What structural function do joists provide in a building?

Joists play a crucial role in the structural integrity of a building by distributing loads across the framework. They are horizontal members that support the weight of the structure above, such as floors and ceilings. By spreading the load evenly across the beams and walls they connect to, joists help maintain the stability and balance of the entire building. This distribution of loads is vital because it prevents concentrated stress on specific areas, which could lead to structural failure or sagging over time.

While other options describe functions that are part of a building's overall structure, such as providing vertical support, enclosing spaces, or connecting roofs to walls, these do not accurately describe the primary role of joists. Joists specifically focus on load distribution, making option B the correct answer in the context of their structural function.