The direct connection between purlins and roof covering is that purlins provide the essential structural support for the roof covering and manage its load.
Purlins are horizontal beams or supports that span across the main rafters or trusses of a roof structure. Their primary role concerning the roof covering is to serve as the direct support system that the roofing material is attached to.
According to the provided reference:
"Purlins provide structural support to roofs... They are instrumental in distributing the roof covering load, such as metal sheets, to primary structural elements, averting overloading and ensuring stability."
This means that purlins act as intermediaries between the roof covering (like metal panels, tiles, etc.) and the main structural framework of the building (rafters, trusses, or beams).
Here's how they work together:
- Direct Support: The roof covering is directly fastened onto the purlins. This creates a continuous surface that protects the building from the elements.
- Load Transfer: Purlins receive the weight and forces exerted on the roof covering, including:
- The weight of the roofing material itself.
- Loads from snow, ice, and standing water.
- Forces from wind uplift or pressure.
- Load Distribution: Instead of the roof covering transferring these loads directly to the main structural elements over large spans, the purlins gather the load from the covering over their length and then transfer it to the stronger, primary supports at specific points.
- Stability and Prevention of Overloading: By distributing the load effectively to the primary structural elements, purlins prevent concentrated loads that could overload parts of the main structure. This action is instrumental in averting overloading and ensuring stability of the entire roof system.
Essentially, purlins are the crucial link in the structural chain that safely carries the weight and forces from the protective outer layer (the roof covering) down to the main bones of the building.
Here is a simplified look at the load path:
| Component | Function | Receives Load From | Transfers Load To |
|---|---|---|---|
| Roof Covering | Protects from elements, initial surface | Elements (rain, snow, wind) | Purlins |
| Purlins | Support covering, distribute load | Roof Covering | Rafters/Trusses |
| Rafters/Trusses | Main structural support | Purlins | Walls/Columns |
| Walls/Columns | Vertical support | Rafters/Trusses | Foundation |
This interdependent relationship highlights why purlins are a fundamental component in roof construction, directly enabling the roof covering to perform its function safely and effectively over the long term.
