What is the term for the horizontal support system that supports a building's roof?

The term for the horizontal support system that supports a building's roof is best described by the concept of a truss. A truss is a structural framework, typically made from wood or steel, designed to support the roof and distribute weight evenly across the building. They are engineered to provide strength and stability, often allowing for larger open spaces without the need for interior columns.

While beams are also horizontal structural elements that support loads, they typically serve different purposes, such as spanning openings or transferring loads between vertical members. Ceilings, on the other hand, are the finished surfaces inside a building, which do not provide structural support. Elevators are mechanical devices used for vertical transportation within a building and do not relate to structural support. In the context of supporting a roof, the truss is the most accurate descriptor.