Leakage in Attic After Wind-Driven Rain
Possible cause: Leaky or inadequate shingle underlayment or deteriorated flashing  
Leakage in Attic After Ice Build-Up
Possible cause: Inadequate shingle underlayment allows water from ice dams to leak into attic
Blistering and/or peeling of interior and/or exterior paint
Possible cause: Excessive temperature or high humidity due to poor attic ventilation
Stains on Interior Ceilings and Walls or Mold and Mildew Growth
Possible cause: Inadequate or faulty shingle underlayment allowing leakage or inadequate ventilation
Exterior Decay, Sheathing, and/or Siding
Possible cause: Poor attic ventilation
Missing, Cracked or Curled Shingles
Possible cause: Shingles have reached the end of their useful life
Dark, "Dirty-Looking" Areas On Your Roof
Possible cause: Environmental pollutants; vegetation, fungus or algae; loss of granules due to age of shingles
Excessive Energy Costs
Possible cause: Insufficient attic ventilation causes heating/cooling system to run excessively

In warm weather, inadequate ventilation will trap hot air in the attic, causing air conditioning systems to work harder, or leaving your home's interior hotter and less comfortable.

Attic Ventilation: In both hot and cold weather, insufficient attic ventilation may cause moisture in the attic to become trapped and condense on the rafters. This condensed moisture can drip down onto the insulation and reduce its effectiveness.

Excessive energy costs can result.