Why penetrations leak first
A roof is a sloped, water-shedding surface. Anywhere you cut a hole in that surface, you're trying to make a waterproof connection between two different materials (typically metal flashing or rubber boots against asphalt shingles). Different materials expand and contract differently with temperature changes, and the sealants and gaskets that bridge them have shorter lifespans than the shingles themselves. So the penetration always wears out first.
Plumbing vent stacks (rubber boots)
Every bathroom and kitchen drain has a vent pipe that exits through the roof. These pipes are sealed with a rubber 'boot' that slides over the pipe and is layered into the shingles. The rubber dries out and cracks from UV exposure — typical lifespan is 15-25 years, less in hot/sunny climates. Inspect: look for cracks, splits, or visible daylight where the boot meets the pipe. Cost to replace: $100-300 each, takes a roofer 30 minutes.
Chimney flashing
Chimneys have the most complex flashing on the roof — step flashing along the sides, base flashing at the bottom, counter-flashing tucked into the chimney mortar joint at the top, and sometimes a cricket (small saddle) on the uphill side to divert water. Bad chimney flashing is responsible for more leaks than any other single roof feature. Inspect: look for caulk-only repairs (always failing), missing or rusted step flashing, and cracked mortar joints around the counter-flashing.
Skylights
Skylights leak from three places: the seal around the glass to the frame (manufacturer's job; lasts 15-25 years), the flashing kit around the curb (installer's job; lasts 20-30 years if installed correctly), and the flashing-to-shingle interface. Inspect: look for water stains on interior walls or ceilings near the skylight, condensation between glass panes (failed seal), and any visible gap or caulk repair in the curb flashing.
Attic vents (turbines, box vents, ridge vents)
Attic ventilation comes in several forms — turbines (the spinning ones), box vents (low-profile boxes), ridge vents (continuous along the ridge), and gable vents (in the gable end walls). Ridge vents are the most reliable; turbines are the most failure-prone (the bearings seize and the cap leaks). Inspect: look for cracked plastic, missing screens (rodents/insects), visible water staining around the base flashing.
Solar panel mounts
Solar panels are mounted with brackets that penetrate the roof deck. Each mount point is a potential leak. Quality installations use flashing-integrated mounts (lifespan ~30 years); cheap installations use deck-screwed mounts with sealant only (lifespan ~10 years before resealing required). Inspect: look at each mount point through binoculars for visible flashing or just sealant.
Satellite dish and antenna mounts
Old satellite TV dishes mounted to the roof are almost always leak sources, especially after the dish is removed. The bracket holes in the deck typically just got caulked over and the caulk has long since failed. If you have an unused dish, get it removed AND the holes properly flashed.