Bad Chimney Flashing
Good chimney flashing includes sections of “step flashing” that run up the sides of the chimney, and “counterflashing.” Counterflashing fits into grooves cut into the chimney and covers the step flashing. Cutting, fitting and installing all those parts takes time, so sloppy roofers take shortcuts.
Improperly flashed chimneys (top photo) cause lots of rotting roof sheathing and framing members. Chimneys need to be properly step-flashed and counterflashed so that water can't run down the face of the chimney and into the attic. You can't rely on caulk or roof cement to keep water out. If you suspect your flashing is shoddy, crawl into the attic after a heavy rain. Look for signs of water around the chimney and downhill from it.
Missing Gutter Apron
When water flows off the edge of your roof, some of it clings to the underside of the shingles and dribbles toward the fascia. If you have gutters but no gutter apron to stop the water, it will wick behind the gutter. Eventually the fascia, soffits and even the roof sheathing will rot. You may see water stains below the gutter on the fascia and soffit. This is a sure sign that the gutter apron is missing.
The best time to add gutter apron is when you're getting new shingles. But it is possible to slip gutter apron under existing shingles. A dab of roof cement every couple feet will “glue” it to the shingles and hold it in place. You'll have to remove gutter brackets or straps and then refasten them after the apron is in place. Gutter apron is available at home centers in 10 ft. lengths.
Vent Flashing Failure
Your plumbing system includes “vent” pipes that pass through the roof. And like any other roof penetration, that sometimes means trouble. There are two kinds of flashing used to seal vents: a “boot” that relies on a snug rubber seal, and all-metal flashing with soft lead that can be bent over and into the pipe. Some versions are made completely from lead; others are galvanized steel with a lead collar. When any type of vent flashing fails, the solution is to replace it. To see how, read more on plumbing vent flashing.