Homeowners tend to consider whether to repair their roof or installing a new one. To make the decision you have to weigh the cost of the repairs against the value you will obtain from them. A roof that is only missing a few shingles can represent an inexpensive job.
On the other hand, a roof that is old means that you should consider replacing it instead of repairing it. This is not an expensive option when you consider that you are extending the life of your roof for another 15 or 20 years. Asphalt shingle roofs last 20 to 25 years, while a roof installed over an existing layer of shingles should be replaced every 20 years.