When your roof starts having problems, you need to have a roofer come and evaluate it for you. The roofer will check out all sorts of things and then let you know your options. Those options include replacing your roof or doing something called reroofing. You may not know the differences between the two things, and until you do, you can't make an informed decision about what you should do.
Reroofing
This process is something that can only be done on asphalt shingle roofs. The contractor will put a new layer of shingles on your roof directly over an existing layer of shingles. Some things mean you won't be able to have the contractor use this process. One is how many layers of shingles you already have on your roof. Most building codes will limit how many layers of shingles you have on your roof, and if you already have more than one layer on the roof, you will have to look at other options.
However, if your roof is a viable candidate for reroofing, there are some reasons why you might want to go this route. Reroofing your roof is less expensive than fully replacing it and is also a quicker process. Drawbacks also exist, including the fact that you don't know what is happening under the shingles since they aren't removed during the reroofing process so you may be sealing rot under two layers of shingles.
Replacement
When you replace your roof, it doesn't matter what the existing material is because the contractor will strip all the roofing from your roofing. That includes all the sheathing and under-roofing material as well. The contractor will replace all that material, eliminating any rot or weak spots on the roof. You can talk to the roofer about what materials they will use on your roof, and you can change anything around at that point. Replacing your roof will cost more and take longer, but you know that everything under the new roof is going to be in good shape and should last for years.
When you have issues with your roof, call a roof replacement contractor. The contractor can climb up on your roof to take a look at it and see what is going on. Then they can tell you what your options are so that you can make a decision that will leave you with a good roof on your house.