Why metal sheds snow the way it does
A metal panel's smooth, low-friction surface lets accumulated snow release and slide as a sheet rather than melting and refreezing in place the way it can on a rougher shingle surface. That's the mechanism behind metal's snow-shedding reputation, and it's also exactly what makes uncontrolled sliding snow a hazard below the eaves.
The hazard snow guards address
Industry guidance on the topic is blunt about the stakes: sliding snow and ice from a roof can damage property, injure people below, and overload a lower roof it lands on. Snow guards exist specifically to break up that slide into smaller, controlled releases instead of one large one.[1]
Why attachment method is harder than it sounds
Attaching a snow guard without penetrating the panel is a genuine engineering challenge. Documented testing of different methods found that some adhesive-based systems failed to survive even a single winter, while certain mechanical fasteners held for a couple of seasons before pulling loose under a heavier snow load.[1]
The same review noted that sliding snow in a roof valley can be forceful enough to roll a standing seam onto its side, compromising the seam's seal in exactly the spot the roof depends on most.[1]
What this means for your installation
Because no single attachment method has proven universally reliable, snow guard placement and hardware should be discussed as a specific decision for your roof's pitch, panel profile, and typical snow load rather than treated as an afterthought added post-installation.[1]
Ice at the eaves is a related but separate problem
Snow sliding off the field of a metal roof is a different mechanism than an ice dam forming at the eaves, which happens when heat escaping through the attic melts snow higher up and the meltwater refreezes once it reaches the colder overhang. A metal roof's smooth surface changes how snow sheds, but it doesn't by itself prevent an ice dam caused by attic heat loss.
Self-adhering ice-and-water membrane at the eaves, the same product used under asphalt shingles, is still worth discussing with a metal roofing installer in a snow-prone climate, independent of whatever snow guard plan you choose for the field of the roof.
Planning ahead rather than retrofitting
Deciding on snow guard type, spacing, and row placement before the panels go on gives the installer more attachment options than trying to retrofit guards onto a finished roof later, when some mechanical methods become harder to execute without risking the existing panel's seal. Raise it as part of the original quote conversation rather than as a follow-up question after the roof is already finished.