A Dry Rotted Seal
The dry rotted sealant around this vent was causing nail pops. The nail pops were lifting the surrounding shingles, creating an entry point for water.
On their own box vents aren't the most efficient type of ventilation. Mixed with a ridge vent they can work quite well.
Just sealing the area around a vent pipe won't fix the problem. As you can see with the sealant around the vent several attempts were made to fix the leak around the vent. It was only a bandaid.
Wood Rot caused by Leaking Vent
The wood around this old vent had been leaking for so long the decking around it needed to be replaced.
Klaus Roofing Systems of Cincinnati Improves a Petersburg, KY ventilation System
Once all of the decking was replaced we put on brand new shingles and properly sealed the vents to the roof. Now Bonnie has a leak free roof that looks really elegant.