So is the chimney guy saying that hardwood burning is ok only...

So is the chimney guy saying that hardwood burning is ok only reason I ask is that I have a wood heater and there is always black flakey looking stuff from the flu sitting on the roof especially when there hasn't been any rain for 3 months. BTW I only use hardwood. Often wondered about the fact that when it does rain that sooty material ends up in the tank.