Welcome to CalcTree's Purlin and girt design tool! You will be able to determine the member capacities for purlins and girts according to AISI S100-16. The purlins are manufactured by AEP. All calculation units are in imperial units.

📝 Note

  1. This calculation only calculates specifically for the AEP's manufacturer table. Other manufacturers may provide different dimensions to their products but follow similar sizing guide. The advantage of this is AEP provides a very extensive selection of section sizes.
  2. Inputs and outputs are all in imperial units as is every calculation step. They may need to be checked.
  3. This calc only calculates section capacities with any assumptions made stated in th


For a detailed rundown of designing purlins and girts, check out our design guide here. But here's a brief summary to get you up to speed.

Purlins and girts are structural members used to support loads from sheathing and cladding in roofs and walls. They can be made from timber or steel, with steel sections being either C or Z-shaped and hot or cold-rolled.
C and Z steel sections
When designing purlins, the designer must consider various loads, such as dead, live, snow and wind loads, and check them against the bending, shear, torsion, buckling and deflection capacity of the purlin in accordance with relevant design codes.

The maximum distance between steel purlins should be around 1200mm, with typical distances in residential buildings being about 600 to 800 mm.

The depth of the purlins is usually estimated using the span/32 rule of thumb. C-section purlins are used in simple, non-continuous spans, while Z-section purlins are used in continuous lines of purlins for lapping at internal supports.

Loads in buildings are concentrated or uniformly distributed over an area, so purlins would pick up different portions of the total load acting over the surface depending on their arrangement. Effective area is taken as the product of tributary width and breadth. Dead loads include the self-weight of building elements and superimposed dead loads, such as cladding, tanking, insulation, plaster, finishes, and services.

The line load for dead loads is obtained by multiplying the tributary width and density. To simplify the design, tributary width is simply the distance between two adjacent members.

In summary, when designing purlins, one should consider the load-bearing capacity, shape and orientation, and the type of loads the structure will bear.

🎁 Bonus info

Purlin dimensions


Web thickness (D)

Flange Thickness (B)


Span of member

Material properties






Member capacities

Shear capacity (kips)

Moment Capacity (kips*ft)

Axial compression capacity (kips)

Nominal tensile strength (kips)

*Determined from manufacturers' tables

💬 We'd love your feedback on this template! It takes 1min


  1. Manufacturer's table: AEP:
  2. AISI S100-16 -