Home » All Calculators » Construction and Civil Engineering » Purlin Size Calculator

Purlin Size Calculator

Photo of author
Published on

In the architectural symphony of a building, purlins play a crucial role in supporting the melody of the roof. The Purlin Size Calculator emerges as a conductor, ensuring that the structural elements harmonize without missing a beat. Designed for architects, engineers, and builders, this calculator streamlines the process of determining the optimal size of purlins to bear the roof's load efficiently.

Purpose and Functionality Explained

Purlins are the horizontal beams that provide support to the roof deck, panels, or sheets. Calculating the correct purlin size is vital to prevent excessive bending, deflection, or failure under load. Given the variables like span length, roofing material, purlin spacing, and environmental loads, calculating purlin size is complex. The Purlin Size Calculator simplifies this complexity using a fundamental formula that accounts for these variables to suggest an appropriate purlin size.

The Core Formula

The backbone of this calculator is the formula for calculating the bending moment (M) and the required section modulus (S):

  • Bending Moment 28M=8w×L2​
  • Required Section Modulus S=σM


  • w is the total uniform load,
  • L is the span length between supports,
  • σ is the allowable stress of the purlin material.

Inputs for Precision

  • Span Length Between Supports (L): The critical distance needing support.
  • Total Uniform Load (w): The combination of dead, live, and environmental loads.
  • Allowable Stress of Material (σ): The stress limit the material can safely endure.

Step-by-Step to Structural Strength: An Example

Let's consider a purlin tasked with spanning 6 meters, shouldering a load of 550 N/m, with the material's stress tolerance capped at 250 MPa.

  1. Assessing the Load: Our purlin faces a total load of 550 N/m.
  2. Calculating the Bending Moment: =550×628=2475 NmM=8550×62​=2475 Nm
  3. Determining the Section Modulus: =2475250×106=9.9×10−6 m3S=250×1062475​=9.9×10−6 m3

The conclusion? Opt for a purlin with a section modulus of at least 9.9×10−6 m39.9×10−6 m3.

Vital Information at a Glance

ParameterDescriptionExample Value
Span Length Between SupportsDistance between supports (meters)6 m
Total Uniform LoadSum of all loads (N/m)550 N/m
Allowable Stress of MaterialStress limit of purlin material (MPa)250 MPa
Required Section ModulusMinimum modulus for safety (m³)9.9×10−6 m39.9×10−6 m3

Conclusion: Elevating Engineering Efficiency

The Purlin Size Calculator transcends being a mere tool; it's a bridge to engineering excellence. It ensures that purlins—the silent sentinels of a building's roof—are chosen with precision, supporting the edifice against the forces of nature and time. For architects, engineers, and builders, it offers a path to make informed decisions swiftly, ensuring that safety and efficiency are always paramount. As with all structural calculations, coupling this tool with the expertise of a structural engineer is recommended, ensuring adherence to local codes and standards. In the realm of construction, where accuracy is everything, the Purlin Size Calculator stands as a testament to the beauty of engineering done right.

Leave a Comment