New roof

I had an addition put on my home about eight years ago. I recently noticed a leak in the garage and cut away some drywall to inspect. Sure enough, There is a leak where the addition meets the old roof line. Since my roof is 18 years old I thought it best to replace the entire roof. My first question is, Can I go over the existing roof since there is only one layer on at this time. My second question is, will this keep the leak from reoccurring. And my third question is, if I need to tear up the existing roof do I have to tear off the whole thing or only the area where I see the water damage. I'm not sure how well the roof seams were flashed but I only see water at the corner of the room addition.
Any info would be helpful.
Thanks,
Mike

 

Its best to take off all the old roof. That way you can see all of the wood there and if you have to replace it.

ED

 

Can't visualize how your older roof and your addition roof "meet" so it's impossible to generalize accurately. But one would think that certainly aftyer a complete tearoff, then reshingling, it shouldn't leak. However......We've seen many many roof leaks over the years which STILL leaked even after a second and third reroof. It's all in workmanship.
If your addition roof has any unusual tie-in with the existing structure roof, make sure you understand HOW someone is going to permanently correct the problem.