This is a decision you have to make after taking all things into consideration.

Q. How old is the roof most manufactures say roof shingles should last about 20 years. In normal situations I would agree?

A. If it is over 20 years old. And you have already spent money making roof repairs. maybe it is time to replace the roof. Otherwise you will be throwing your money away.

Q. Did the water cause any damage to the plywood or struts in the crawl space from the roof leak ?

If you  are lucky enough and did not have any deck or structural damage. Then your roof repairs will be easy and inexpensive. Most likely you can do this yourself .

Tools you will need to do the job

Tool-1-A flat Pry bar shown in picture three.

Tool-2--A bent Pry bar shown in picture four.

Tool-3-A hammer shown in picture five.

Tool-4-Straight cutting tin snips shown in picture seven.

Tool-4-A tube of roofing adhesive shown in picture fifteen.

Picture- One
The first step is to carefully separate the roof shingle tabs above the the ones that you are going to replace. Keep in mind the roofing shingles were designed to stick to the ones below to prevent strong winds from separating and or ripping them and expositing the roofing nails. Exposed nails will eventfully leak.

Picture-Two
Now you go up one more layer and you do the same thing. Because this roofing shingle covers the shingle that you are going to lift. In order to expose the nails you will need to remove. If  you try to lift the lower roofing shingle without separating the one above you will not get to the nails and most likely crack or break the one you are trying to lift. Then you will have to start all over and have to replace a another one.             

Picture-Three
Take your flat pry bar using the angled end to prop up and hold the shingle tab up.

Picture-Four
Then  positioned your bent pry bar at the edge of the nail head, and give it a tap with your hammer. Push down on it and it will loosen your nails.

Picture-Five
Now take your hammer using the claw and remove your roofing nails. That wasn't so hard was it.

Picture-Six
After removing the bad roofing shingle. Take your flat pry bar using the angled end to prop up and hold the shingle tab up. Directly above the broken roof shingle you have just removed. Now remove the roofing nail to the left of your bar. The other roof shingles can stay.
 
Picture-Seven
Now repeated the above step on the next tab to the left. After the two roofing nails have removed. Now take your tin snips and cut the bad roof shingle.

Picture-Eight
Ok now that the damaged roofing shingle is loose pull it out.

Picture-Nine
If you do not have enough extra roofing shingles. You can just cut out one of the tabs instead of the entire roofing shingle.

Picture-Ten
Weather using a single or a full shingle. Just slide it into place under the above ones. 

Picture-Eleven
Make sure you line up the the new shingle or tab evenly with the old ones. Now you are ready to nail it in.
 
Picture-Twelve
Now hold up the shingle above with your pry bar take your hammer and two 1-1/2" roofing nails. Put one in here.

Picture-Thirteen
Now take your other roofing nail and put it in the other edge.

Picture-Fourteen
Now take your tube of asphalt roofing tar to coat the nail heads and the gaps between shingles. This will helps hold the new roofing shingle from being blown up by strong winds.

Picture-Fifteen
The tube is the most convenient, although the most expensive per ounce. You can also buy it by the gallon if you would to save a few dollars.

After you have applied roofing tar adhesive. All you have to do is press down the roofing shingles on to the tar adhesive. This will hold them in place. Congratulations you are done. Thank you for letting me help.

The Roofing Doctor : If you would like to see how to repair a hole under a vent flange you do not need.    click here Roof Hole Repair Part One