I suppose it would be ideal to use an antenna that's about 2-3 inches (or whatever feels safe enough to you) above the roof. Or perhaps have a mark on the antenna at the acceptable height. Does such an antenna exist?
Would also be cool if there was some sort of roof radar collision warning system. Something that could be adjusted to accommodate those who have after market whatever's on the roof.
I went with a 24” antenna from Amazon. At 24”, it protrudes about 2” past the highest point of the roof, which is a really good indicator for when you should be cautious about entering a garage.