What is defined as the external frame for a floor platform?

The rim joist serves as the external frame for a floor platform, connecting the ends of the floor joists and providing structural support. It is typically located at the perimeter of the floor structure and plays a crucial role in maintaining the integrity of the floor by helping to bear loads and preventing lateral movement.

The rim joist also contributes to the overall energy efficiency of the building by helping to seal the gaps around the edges of the floor and can be used to attach insulation. The framing of the building includes the rim joist as a key component, ensuring that the floor platform is securely anchored and stable.

The other options refer to different structural elements: joists are the main supports running perpendicular to the floor, the band joist is often used interchangeably with rim joist but can refer to different contexts, and subfloor refers to the layer of material installed over the joists that provides a base for the finished floor surface.