Pole barn purlin spacing matters because purlins help hold your roof together. Purlins are pieces of dimensional lumber that attach to the top chords of the trusses. They connect and brace the trusses, and they also give the roof steel something to attach to.
When builders install them correctly, purlins create a diaphragm effect. In low snow load areas, they usually sit 24 inches on center. However, they go closer together where snow loads are higher.
How to Install Purlins Flat
Once you know the right pole barn purlin spacing, you can choose an install method. The most common method is to lay the purlins flat. Builders typically do this with 2x4s. Then they nail the purlins to the trusses with two 20D galvanized ring shank nails.
This method works when the trusses are two to five feet apart. In most cases, only use five foot on center trusses for snow loads of 30# or less.
Pole Barn Purlin Spacing for Higher Snow Loads
For higher snow loads, the purlins need to sit closer together. For example, a four foot on center design can handle snow loads up to 60#. Also, for even higher snow loads or longer spans, space the purlins closer than 24 inches in the drift area.
If snow loads go above 60#, the trusses also need to move 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. With trusses two feet apart, you can also use 7/16 inch OSB or plywood sheathing instead of purlins.
That sheathing adds extra strength. It also deadens the sound of rain or hail, and it makes the roof easier to walk on.
Fasteners and Hangers for Each Connection
First, if the purlins lie flat and sit two to five feet apart, use two 20D ring shank nails for each connection. For wider spans, use LU26 hangers for 2×6 purlins and LU28 hangers for 2×8 purlins.
Finally, the purlins may overlap without butting and sit eight or nine feet apart. In that case, use one 60D galvanized ring shank nail.
