What does the term "purlin" refer to in roof construction?

The term "purlin" in roof construction specifically refers to the horizontal supports that provide structural framework for roofing materials to be laid upon. These horizontal members are crucial as they help distribute the weight of the roof and provide support for the roof deck and any additional roofing materials. In many cases, purlins are placed at regular intervals and are integral to the overall stability of the roof structure.

Understanding the role of purlins is essential for recognizing how roofs are constructed and how they function. They are typically used in buildings with sloped roofs, where they sit atop the rafters and provide a nailing surface for the roofing materials, enhancing the integrity of the roofing system.

The other options refer to different components of roof construction but do not define purlins. Vertical supports, for example, are referred to as posts or columns, while insulation serves a different purpose by improving thermal performance. Sheathing, on the other hand, typically refers to the boards or panels that form the outer layer of the roof deck, providing a surface for roofing materials to adhere to.