Many houses utilizes trusses to distribute both the live and dead load of a roof. The dead load is the weight of the roof components themselves. The live load is the load that changes over time. In the case of a roof this would generally be the weight of a layer of snow. In the case of floor trusses, the live load would consist of factors such as furniture and house occupants. In this video, Paul Kassabian provides an excellent tutorial on how trusses work. I recommend checking out his Youtube channel and liking his videos to help support his good work.