As soon as I'm in charge, there will be no more venting of clothes dryers through the roof. I think the roof is a stupid spot to terminate clothes dryer vents because it's difficult to clean and it's a major contributor to ice dams. Clothes dryers should be vented through the side of the house, preferably within reach from the ground.
Ice Dam Contributor
When bath fans and clothes dryers vent through the roof, they melt a bunch of snow. After the snow melts below the vent, it'll probably freeze again farther down on the roof. If this happens enough, an ice dam will form that's large enough to cause roof leaks. These areas of the roof are especially susceptible to roof leaks because there is no ice and water shield installed in these locations. I blogged about this earlier this year: Advanced Ice Dam Prevention.
Tough to Clean
As I mentioned in my blog post about keeping your clothes dryer safe, the terminals for clothes dryers need to be cleaned on a regular basis. These terminals can get extremely dirty with lint over time, and eventually, the dampers get stuck open.