Which structure provides midpoint support for rafters?

The structure that provides midpoint support for rafters is knee walls. Knee walls are short walls that are typically found in the attic space or the sloped part of a roof. They help support the rafters by connecting them to the principal frameworks of the building. By providing this support, knee walls help to distribute the load of the roof and create a stronger structural system, reducing the risk of sagging or failure in the rafters.

Ridge beams, while important, primarily serve as the top support structure where the rafters meet at the peak of the roof, rather than providing midpoint support. Ventilation ducts are not structural elements; their purpose is to allow air to flow through the building for ventilation, not to support weight. Floor joists are responsible for supporting floors and do not interact directly with the rafters in the way that knee walls do. Thus, knee walls play a crucial role in the integrity of roof structures by enabling proper load distribution and stabilization of rafters.