Replacing an entire roof is an expensive investment so it is important to know exactly what problems the roof has. A thorough inspection of the roof surface and often of the attic should give you a good idea whether or not you need a whole roof replacement or if repairs can be made. If the latter is the case, a roofing repair contractor should be consulted to determine the cost of the repair.
Depending on the material of your roof, there are many simple problems that if left untouched, will cause lots of damage in the future. For example, a small water leak is not a reason for a whole roof replacement, however, in a clay tile roof it could cause serious problems; the rounded shape of the tiles creates a path for water to flow down. This could cause mold to grow, tiles to deteriorate and even cause damage to the frame of your home. Replacing one or two broken tiles is a job that can be done by most homeowners and is something that will add years to the life of your roof.Roofing repair on metal roofs is rarely necessary. If needed, you can re-caulk seams between panels and around skylights and chimneys, to stop leaks. If your roof's problems are greater than a few loose shingles or nails, or a few small leaks, you should get a contractor's opinion about the need for roof replacement.
Any roofing repair job that is going to require removal of much of the shingling, metal panels, or tiles should be upgraded to a roof replacement job. Sometimes, making repairs to one half of the roof and not the other causes cosmetic issues. Also, because roofing systems are designed to work together, mixing and matching is not a good idea. At this point, a roof replacement is the best option. At some point, it becomes more cost effective to just tear the whole thing off and replace it than to spend hours or days trying to fit two different roofing systems together.