Dryer vents normally are not in excess of about eight feet and terminate through a wall to the outside.There,a standard dryer vent with cover is installed.Now that I think about it,I may have seen these on a roof before but it is not too common.You can pick these up at home improvement stores for a few dollars.Install the vent flush with the pitch of the roof and use plenty of caulking or roof cement on the underside which rests on the roof.

Most dryer vents have a hinged cover that is supposed to keep stuff outside from coming in. If this is not working, you should replace or repair it. It may be, however, that the leak is around the edges, where the pipe comes out of the roof. If this is the case, seal it with caulk or roof sealer on the outside - just sealing from the inside will allow rain water to get under the shingles and let the roof plywood get wet and rot.
I just copied the following statement from another post. (Some caulk is silicone.)
"never use silicone on asphalt or fiberglass shingles, silicone will damage the shingles, I have seen silicone eat a hole clear through a shingle."

[This message has been edited by rpxlpx (edited November 17, 2000).]

While silicone based caulk is very common there are others such as asphalt based,Nitrile,Neoprene,polybutane,rubber,etc.To be sure,I would consider roof cement if unsure of the application.