Also, after a bunch of times playing it on casual, I would suggest clipping that yellow building's roof entirely. I often see one of the teams taking position there, which makes other team's efforts that much harder (fighting from low ground is a big disadvantage). Defending team is just forced to stand near their health and ammo spots.
You can argue that a decent sniper can snipe foes on that roof, but power imbalance is everpresent in casual. There may be no snipers on one of the teams, or no good enough snipers, or people camping on roof can be too good, etc.
If clipping the roof seems too radical, maybe placing func_nobuild there? It won't fix the problem, but at least there will be less wrangled sentries placed there.