What is it that is cracked? If it's an EPDM rubber roof, the rubber just needs sealing. But if the crack is in the wood structure underneath, you may have to get under the rubber and do some carpentry.
If a fiberglass roof, it will need some reinforcing, maybe by building up the roof in that area with glass cloth and resin.
Cracks in the existing sealant just need more sealant.
2004 American Tradition; 2013 Buick Verano
Homebase in the Ocala Nat'l Forest near Ocala, FL