Final answer:
An inclined member of the roof that serves a similar purpose to joists in the floor is called a rafter. Rafters form the structural framework of a roof, bearing its load and shaping its form, much like joists support floor systems.
Step-by-step explanation:
An inclined member of the roof that serves the same purpose in the roof as joists do in the floor is known as a rafter. Rafters are the structural components of a roof that provide its shape and support the roofing materials. They extend from the peak of the roof, called the ridge, down to the outer edges of the roof. These are crucial in forming the framework that supports the sheathing, underlayment, and shingles or other roofing materials. Similar to how joists support the floor, rafters bear the load of the roof and help distribute the weight down to the walls and the foundation of the building.
Structural designs like the sharply pitched roofs of long houses, which resembled inverted boats, make use of rafters extensively, taking into account factors such as climate where pitched roofs help snow to fall to the ground and prevent collapse. In Gothic architecture, which is characterized by its interesting use of vertical lines and structural ribbing, the ceiling is often composed of four-part ribbed groin vaults that draw the eye upward, hinting at an interior structure that may include rafters.