Several factors will decide whether you should go for repair or a complete change of your roof. If it is a matter of a few leaky patches or damaged areas, a repair of those areas is an obvious solution. If a few of the shingles have blown away as a result of a storm or strong winds, you should definitely go for replacing only that part of the roof. Repairing and replacing a few shingles will surely be a cost efficient solution as well. However, if your entire roof shows signs of damage, it would be a better decision to go for complete roof change to save on recurring repairs.
Asphalt being the main ingredient in shingles, it can be extracted to be used in road construction, maintenance and repair. It is also possible to create new shingles from the old ones. To give you an idea, the shingles from one average-sized home can be used to surface about 200 feet of a two-lane highway. In recent times asphalt has also been used as a source for alternative energy generation.