Final answer:
A flying buttress is a horizontal structural member used in Gothic architecture to support roof loads and transfer them to structural supports. It helps counterbalance the thrust of the vaulting and ensures the stability of the building.
Step-by-step explanation:
The horizontal structural member that supports roof loads and transfers them to structural supports is called a buttress. A flying buttress is a structure commonly used in Gothic architecture that transfers the weight loads from roofs and upper stories to the ground.
It consists of an arched exterior support that extends from the wall of a building and counterbalances the thrust of the vaulting, ensuring the stability of the structure.