Roof vent leak
I have a 5 year old roof with a leaking water heater vent. During the last huge rainstorm the vent leaked onto my on demand water heater. Under further inspection the vent appears to have some kind of roofing mastic around half of the circumference where two pieces form a seam. I'm guessing the mastic is now dried out and allowing water in or possibly the leak came from the seam area without the mastic (but I doubt is as I think it would have leaked in the last 5 years.) There was a lot of shoddy work done on the house so nothing would surprise me. Also, the lower skirt part of the unit is on top of the tube, that is the tube portion does not come over the skirt. It's the only vent on my roof that is like this so obviously more prone to leaking. Should I try to scrape the old mastic off and reapply and if so what would be the optimal material to seal the seam? This stuff doesn't look like it would be easy to remove. Any advice much appreciated.
Here is a link that might be useful: Vent pic