The act of scaling and traversing rooftops secretly and illicitly. Best done under the cover of darkness on public property. The glory of skulking is derived from the adrenaline rush associated with trespassing, so there is no need to combine roof skulking with other illegal activities, i.e. vandalism.
Will: "Susan and Kate plan to skulk all buildings on their college campus prior to graduation!"
Josh: "Wow, what intrepid skulkers!"
Will: "But wait, aren't some buildings unskulkable?"