I have a hard time believing that he could get on the roof and off without being noticed and then get up the street to Leisure World and set up his next shot 25 minutes later. Anythings possible I guess... Just had a thought. Wasn't there a witness at the gas station that saw the white truck leaving? If so, then he probably didn't do it from the roof, because he would have to exit from the back of the building. If the truck is parked behind in the alley, then the witness at the gas station wouldn't see the truck leaving.
There are a lot of those white trucks, but if they saw one leaving quickly, chances are the shooter was in a nearby parking lot.
I think the police have to check the roof because they couldn't find anything in the parking lot and they want to cover all their bases. I would love to get a picture from the gas pumps looking out across the street to see what is visible and the bench at Leisure World (victim #4) but, alas, I'm 300 miles away...