Typically, in a one story house, if the ceiling joists are running parallel to the wall it is not load bearing, but this isn't always true. Assuming you meant ceiling joists vs. rafters (the slanted 2x lumber that the roof sheathing is nailed onto), I would say your wall isn't load bearing. The safest thing to do would be to spend the money on a structural engineer, but if you add the header you will be erring on the side of safety without spending the $$ on the SE.