This was for sure one of my favorite adventures, but it's tricky to duplicate. To access the roof, you've got to walk in the front doors of the Tannery, which is an office building. A really, really nice office building. Once in there, you can take the stairs to the sixth floor and go into the (unlocked, almost always) boiler room. There's a window in there where Riley and I rigged the screen, so it comes off if you just pull. Then, climb through the frame and descent onto the roof. Voila - you're welcome.