Great Question and thanks for taking the time to inquire here.
Code requires a termination for a dryer to have a damper, and no screen.
Venting to the roof is very popular especially in the souther states.
Venting to a flat roof; options are not always clear.
If the flat roof, or the location of the vent on the flat roof will not have any chances of greater than 2" of puddling, then some of the larger gooseneck vents will work fine as they usually incorporate a 2 to 3" vertical riser within the gooseneck, that is sealed with silicone to the flashing pad. The very large 8" x 8" hood goosenecks are eye sores from the road and the damper is very heavy to lift up which creates some resistance. A new vent called the DryerJack Zero which came out in January of 2012 is pretty impressive. It has the raised collar so it works fine on flat roofs, and it is pretty dang efficient. It has a 6x8 inch structure in a gooseneck style.
For flat roofs with the potential of more than 2 inches of flooding due to scuppers or drains getting clogged, a vertically extended vent will be required. The vent you found is not the best for venting the dryer as it is hard to clean, hard to see if it needs cleaning, and has lots of obstacles that promote lint build-up. But it would work for your bath fans.
The DryerJack Zero (6x8 inch gooseneck) introduces an industry first, a curved damper. This vent is so fine tuned that it creates almost zero back pressure. It works on flat or sloped roofs and is now available at this link:
http://store.dryerbox.com/DryerJack-Zero--Gooseneck-Roof-Vent-4_p_83.html This link for more information:
http://www.dryerjack.com In either case, make sure you watch the video right under the pictures in the "store" link, called Pressure Testing of 7 Dryer Roof Vents in HiDef
If you remember, please revisit this thread and update on how the install went so that others can learn from your experience. Thanks
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