Improper or worn flashing around a chimney along the roof is the most common source for leaks. A hairline crack above the flashing will allow a vast amount of water to pour into your home during heavy rains. Flashing can be worn, or corroded over time. Aluminum flashing tends to wear fastest, lasting about 5 years. Your flashing might also be improperly installed. Flashing should not just be glued to the brick, there should be a stepped flash or counterflash slipped into a one inch slot. Whatever you do, do not use caulk to repair flashings and instead use a high pressure polyurethane sealant. You can test the resistance of your flashing by running a low pressure hose around the chimney to roof intersection and see if any water comes inside.