Will Creosote poison roof collected drinking water?

Creosote causes neurological problem if inhaled in higher concentrations, so while being diluted from the water over time it might cause problems. Of course it all depends on the strength of the preservative also and the concentration that actually gets into the water.

Your best bet is to run any water collected through a filter if you plan to drink it. If you're using it for a shower or non-internal use it should be okay.