If your roof is over 10 years old, you may want to look for other signs of trouble...
1. Algae Growth
Caused by growth of airborne algae deposited on roof. This does not affect the service life of the material. A common problem in warm, humid climates. Algae resistant shingles available regionally.
The bending and twisting of shingles can be caused by lack of ventilation. Can also be caused by improper number of fasteners.
Caused by moisture in shingles. Caused by excessive use of asphalt plastic cement.
4. Missing Shingles
Caused by improper fastening and, also, from exposure to high winds. This problem can usually be solved by replacing individual shingles.
Decaying shingles caused by absorption of moisture by mat at the core of the shingle. This is a common problem with organic-based shingles.
6. Missing Granules
Excessive loss of grains and particles in aging shingles indicates the need for replacement. On newly applied shingles, a certain number of loose granules is normal.
7. Damaged Flashing
Damage to the protective layer around the base of the chimney can be caused by drying and cracking, and, also, by improper installation. This can usually be repaired without replacing roof.
Several things contribute to warped shingles. Improperly applied felt, lack of ventilation and/or movement in deck can cause buckling.
9. Ceiling Spots
Dark water spots on the ceiling or in the roof are caused by leaks in the roofing system. Need to check roof for damaged shingles and attic/crawl space for leaks in deck. Look for cracks in flashing around chimney and vents. On rafters, check for leaks that travel away from original source.