Purlin vs. Girt — What's the Difference?
Edited by Tayyaba Rehman — By Urooj Arif — Updated on February 27, 2024
Purlins are horizontal beams that support the roof of a building, while girts are horizontal beams that provide lateral support to the walls. Both are structural components, but purlins focus on the roof and girts on the walls.
Difference Between Purlin and Girt
Table of Contents
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Key Differences
Purlins and girts serve as crucial structural elements in buildings, particularly in steel construction and pole barns, but they support different parts of the structure. Purlins are installed horizontally across roof trusses or rafters to provide support for the roofing materials, such as metal sheets or tiles. They help distribute the roof loads to the primary frames and walls. Girts, on the other hand, are horizontal supports placed between wall columns to strengthen the wall system and provide a surface for attaching wall cladding.
The placement of purlins and girts is dictated by their respective functions. Purlins run parallel to the building ridge, offering a structural platform for the roof. Girts are aligned horizontally along the walls, enhancing the structural integrity of the wall system and supporting exterior finishes. While both are designed to resist bending under loads, purlins primarily resist the load of the roof, and girts counteract forces impacting the walls, such as wind.
Purlins and girts are often made from the same materials—steel, wood, or engineered wood products. The choice between these materials depends on the building design, structural requirements, and cost considerations. In metal buildings, for example, steel purlins and girts are common due to their durability and strength.
The engineering design for purlins and girts takes into account the load-bearing requirements and the type of cladding or roofing to be supported. Purlins must be strong enough to support roof loads including snow, wind, and the weight of the roofing material, while girts must be capable of supporting the wall cladding and withstanding lateral forces.
Despite their different applications, the installation of both purlins and girts requires careful planning and precision. Their spacing, size, and orientation are critical to the building’s overall structural performance and aesthetic appearance. Ensuring proper installation is key to maximizing the effectiveness and longevity of both purlins and girts in the structural system.
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Comparison Chart
Structural Support
Supports the roof.
Provides lateral support to walls.
Placement
Installed horizontally across roof trusses.
Placed between wall columns.
Main Function
Support roofing materials and distribute roof loads.
Strengthen wall system, support wall cladding.
Orientation
Parallel to the building ridge.
Aligned horizontally along the walls.
Materials
Steel, wood, engineered wood.
Steel, wood, engineered wood.
Load Resistance
Resists roof loads (snow, wind, roofing weight).
Counteracts forces on walls (mainly wind).
Compare with Definitions
Purlin
Horizontal beam supporting the roof.
The contractor installed steel purlins to support the metal roofing.
Girt
Placed between wall columns.
Installation of girts involved precise placement for maximum structural integrity.
Purlin
Installed across roof trusses.
The purlins were spaced evenly across the rafters for optimal support.
Girt
Counteracts wall forces.
The girts were designed to resist the lateral wind forces impacting the building.
Purlin
Resists roof loads.
The purlins were engineered to withstand heavy snow loads.
Girt
Aligned horizontally along the walls.
Horizontal girts ensured a uniform look and added strength to the walls.
Purlin
Supports roofing materials.
Wooden purlins were chosen for the traditional barn roof.
Girt
Horizontal beam for wall support.
Girts were added between the columns to enhance wall stability.
Purlin
Parallel to the building ridge.
Purlins ran parallel to the ridge, providing a structural platform for the roof.
Girt
Supports wall cladding.
The steel girts provided a secure attachment point for the exterior panels.
Purlin
A purlin (or historically purline, purloyne, purling, perling) is a longitudinal, horizontal, structural member in a roof. In traditional timber framing there are three basic types of purlin: purlin plate, principal purlin, and common purlin.
Girt
A horizontal structural member of post and beam architecture, typically attached to bridge two or more vertical members such as corner posts.
Purlin
One of several horizontal timbers supporting the rafters of a roof.
Purlin
A longitudinal structural member bridging two or more rafters of a roof.
Purlin
In root construction, a horizontal member supported on the principals and supporting the common rafters.
Common Curiosities
How are purlins and girts similar?
Both are horizontal structural members that provide support, but they serve different parts of a building—purlins support the roof, while girts support the walls.
Can purlins be used for purposes other than supporting roofs?
Primarily, purlins are designed to support the roof, but in some designs, they may also contribute to the overall rigidity of the structure.
What factors influence the choice between wood and steel for purlins and girts?
Factors include cost, structural requirements, durability, and the specific design or aesthetic goals of the building.
What is a purlin?
A purlin is a horizontal structural member that supports the roof of a building.
What does a girt do?
A girt provides lateral support to the walls of a building and serves as an attachment point for wall cladding.
Are girts necessary in all types of construction?
Girts are essential in constructions where wall stability and support for wall cladding are required, especially in metal and pole barn buildings.
Is it possible to retrofit girts or purlins for additional support?
Yes, additional girts or purlins can be added to an existing structure to enhance support or accommodate new loads.
Can girts be used in non-structural applications?
While primarily structural, girts can also serve aesthetic purposes or as a framework for insulation and internal finishes.
What considerations are taken when installing purlins on a sloped roof?
Considerations include the angle of the slope, the type of roofing material, and the load distribution to ensure proper support.
How do environmental factors like wind and snow affect the design of purlins and girts?
These factors dictate the strength and spacing requirements to ensure the structure can withstand environmental stresses.
How do purlins contribute to the thermal performance of a building?
Purlins can affect thermal performance by providing support for roofing insulation and creating an air gap, enhancing energy efficiency.
What materials are used for purlins and girts?
They can be made from steel, wood, or engineered wood products, depending on structural needs and design preferences.
How do the roles of purlins and girts differ in a building’s design?
Purlins distribute and support the load of the roof, while girts reinforce wall systems and support wall cladding against lateral forces.
Can the spacing of purlins or girts affect a building's stability?
Yes, incorrect spacing can lead to structural inadequacies, affecting the building’s ability to support loads and resist external forces.
What advancements have been made in the materials used for purlins and girts?
Advances include the development of lighter, stronger materials and engineered products that offer improved performance and ease of installation.
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Written by
Urooj ArifUrooj is a skilled content writer at Ask Difference, known for her exceptional ability to simplify complex topics into engaging and informative content. With a passion for research and a flair for clear, concise writing, she consistently delivers articles that resonate with our diverse audience.
Edited by
Tayyaba RehmanTayyaba Rehman is a distinguished writer, currently serving as a primary contributor to askdifference.com. As a researcher in semantics and etymology, Tayyaba's passion for the complexity of languages and their distinctions has found a perfect home on the platform. Tayyaba delves into the intricacies of language, distinguishing between commonly confused words and phrases, thereby providing clarity for readers worldwide.
