The two most frequently used terms in metal or sloping roof construction are purlins and rafters. Both are parts of a roof framework, although they differ in functions. In this article, you will learn about purlins and rafters, their usage in construction, and how they are different.
What is a Rafter?
A rafter is one member of a group of parallel ‘beams’ that are laid along the slope of a roof and forms its main structure. Rafters are laid parallel or perpendicular to the columns or load bearing walls and transfer loads to them. They bear and transfer the major portion of loads that act on roofs, including wind and rain loads and the self weight of the roof.
Key Features of Rafters:
- Slope: Rafters are installed with an inclination that corresponds to the designed roof angle. In this case, it would be clear that the positioning of the rafters has some effect on the pitch of the roof.
- Primary Support: Rafters may be considered the main components of the roof because the majority of the roof load is transferred to these members.
- Material: In small projects, rafters can be constructed from wood, but the use of metal can also be encouraged where large-scale construction or higher durability is required.
How Rafters Work:
The rafters are placed equidistant from each other to form the roof’s framework and fastened at the top of the ridge board and at the bottom of the wall plate. They form a triangular shape in conduction of the weight of the roof evenly along the supporting walls of a building (in case of pitched roofs). In the absence of rafters, the roof would collapse because the supports would not be good enough to carry the roofing materials and environmental loads.
What is a Purlin?
A purlin is a horizontal member laid above the rafters to support the roofing material. Purlins run perpendicular to the rafters and give support to the joints between roofing material components, such as joints between tiles. They also provide additional support to the rafters and the overall roof framework.
Key Characteristics of Purlins:
- Position: While purlins are also laid across the slopes of roofs, they are perpendicular to rafters and above them.
- Secondary Support: The main purposes of purlins are to support the roofing material and provide secondary support to the roof.
- Materials: Purlins can be made in wood, in steel, or even aluminum, depending on the size and the structure of the building.
How Purlins Work:
Purlins are oriented perpendicular to the rafters, their purpose being to evenly distribute the weight of the roofing material across the roof. They are like additional secondary rafters that would be utilized on larger-scale buildings, where the size of the roof requires extra support or where the size of the roofing material is small, such as with tiles.
Key Differences Between Purlins and Rafters
While purlins and rafters are indispensable in the construction of a sloping roof, their purpose and applications vary. The major differences are listed below:
| Feature | Rafter | Purlin |
| Primary Function | It provides the main support for the roof structure | It provides secondary support for rafters and roofing material |
| Size | Has a minimum width and depth of 50-75mm | Much smaller in size, can be around 20-25mm |
| Load Bearing | It supports the weight of the roof directly and transfers load to columns or load bearing walls. | Helps in load distribution across the roof area and transfers load to the rafters. |
| Importance/ necessity | Is an important structural element in most sloping roof frames. | May not be necessary in a few cases, like small roofs with large roofing materials such as metal sheets. |
| Cost | Cost is higher as the thickness of the members is more and because they also require higher load bearing capacity. | Lower cost due to smaller size and load bearing capacity. |
Comparison Table: Purlins Versus Rafters
| Aspect | Rafters | Purlins |
| Main Function | Support the weight of the roof | To provide additional support to the rafters along with the roofing material |
| Installation Position | From ridge to eaves | sloped Horizontally, across rafters |
| Common Uses | Residential and small structures | Large buildings like warehouses or barns |
| Cost | Cheaper for smaller projects | Adds to the cost of larger or more complicated roofs |
| Material Options | Almost entirely wood, but can be metal | Wood, steel, or aluminium |
| Support Role | Main supporters of the roof structure | Secondary supporters, which assist rafters and the roof |
Framework of Rafters and Purlins
In most cases, both rafters and purlins are used together in one roof structure. It usually happens in cases when the building is larger or when the roofing material is small and needs support.
An example of such usage is a simple pitched roof that can be found in parks. Here, rafters support the main roof structure and transfer loads to the metal columns, while purlins are required to hold the terracotta tiles in place. Both together create a much stronger and more stable roof that can bear all the loads and environmental stresses such as wind or snow.
Conclusion
Despite both being important, purlins and rafters do different work: rafters are the basic roof-supporting elements, while purlins are secondary support elements that hold the roofing material in place. Being able to know the differences between purlins and rafters will finally lead you to make better decisions for your building project by ensuring the roof is not only safe but also functional.
FAQs:
A rafter is a beam that provides primary support for a sloping roof, supporting the self weight of the roof and bearing environmental loads
Purlins: These provide extra support to rafters and roofing material at intervals when the roof spans over a distance, particularly in larger constructions
Yes, purlins and rafters are often used together for roof assemblies, providing additional strength and stiffness.