg/m³) holds the two steel sheets together better than a light EPS core (10–12 kg/m³). The core's job is to stop the sheets from sliding, and some cores do that better than others.
Span
This is the big one. Double the distance between supports, and the panel becomes sixteen times easier to bend. A panel that works fine at 3 meters may sag badly at 6 meters.
Steel thickness
The steel sheets carry the bending force. Going from 0.3mm to 0.5mm steel makes a real difference to how much the panel can take.
The frame usually gives way first
A point that surprises most buyers: the building frame often fails before the panel does.
Research on industrial buildings with sandwich panel roofs found that when roof load goes about 11% above design, the main structure is already at its limit. At around 36% above design, major structural damage starts. In plain terms - if something heavy lands on the roof, the frame can collapse before the panel shows any problem.
So the panel's strength is rarely the weakest link. The purlins, the frame and the foundation matter more.
