This might be intentional, but Graves can place her ult on top of the buildings that deal shock damage. McGinnis turrets can also be deployed up there, but they die quickly due to taking the damage from the shocks. Graves ult only takes damage from heavy melee so it is unaffected by being on the shock damage roof. This may just be skill issue, but it seems unintentional as a mechanic since the roof is designed to keep players from being there for extended periods, but this ult is unaffected. The zombies fall off the roof quickly so they aren't taking damage from the shocks.