To find a leak source, you need to know what water is likely to do. Water will always flow directly down with gravity. Obstructions, skylights, and penetrations are the most common leak sources. Start by examining chimneys, skylights, and plumbing vents. Then inspect roof-to-wall transition areas where flashing takes the place of roofing, such as dormers, valleys, and eaves.
Many times owners think they have leaks, when actually their roofs just do not have adequate ventilation. Look for algae stains on interior plywood, soaked insulation when it hasn’t rained, and black rings or rust around nails—the first sign that moisture is condensing in attic spaces.
Always repair leaks promptly to avoid adverse effects, including mold growth.