Earth Worm Found on a 30m High Clean Roof Slab After Rain

He was looking for high spot to get out the water that flooded his home. Your roof looked safe.

Don't know what the support structure of your home is could have been residing in one of the cracks and crevices. Which would collect soils and dead plant matter deposited by the wind and rain. SimpleMind's explanation of a bird as how he got there in the first place is my best guess also.

Here it rains cats and dogs. Clearly it rains worms in your location.

I found this.

Over here we discover them by the can.

It may not apply here. My experience with earth worms is this. My house set on a concrete slab with a concrete walkway. Flower bed and dirt from front door 1 meter max.

After heavy rains or several days of rain "ground saturated" the sidewalk would be covered. How or when they figured out to do this... The aluminum screen door they would work there way up the hinge side of the door in the space of the edge of the door and face of the frame to the top. Then when you open the door they would "rain down on you" from the top. The ones in/on the side would fall out. All required you to sweep them out. The only space I could find was a wear/tear spot on the sweep on the bottom of the door.

In your building could there be any moulding or trim or some other path that could lead to the roof without causing problems or leaks on the interior? Just a thought.

But I agree a bird could have dropped it. May be just as you came up it left and you did not notice.

When pigs can fly, so can worms....