Purlins and Snow Loads
Purlins are pieces of dimensional lumber attached to the top chords of the trusses in a pole barn to connect and brace them. Purlins are used to attach the roof steel to the building. When installed correctly, they create a diaphragm effect. Purlins are usually spaced 24 inches on center in low snow loads, but they are put closer together if snow loads are higher.
The most common method of installing purlins in a pole barn is to lay them flat. This is typically done with 2x4s. The purlins are nailed to the trusses with two 20D galvanized ring shank nails. This method is used if the trusses are two to five feet apart. Only use five foot on center trusses for snow loads of 30# or less in most cases.
For higher snow loads, the purlins will need to be closer together. A four foot on center design can be used for snow loads up to 60#. For higher snow loads or longer spans, the purlins will need to be closer than 24 inches in the drift area.
If the snow loads are more than 60#, the trusses will need to be closer together. Trusses spaced three feet apart for nine foot on center post spacings can support most snow loads. For an even stronger design, you can use two foot on center trusses with eight foot on center posts. If the trusses are spaced two feet apart, 7/16 inch osb or plywood sheathing can be used instead of purlins to provide extra strength, deaden sound if it rains or hails, and make it easier to walk on the roof.
If the purlins are laid flat and two to five feet apart, use two 20D ring shank nails for each connection. For wider spans, use LU26 hangers for 2×6 and LU28 hangers for 2×8. If the purlins are overlapped but not butted and placed eight or nine feet apart, use one 60D galvanized ring shank nail.
When you are ready contact CHA Pole Barns to talk about your pole barn project.