What is the upper member of the truss system that acts as a common rafter?

The upper member of a truss system that acts as a common rafter is typically referred to as the "top chord." It is the horizontal or sloped structural element that supports the roof load and connects the apex of the truss to its endpoints. The top chord works in tension or compression, depending on the design and loading conditions, and contributes to the overall stability and strength of the truss structure.