What are the common signs of a roofing leak and how can it be repaired effectively?

Common signs of a roofing leak include water stains on ceilings or walls, damp or musty odors, and missing or damaged shingles. To repair a roofing leak effectively, it is important to identify the source of the leak, replace any damaged shingles or flashing, and seal any cracks or gaps with roofing cement or sealant. It may be necessary to hire a professional roofer for more extensive repairs.