One thing I've noticed about my new roof mount is how flimsy the sheet metal is up there. I put the mount in the flat spot between the stiffening ribs in the roof, giving it about 6" of flat, un-reinforced sheet metal between the ribs and nothing ahead or behind it. (it looks pretty good
When I put a tall, stiff, and rather heavy antenna on the mount and moved it back and forth (as the wind might do while driving), I saw how it flexed the roof's sheet metal. This caused me to wonder what would happen if I accidentally drove into a garage or drive-thru with a stiff (no-spring) antenna. I'm guessing that it would be bad news for both the antenna and the roof.
For now, I have a stubby, 12" antenna up there, so I don't have to worry. But, I don't think I want to drive with anything longer that isn't spring loaded.
I have seen people use circular saw blades to reinforce the roof using SO-239 type mounts....