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I am not a plumber but I have "been there" with my in-law's home. If the dip runs only for a short run of pipe, you can dig it and put a steel sleeve over the pipe. The sleeve we used was stainless steel with some, IIRC, rubber gasket material. It clamped on over the pipe much like the sleeves you may recall being used in submarine movies. You would also want to sleeve the areas where the roots get into the pipe - hopefully the dip and the root intrusion are in the same location. After you have sleeved the pipe pour some loose dry concrete under and over the pipe. A bag or two will do. Then place another bag of concrete 'in the bag' on top of the pipe to prevent roots from getting back into the line. At least that is the way the guys who fixed the in-laws line did it :)