Single Truss System: Purlins Laid Flat

Before any of my readers drop their teeth due to me saying something I don’t exactly extol… is easy….read on!

I’ve spent the last few days in Michigan – where I am being told no Building Official would EVER approve of a pole building designed with double trusses spaced 10 or 12 feet on center and 2×6 or 2×8 purlins on edge. To the contrary, I’ve been involved in the design of about 15,000 buildings designed exactly this way. Throughout the Western United States, the typical post frame construction utilizes the concept of pressure preservative treated columns, spaced 10, 12 or even 14 feet on center, with two ply trusses, aligned with the sidewall columns. Literally hundreds of thousands of buildings have withstood snow loads to upwards of 200 pounds per square foot (psf) and wind speeds far over 100 miles per hours (mph).

The originally patented post frame (pole building) design concept utilized columns spaced every 12 feet. Even the unimpeachable source of all knowledge (Wikipedia) references columns spaced every 12 feet (

In post frame construction, roof purlins are the members running the lengthwise direction of the building, either placed on top of, or between the roof trusses (or rafters), to attach the roof steel or other roof sheathing to.

To the good folks in Michigan, the only way to construct a pole building, is to place the columns every eight feet. Truss carriers (basically headers) are then run from column to column. The trusses are installed on top of the carriers, every four feet.

This four foot truss spacing, allows for 2×4 roof purlins to be placed flat on top of the trusses. These purlins are attached by driving nails through the wide face of the purlin, into the tops of the trusses. It does make for a fairly easy purlin install (other than dimensional lumber typical runs about ¾ of an inch over length, so every purlin must be trimmed), and it gives a big wide surface (3-1/2 inches) to run screws through the steel roofing, into the purlins.

Like any system, there are some downsides – and my opinion is this method has some significant ones, which outweigh the positives.

In comparing to my idea choice of construction, this method requires 50% more holes to be dug. It takes half again more trusses and involves handling about double the number of framing members. Granted, often the framing members are smaller, but handling is still handling.

Structurally, there do exist some issues. The great majority of building failures are from connections – either inadequate, or improperly installed. The fewer connection locations, the less places for a potential error.

The trusses every four feet, columns every eight, results in purlins having to be connected to trusses, trusses to truss carriers, truss carriers to columns. Lost is the direct truss to column connection provided by them being aligned with each other.

In a single truss system, if one truss fails due to extreme loads, the balance of the roof is sucked down behind it. The face-to-face connected double truss system, creates a redundancy where no two adjacent trusses share the identical weak link. This load sharing prevents many would be failures.

With the single truss system, as each truss is only 1-1/2 inches wide, requires double the lateral bracing against buckling of a double truss (three inch wide system). Again, more pieces, more connections, more possible locations for failure.

And one more thing – I mentioned nailing the steel to the purlins on the 3-1/2” face as being an easy target to hit.  However – what about nailing the 2×4 purlins where they come together over a single truss?  You now have two purlins butting into each other, sharing that 1-1/2” space.  The chances of missing the truss beneath, or splitting either the end of the 2×4 and/or the ¾” space on the truss are….pretty high.  Another connection failure.

And back to the Building Official issue – our company has dozens of clients all across the state of Michigan who have been granted Building Permits and are enjoying their buildings with double trusses and wall columns spaced every 12 feet. Never be able to obtain a permit? Never is a big word, and it just is not the case.

7 thoughts on “Single Truss System: Purlins Laid Flat

  1. Hello Mike, thanks for this post. It helps to confirm what I had in mind while I’m still designing my pole barn. I intend to build one that is 18×24 in size. My question is this: is there any benefit in building a truss out a 4×4’s as opposed to building a double truss out of 2×4’s? Either way, I like the design idea that you present here; with wider trusses to lay purlins flat against, with connections of the trusses on a notched out 6×6 or 8×8 post. I’m eager to learn any answer you have for me. Thanks!

    1. Justin ~ I’m hoping you are not considering building trusses onsite. The engineering alone would make it cost prohibitive. Prefabricated meal connector plated roof trusses are truly the only practical design solution. Truss companies have equipment which is designed to press the metal connector plates into nominal two inch thick lumber – therefore, four inch material is just not an answer. Assuming you will be having two 12 foot bays on the 24 foot length, it will take at least 2×6 roof purlins on edge to make the span (could be higher depending upon snow loads). It will be much easier to attach the purlins into the side of the trusses, if the truss top chords are at least 2×6.

      1. Thanks Mike, if there are three 8 foot bays on the 24 foot length, could 2×4 roof purlins be used instead of 2×6. I live in south Mississippi, so snow is not a problem. Last year it snowed the first time in 50 years, and it was about an inch total that melted the next day! If 2×4 purlins are used on this configuration, and even if they are not (use 2×6 instead), would you still advise attaching them into the side of the double truss, or along the top like your article above mentions? Thank you very, very much for your input!

    2. So I took down two hog barns, one was a 2×4 structure on concrete and one was a 2×6 structure on concrete. The trusses were the same size with slight variation. They were all spaced 4 feet apart from one another but they were also on a stick built wall. My plan is to build one BIG pole shed. I plan on using 4×6’s for posts spaced 8 feet apart. Then I plan on using a double 2×10 for a header and spacing my trusses every 4 feet. One on the post and the next on the header. I planned on using 2×4 purlins flat to cover the gaps and secure the roof to that. I have extra wood so I did plan on adding a short chunk of 2×4 on trusses where the purlins meet on the single board. Does anyone have any thoughts on this method??? I see a lot of double truss talk but not much double header talk. Should I sandwich them together and secure it to the posts or I have also seen guys put one on each side of the 4×6 but I have read a lot of negative things on that. The last thing I thought of doing was adding bracing from the post to the center of the header where the truss will be sitting. I just don’t know if its necessary. Again with the two sheds turning into one I have extra wood so that isn’t really an issue. Thanks

      1. You are a brave man, as I personally would not take on using a bunch of recycled building materials. I also don’t know what your new building will be used for, but most pure agricultural buildings were designed for minimal roof loads – so the trusses may not be adequate for your needs.

        Where I would start – hire a local Registered Professional Engineer who has experience with trusses and post frame construction to examine what you have to determine if it will be adequate for your needs. It is possible you might have invested a lot of effort into a pile of kindling. In the event what you have will work, I would consider having your engineer design a building with a pair of those trusses bearing directly upon each column to eliminate those truss carriers. The engineer can verify what size your building columns should be as well as all of the necessary connections and framing sizes and spacings.

    1. Thanks Rich,
      We do not engineer trusses for the DIY’er. I don’t know of a truss manufacturer that would supply an individual with the sealed truss plans, and not supply the trusses. Too many liability issues. You might be able to hire an engineer proximal to your location to design them, though the cost would far outweigh the savings in labor. My recommendation would be to source a local engineer and have them give you an estimate on the cost of the design and seal. Then you could calculate the total costs in seals and materials.
      Good Luck with your endeavor.


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