If the roof covering is more than 20yrs old the look at replacing rather than repairing.
Methods of repairing cracks or lifting lap joints
USE Trowel on mastic.
OR
Flash band and tin/s of primer if it's the lap joints. primer is required to assits sticking. Use hair dryer to warm up the flash band as you lay it, use wall paper joint roller to roll the flash band down.
Both methods have limited life of around 6-10yrs. Ensure all loose material has been removed and surface is DRY. If it's too bad a state its a profesional replacement job as it has to conform to building regulations check with your local council building control office.
Bob
January 2011