No Chimney Cricket
A wide chimney forms a dam on your roof. Debris builds up behind that dam and holds moisture, which leads to rusted flashing and wood rot. Any chimney wider than 30 in. needs a “cricket,” or “saddle”: basically a small roof built behind the chimney. A properly installed chimney cricket will direct water and debris around the chimney and off the roof. If your chimney doesn't have one, watch for holes rusting through the flashing. If you're getting a new roof, be sure the contractor's bid includes a cricket.
Missing Kick-Out Flashing
Kick-out flashing is critical where a roof edge meets a sidewall. Without it, roof runoff flows down the wall and possibly into the wall. This is worst when there is a door or a window below and water can seep behind the trim. You might not notice it for years, but eventually rot will destroy sheathing and framing. In extreme cases, the stucco is the only thing holding up the wall! Don't wait for that to happen to you. To see how to add kick-out flashing, read Use a Kick-Out Flashing to Stop Rot.
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.