My knee-jerk reaction is to say plastic sheeting, but that can block air flow. I suggest cotton duck fabric stapled or hung at an angle to the ceiling joists. If leaks are a regular problem, put a weight on the lower edge to lead the water into drainage. This can be as simple as tying some washers to a string and securing the loose end to the middle of the fabric sheet edge. Dangle the washers into a PVC or flexible pipe and the water will follow accordingly.
Cotton duck should be available at any craft store selling fabrics – just call and ask first.