Home roof replacement services generally encourage you to replace the entire roof. Replacing only part of the roof (i.e., just the part or parts over which there are leaks) brings a new set of problems with it. The top three problems with replacing just part of your roof are as follows.
In the grand scheme of things, this does not seem like much of a reason for only doing part of your roof. However, once those completely different-looking shingles are on your roof, and they stand out like a sore thumb, you are going to wish you did the whole roof. On top of that, if you choose to sell the house in the next decade, buyers are going to see those mismatched shingles and immediately wonder what was wrong with the roof before that caused you to only "patch" certain areas.
Leaks May Be Located Higher up Than You Think
The problem with replacing the part of your roof that is just over the area where you are experiencing leaks is that the actual location of the leak may be much higher up. If you have a leak over the front door, the leak may be at the peak of the roof, and not over the front door. This means that the leaks could continue, only you will notice the leaking only slightly less because you have had part of the roof replaced that would otherwise block some of the path of the water. Do not be too surprised if leaks and water damage are noticed in your ceiling a few years from now at a point higher up than the roof areas you had fixed.
If You Resort to Doing the Entire Roof in a Couple of Years, It Is Like Paying Double for the "Patched" Areas
Just pause for a second and let that thought soak in. So, you decide seven years from now to replace the whole roof. You have already paid the first contractor to fix and re-shingle specific areas of the roof. Then you hire a contractor to do the entire roof. At that point in time, you have paid twice for the areas you just had patched a few years earlier. You see, the second contractor cannot simply lift the newer shingles to remove older shingles and then re-roof everything but the previously patched areas. The whole roof has to be torn off to be redone when you choose to do the entire roof.