Before giving you an estimate, a contractor should assess all components in and around your roof, including the drip edge and gutters that steer water runoff. In a complete roof repair or redo, you’ll typically need to replace the underlayment. That component, usually treated paper or synthetic sheets, serves as a weather barrier atop the decking, the flat surface—typically plywood—that lies on rafters. In a cold climate, a roofer may suggest applying an ice-dam protection membrane in some areas to prevent ice dams that can cause water seepage. Adding or updating the ridge vent and vented soffits can improve attic airflow and prevent harmful moisture buildup. Replacing or fixing flashing, the material around the chimney and where roofing planes meet, can prevent leaks; ask about alternatives to copper, which is durable and pretty, but costly.