When the dryer vents through the roof - very difficult at best.
This has become more and more common in recent years. And it is a pet peeve of mine.
Dryer vents through the roof have built-in problems. Homeowners are set up for problems from the start.
And this is the problem - dryer vents always clog.
Sooner or later!
They must be cleaned regularly.
However, when they vent through the roof they are essentially out of sight.
Out of sight, out of mind.
And I find that many homeowners have NO IDEA their dryer vents through the roof, or that they need regular cleaning!
Why am I peeved by the roof venting technique?
Common sense!
1. It is hard to see. Being on the roof sometimes makes it well over 100' from where it is visible from the ground.
2. Being on the roof, sometimes very high or on a very steep location, it is difficult at best to clean. 
The homeowner has to schlep up there, or hire someone else to do it. Then the contractor must be trusted to actually clean the opening well.
3. The vent through the roof has the exhaust opening very close to the roof surface. A light snowfall can stop it up!
To me this seems to be common sense. But not to others, especially to builders!
But what's other the problem on this roof?
Unfortunately here the "ventilation professional" did not understand that bathroom vent covers SHOULD NOT be used for dryers.
Key words: "ventilation professional."
Can you see the clog inside?
Obviously that is full of lint.
This house is 11 years old. What do you bet there is 11 years of lint build up in there!
People ask HOW OFTEN their dryer vent should be cleaned.
The answer is simple. When it looks like there is lint build up!
And it depends! It depends on what kind of clothing people wear, how many people are in the house, how often they do wash, etc. It depends on lots of things!
But when the dryer exhausts onto the roof it is plain and simple:
DIFFICULTY IS BUILT IN!
So, with all this difficulty why put the dryer vent through the roof?
Because it is easy. It is quick. It is more convenient.
For whom?
Think carefully.
My recommendation: if you see that your dryer vents through the roof, then, after pulling your hair out, check it regularly! And make sure the exhaust port is not one made for bathrooms! Then, if it needs cleaning, clean it! This is according to the National Fire Prevention Association, which article you can read HERE in its entirety: "In 2010, clothes dryers and washing machines accounted for 4.5% of all reported home structure fires, 1.9% of associated civilian deaths, 2.8% of associated civilian injuries, and 3.1% of associated direct property damage." Reason enough.
Jay Markanich Real Estate Inspections, LLC 
Based in Bristow, serving all of Northern Virginia.
Office (703) 330-6388 Cell (703) 585-7560
