This is very unusual and unsafe for a contractor to vent a 2nd floor dryer vertically through the attic to a roof vent. We see dryer vents from time to time that have this amount of lint build up. It is more typical for a dryer, at any level of the house, to be vented horizontally out the exterior wall. There is less resistance for air and un-filtered lint to find its way out of the vent pipe when the dryer is vented horizontally.
The dryer is designed to exhaust the heat and moisture while the dryer is drying. If the heat and moisture cannot get out of the dryer then it has to work harder and heat builds up in the dryer and vent which can lead to a fire. Statistics show that clogged dryer vents are the number one cause of house fires.
The first sign that you might have a clog is when your dryer stops drying your clothes in one cycle. Another sign is if the clothes seem unusually hot to the touch after a drying cycle or if excessive heat is noticed in the room where the dryer is located.
Be sure to have your dryer vent inspected often and if required, have the lint cleaned out. It is also important to remove the lint from the dryer filter after each laundry load to prevent build up in the vent pipe.