If you are getting a leak into your attic or inside your house from a dryer vent, maybe the vent on the roof is not really made for dryers. The lint from dryers will eventually clog up the roof vent and not allow the moisture to escape properly. The moisture will then be forced back down the pipe joints and out on to the sheetrock ceiling below or on to the plywood sheathing under the shingles.
This is what a new dryer exhaust roof vent looks like.
The picture below shows what a bath fan vent looks like when dryer lint accumulates in it and gets clogged up.
Here is another bath vent wrongly used as a dryer vent cap on the roof: