I'm not really sure at what point a wall transitions from "a kinda tilty vertical" to "a not-too-bad overhang"

Anything that's more-than-vertical is technically an overhang, but overhangs can be more or less "steep".

If it follows the pattern, I suppose "horizontals" would make sense.

But instead, they're "roofs". *g*

Once you have the strength to stay on at all, roof climbing is exhausting but ridiculous amounts of fun.