Wind
wind is a huge enemy of older old interlock style (t-lock) roofs, its good to go out and look for wood or black spots on your roof if you have this type of asphalt roof. also if you can check flashing around chimney and skylights, if you have a flat roof, its good to have it inspected for damage from branches. a slow roof leak can do a lot of damage before getting noticed. http://www.pwmccallumroofing.com for more info