What must rafters be covered with before applying finishing roof material?

Rafters must be covered with sheathing or decking before applying finishing roof material because this step provides a stable and secure substrate for the roofing system. Sheathing, typically made of plywood or oriented strand board (OSB), creates a continuous solid surface that helps to support the weight of the roofing material and contributes to the overall structural integrity of the roof. Additionally, sheathing serves as a barrier against wind and rain, preventing water from infiltrating the rafters, which can lead to damage and rot.

While insulation, a vapor barrier, and roofing felt are important components in roofing systems, they serve different purposes and are either applied after or are supplemental materials. Insulation helps with temperature regulation and energy efficiency but is not the first layer applied over rafters. A vapor barrier is used to prevent moisture from penetrating certain areas and is not specifically required for the initial covering of rafters. Roofing felt is a protective underlayment that is typically applied over sheathing before the final roofing material is installed but does not replace the need for sheathing itself. Thus, the correct answer highlights the essential first step in the roofing process: the installation of sheathing or decking.