Metal flashing prevents leaks where shingles meet other surfaces, like walls and chimneys. Proper flashing work takes time and know-how, so sloppy roofers sometimes slather on roof cement instead. It seals out water long enough for them to cash your check, but it soon hardens, cracks and leaks. In the end, all it does is make a proper repair more difficult. So if you see heavy 'tar' patchwork on your roof, fix it right—before it leaks and leads to interior damage.
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.