Which component does NOT perform the same function as a knee wall?

A knee wall is a short wall that typically supports the roof structure, particularly in scenarios where the ceiling height is limited, like in attics. It plays a critical role in providing lateral support and stability to the roof system and can also serve as a space for insulation or storage.

Birds mouths do not serve the same function as knee walls because they refer to a specific cut or notch made in the ends of roof rafters that allows them to rest on top of the wall plate. This design helps to secure the rafter and ensures proper alignment with the wall but does not provide the same structural support as a knee wall does.

In contrast, purlins, struts, and strongbacks are all components that contribute to structural support and stability. Purlins are horizontal members that provide support for roof sheathing and help distribute load. Struts are diagonal supports that help resist compressive forces and maintain structural integrity. Strongbacks refer to horizontal braces that reinforce beams or trusses, further enhancing stability. Each of these components plays a significant role in maintaining the overall strength and function of the roof system, unlike birds mouths, which focus primarily on the rafter connection.