Cracked, curling or buckling shingles
This could be a sign that your home has inadequate roof ventilation or that your shingles have reached their useful lifespan.

Dark or dirty-looking spot
This could be due to granule loss on the shingles from aging. It also could be from algae or fungus growth which can mean there are other, unseen problems with your roof.

Drastic increases in energy costs
This could be a sign that your home has inadequate roof ventilation which is causing your heating/cooling system to run excessively.

Leaks after heavy rain
This could mean that you have deteriorated flashing somewhere that is not allowing your home to be sealed properly. It also could mean that the shingle underlayment is damaged or inadequate.

Leaks after driving snow
This could be caused from inadequate or improperly installed shingle underlayment allowing water from ice dams to leak into the attic. It could also be clogged vents, poorly installed gutters, or an improper balance of static and soffit vents.

Mildew growth or water stains on interior ceilings
This could be a sign that your home has inadequate roof ventilation or that there is faulty shingle underlayment that is causing a leak.

Shingles or siding decay
This could be a sign that your home has inadequate roof ventilation causing them to deteriorate.