No, to the person who ignorantly answered you. The root is not taken out. They take the nerve (pulp chamber) out of the center of the root, blood supply. You need the root. Re-treatments are done all the time. You may have had a root canal and years later either need it re-treated or extracted, and the cause of this would be infection present. I would not say that infection is caused by a poorly done root canal, because sometimes you just cannot get to the apex of the tooth. Could be pulp stones, decalcified canals, or just accessory canals that are impossible to find. So if you cannot completely clean all the nerve out, you seal it up and hope for the best. It could stay like that for the rest of your life, with no probs, or it could re-infect in a few years, few months, few weeks. You never know. Not all of them can be re-treated, often times people opt to extract. You can't leave infection in your mouth like that so you have to do something. But yeah, re-treatment of root canals are common.
Active 32 yr. dental professional....clinical