Final answer:
A knee wall is a vertical wall used for partitioning, while a purlin is a horizontal member used for supporting the roof covering.
Step-by-step explanation:
A knee wall is a short wall that is built vertically from the finished floor to support a roof rafters or joists. It is typically found in attics or crawl spaces and serves as a partition.
A purlin, on the other hand, is a horizontal structural member that supports the roof covering and transfers its weight to the load-bearing walls or columns of a building. Purlins are commonly used in industrial or commercial buildings with metal roof systems.
In summary, the main difference between a knee wall and a purlin is their orientation and purpose. A knee wall is a vertical wall used for partitioning, while a purlin is a horizontal member used for supporting the roof covering.