We recently inspected this house that had a new roof put on. Up in the attic they had used metal flexible vent for the dryer line and combined it into the bathroom exhaust line. There’s a few problems with this. First leave the dryer vent line in the attic should be constructed of smooth rigid metal so that lint does not get caught on the interior ribs which can lead to a dryer fire. Secondly, the bathroom and the laundry needs separate vents through the roof so that the dryer does not force humid air back down into the bathroom.