A root canal only needs to be done if before crown/bridge work if the tooth requires that treatment. It is not part of a routine bridge pr crown. The reason you would need a root canal is if there is not alot of tooth structure or decay is close to the nerve and treatment would aggrivate the nerve, or if there is an infection.
A root canal does not always have to be done before crown or bridge work. However, depending on how much tooth structure is present and the condition of the tooth a root canal may be indicated. If you are having symptoms that include hot and cold sensitivity and or pain upon chewing in the area, your dentist may recommend a root canal. Also, even though a tooth might be completely healthy, a root canal is sometimes done in order to achieve more of a foundation for the crown or bridge to attach too. Hope that helps.