Purlins

Purlins are horizontal structural members used in roof framing to support the roof covering and transfer loads to the primary structural frame of a building, such as rafters or trusses. They are typically installed parallel to the slope of the roof and are spaced apart at regular intervals.

Purlins provide a stable base for attaching roofing materials such as metal panels, shingles, or sheathing. They also help distribute the weight of the roof covering and any imposed loads, such as snow or wind, evenly across the roof structure, and purlins add stiffness and strength to the roof framing system, reducing deflection and increasing resistance to wind uplift.

Purlins are commonly made of wood, steel, or engineered wood products such as laminated veneer lumber (LVL) or oriented strand board (OSB), depending on the structural requirements and design of the building. The size and spacing of purlins are determined by factors such as the span of the roof, the type of roofing material, local building codes, and the anticipated loads the roof will experience.

The size of lumber used for purlins can vary depending on factors such as the design of the roof, the span of the purlins, the loads they need to support, and local building codes. However, common sizes of lumber used for purlins in construction include 2x4, 2x6, and 2x8 dimensional lumber.

The specific size selected will depend on the engineering requirements of the project, with larger buildings or those subject to higher loads often requiring larger purlins to provide adequate structural support. Additionally, the spacing between purlins will also impact the size needed, as wider spacings may require larger lumber to maintain structural integrity.

In some cases, engineered wood products such as laminated veneer lumber (LVL) or laminated strand lumber (LSL) may be used for purlins, offering higher strength and stability compared to traditional dimensional lumber. The choice of material and size should be made in accordance with the project's structural design and local building regulations.

Girts are similar but differ from purlins in that girts are primarily used to support the exterior cladding of a building, such as siding or sheeting. Girts help to stabilize the cladding and distribute wind and other lateral loads to the building's main structural frame.  Purlins, on the other hand, are used to support the roof covering and transfer loads to the primary roof framing system, such as rafters or trusses. They help to distribute the weight of the roof covering evenly across the roof structure and provide stiffness to the roof system. Furthermore, girts are typically installed horizontally between the vertical structural members, such as columns or posts, on the exterior walls of a building. Purlins are installed horizontally parallel to the slope of the roof and are typically spaced apart at regular intervals along the length of the roof. Girts provide support for the exterior cladding and help to transfer wind and other lateral loads to the building's main structural frame. Girts also help to stiffen the walls of the building. Purlins provide support for the roof covering and help to distribute the weight of the roof evenly across the roof structure. They also help to stiffen the roof system and resist wind uplift.

Purlin and Girt Calculators

• Purlins for a Gabled Roof: Computes the total number and length of purlins for a simple gable roof based on the ridge length of the roof, the roof pitch and the span of the roof.
• Purlins for Dormers: Computes the total number and length of purlins for needed for one or more roof dormers based on the dimensions.
• Purlins for a Gambrel Roof: Computes the length and number of rows of purlins needed for a gambrel roof based on the dimensions.
• Purlins for a Hip Roof: Computes the total number and length of purlins for a hip roof based on the ridge length of the roof, the roof pitch, the span of the roof and purlin spacing
• Purlins for a Pyramid Roof: Computes the total number and length of purlins for a pyramid roof based on the span of the roof, overhang, the roof pitch and the purlin spacing.
• Purlins for a Shed Roof: Computes the total number and length of purlins for a single sloped roof based on the span, ridge length and the front and back overhangs and the purlin spacing.
• Purlins and Girts for a Shed:  Computes the total number and length of purlins for the roof and girts for the walls based on the purlin and girt spacing and the ridge length of the roof, the front and back heights, the width (span) of the building and the front and back overhangs.
• Girts for a Building: Computes the number of rows of girts and their lengths for a building
• Pole Barn Materials: Computes the top to bottom framing (including purlins and girts), siding and roofing materials needed for a pole barn based on a few dimension.