Ladders, ladder spam, teamwork, MotF, Jumping horses, sniping, not getting shot, having mad skillz, ingenuity, QMD the map, kill all the groundpounders to force the roof campers to jump etc.
To get back a bit on topic...
Ladders=they break
ladder spam=same
teamwork=? What is that on a public game
MotF=can work except those times where the flag is by the roof
Jumping horse=yeah get me horse that can reach a roof
Sniping=can work, but their archer have the advantage of not having to fear cavalry, so advantage roofers
not getting shot=they're alive too
mad skillz=can work if you're lucky and noone watch the ladder otherwise you can be the best meleer in the world that wont do squat
ingenuity=map dependent
QMD=one whole team wont QMD it
kill all=unless there's an admin that actually kick the roofers, that does not do anything
And don't you dare say "then use an item you dont have that people using a roofs force you to" because the instant you demand that I change my playing style, I get to as well and tell you "Up yours, change your style if you're forcing me to change mine"