Sometimes a badly placed ladder has to be destroyed to make place for a better one or to prevent less observant teammates from wasting their time with it.
Greifers could easily just place a ladder in the doorway of one of the harder maps and ruin the game for the attackers. Maybe cut the damage into quarters, so that it would be rare for them to be destroyed by a mishit or something.
Sometimes a badly placed ladder has to be destroyed to make place for a better one or to prevent less observant teammates from wasting their time with it.
Roofcampers only have to come down once all other enemies are dead.
So it's generally not smart to do this. Once people figure out that only enemies can destroy ladders, this behaviour will change.
I doubt it, shielders might not destroy them, but two handers would still be more likely to break the ladder and hide till they come down than try to charge up. And they're the ones that can take the ladder down in a couple hits.
If all the above possibilities are stricly impossible, then i wouldn't mind taking personnaly a look to ALL the maps in the battle server current rotation, and add an invisible wall preventing access to ALL roofs and other places unreachable by foot.I highly doubt they're going to let you remove access to roofs. However, if you're really willing to personally look at all the maps in rotation, you could probly convince them to let you add drop ladders for every campable area. Since drop ladders can't be destroyed, anywhere they're set up will become reachable. However, this would also mean nobody has to bring ladders to battle and has more free slots.
On the 90 degree idea: What if someone on the attacking team (accidentally or otherwise) places a ladder parallel to the front of a door so the ladder passes 90 degrees and settles in a way that blocks off the middle of the doorway making it impassible for the attackers?