Need help on trusses 9' on center or 4' on center for pole barn

s219 · Feb 21, 2015

There is so much to this decision, including snow loads, uplift loads, etc. Normally what I do is start with the desired metal, which will dictate spacing of rafters and purlins for loads (wind uplift in particular is sensitive to screw retention and spacing). Then see if those loads transfer down to the header beams and post footings OK. In the end, it all has to be right.

For my recent boat house project, I used 26 gauge PBR panel. Knowing the typical uplift (in PSF), needed to handle wind loads, dictated the screw spacing, and that in turn dictated 24" spacing of purlins and rafters.

JerryK · Feb 21, 2015

You have to think ahead on how you want to work up in the ' attic ', too. I have walkways up in my 80' long 4' CTC so I can work up there, and the 4' help support wiring/conduit. If you plan on using a lift to access that area, then the 9' openings would be nice... The 4' give you more things to try to grab as you plummet towards the hard concrete ..!!!

My barn is also striped with 2x4's 2' CTC length wize that my insulation is screwed to. The 4' CTC is better if you ever plan on installing a ceiling. If you do, you will be amazed how huge the ' attic ' is when you go up there... [ and all the $'s worth of 2x4's and trusses that's up there ]

EddieWalker · Feb 21, 2015

I like to put my trusses every 4 feet. That gives me a very solid roof, allows me to put my purlins on the flat, and I can still insulate between them or span that distance for a ceiling using metal. While a lot of commercial buildings will go farther, I think having the purlins on the flat adds enough strength and ease with installing the metal that's it's worth the extra expense.

For posts, I feel that a pressure treated 6x6 is still the best choice. I'm not a fan of using 2x6's at all. Having a post in the ground 3 to four feet gives you a lot of strength. Finding 2x6's that are rated for going into the ground is going to be difficult. I've never seen them, but from what I've read on here, you can special order them. How long do you have to wait, how much do they cost and how straight are they when you get them? If you go with regular lumber and install them on top of concrete, you are creating a hinge at the mounting point that you have to compensate for in your framing with additional bracing to make it stronger. More money, more work instead of just using a treated 6x6.

Eddie

rrrkkk · Feb 21, 2015

mine is 36 x 40 x 14 it has 8' between trusses. I installed a metal liner panel and had no trouble at all with 8' spacing. On one side I put a 40' LVL beam and supported an 8' loft on one side

TBarD · Feb 21, 2015

If the building is not too tall 6x6 treated will work fine, but for taller buildings (eg 14' or more) it may be necessary to go with 2x6 or 2x8 laminated posts. These should be engineered glulams however, treated on the end that goes into the ground. They are likely to be a special order. Re truss spacing, any spacing will work so long as the building is engineered for the local snow and wind loads. If the building is not engineered, for sure you want to overbuild it, e.g., the usual 8' post spacing with trusses 4' on center. Personally, for any larger building in snow country I would want it professionally designed. For a smaller building I would be tempted to just go with a conventional design and overbuild it. Lots of places though, will require an engineered design, to get a building permit these days.