I don't see how a roof can be "open". Does it refer to a skylight or something like that? The translation sounds really odd. This is not a criticism, I would just like to know when a roof could conceivably be "open".
Perhaps in a domed stadium with a retractable roof. Those are common in the U.S. They open the roof when the weather is good and close it when it is bad (rain, snow, hot, cold). Or maybe it could mean a stadium with a hole in the roof (that is supposed to be there).
"Light" could be used for "ανοιχτή" only when talking about the roof colour. For example, "Το χρώμα της σκεπής είναι ανοιχτό κόκκινο" (The roof colour is light red) or "Η σκεπή είναι ανοιχτόχρωμη" (The roof is light-coloured). :)