20lbs is nothing for a strong gust of wind on a roof.
Yes, in a slightly different situation this would be an issue. But, wind is not magic fairy dust that can lift arbitrary loads.
Just stop with the flawed logic.
My point is the specific bad thing described is unlikely to happen. And sure I got a lot of knee jerk reactions but nobody actually did any kind of meaningful calculation demonstrating an issue.
Your sole argument is, historical wind speeds won't lift it. That is not realistic, naive and outright dangerous thing for which people actually get badly hurt or die.
But, I don't see you slowing down in this thread, so I doubt safety concerns will reach you.
Followed by a knee jerk reaction without any facts backing it up...
> That is not realistic, naive and outright dangerous thing for which people actually get badly hurt or die.
PS: Potted plants are often placed on the top of flat roofs with a lip and don't get tossed off it by the wind. The reason has to do with how the wind is deflected over the building and the obstructions around it. Often pushing them in opposite directions from the wind because of a giant counter rotating vortex of air. But, be happy with your inaccurate and over simplified model of how the world works.