Your problem is looking quite serious. There are two options for you. First is to change your roof and second is to repair your roof. I would like to suggest you that you should go for repairing process. It 'll be more economical for you. I have also faced this problem. I would like to suggest you that you should try liquid rubber coatings for your **EDIT** I hope that it 'll prove to be the best solution of your problem. Try it.
I am new to this forum. I have just joined this forum. I just come here to say hello to all the members of this community. Hope it 'll be a very nice experience for me.