I only have AC 9. But even in AC 9, I drew a quick polygon roof, then, I selected the roof on the floor plan, and with it selected, chose SPLIT from the EDIT menu. The dialog box asks for a reference line to be drawn where the split will be. I drew the line and then the dialog asks to indicate slope. The split was executed. Good luck....I guess if it doesn't work, you can always just draw separate poly roofs instead of splitting the one...