Google says residential joists are designed to hold 40-50 lb per sq ft. I think that's mostly in one direction, parallel to the joists, and some perpendicular distribution as well. A cubic foot of water weighs 63 lbs. You can do your own math.

While you're waiting for the roofer, you should put a large bucket in the attic to catch the dripping water so it doesn't cause anymore damage. Then try to dry everything before it starts molding.