Pole Barn Post Spacing Revisited

Pole Barn Guru Blog

Pole Barn Post Spacing Revisited

By far, my most read blog has been on, “Pole Barn Truss Spacing”. With nearly 50% more reads than any other blog I have written, it clearly is a fan favorite. I’ve had it referenced by clients, building contractors and code officials.

So when one of our clients wrote: “After talking with the building inspector he does not like the idea of post spacing 12ft apart with double rafters. Can you quote a standard setup so I can compare apple to apples, post 8ft on center with double 2×12 header and trusses every 4ft, no joist hangers needed for perlins?”, I felt obligated to answer.

(For clarification of the above, “double rafters” are “double trusses”.)

My response: While the building inspector may not “like the idea” of using post spacing 12 feet on center, it is not only a tried and true method, but it is also one which our engineers recognize as being structurally superior and will engineer seal. We have thousands of buildings in all 50 states, done with the exact same design. It affords the benefits of fewer holes to dig, fewer pieces to handle and install, engineered connections and the reliability of doubled trusses.

Given the correct loading criteria and an engineered building, we will guarantee the ability to obtain a structural permit from our plans. The post spacing every 8′, single trusses every 4′ resting upon headers is a system our engineers are not interested in risking their careers on. In the event of a single truss failure, this system will result in a domino effect and the collapse of the entire roof system.

After the winter ice storm of ’96-’97, I spent quite a bit of time studying roofs that collapsed -some of which were on my own buildings!  At that time, we were using two trusses on a pole, so we had this part “right”.   But instead of putting the two trusses together to act as a single unit, we put one on each side of the column with blocking in between.  The trusses were notched in, so this was another part we were doing well – transferring loads into the ground.  But the one part we missed, was putting the two trusses together.  Lumber is after all, a tree…with inherent knots and defects.  With the huge loading of ice that year…the truss “weak spots” gave way – and if one truss fails, it pulls the rest of the roof down.  This is when I asked an engineer to evaluate the truss system I was using as well, and he concluded the “probability of a second truss adjacent to the first one having the same exact ‘weak spot’ – just could not be calculated.”

From then on, I started using double trusses, nailed together according to a specific nailing pattern (supplied on all our plans).  Since then we’ve had other winters with similar ice/snow loadings, and…no more failed roofs!  I had the engineer’s word, but even better – I had real proof from thousands of buildings which survived nature’s “test”.  We’ve been using the double interior trusses ever since.  Only one roof I’ve seen fail since then…and it was determined to have been due to the two trusses were not nailed together adequately – they had few nails holding them together.   Again – the trusses acted as “singles” and pulled part of a roof down.

We get the requests for abnormal (for us) truss spacings or post spacing every once in awhile. Instead, I prefer to turn it around and stress our advantages –

(1) Fewer holes to dig, digging is always the worst part, and the one which is outside of anyone’s control. If they hit a Smart Car sized rock on the next to last hole, are they going to move the whole building?

(2) Fewer posts to set, trusses to raise, purlins and girts to handle. The real advantage of pole building construction is having the least number of pieces, in order to do the job structurally. By using slightly larger pieces (generally 2×6 instead of 2×4, where it takes only 50% more wood, to be 246% stronger) we are being material efficient.

(3) Wider sidewall door openings without the need for structural headers. In the event someone wants to add a door or window at a later date, they have far more flexibility to do so.

(4) Does anyone REALLY want to stand on a 2×4 roof purlin 16, 20 or more feet up in the air? When the 2×4 purlin snaps, it is a long drop to the ground.

(5) Most building collapses come from connection failures. In our case the purlins connect to the trusses with engineered steel hangers (not just nails); the double trusses bear directly on the posts (not on the sides of the posts, or nailed onto a header). The load is transferred down into the ground and is not dependent upon nails to hold the entire weight of the roof.

(6) If you think about it, if you have a 48’ long building, with single trusses every 8’, you have a total of 5 interior trusses.  If you have double trusses every 12’, you have a total of six interior trusses, but only have to dig 3 holes instead of 5. Which would you rather do?

By the way – our client DID get his building permit issued, using our plans, by the very same inspector who originally “did not like the idea”.

10 thoughts on “Pole Barn Post Spacing Revisited

  1. Nancy Young

    Hi Mike,
    I am building a shed roof pole barn for donkeys.
    It will have a metal roof over purlins. I am wanting to make it 14′ deep and 22′ wide with no center post in the 14′ span.
    I am wondering if a 2 x 6 x 16 rafters 2′ OC will work.
    The PT posts are 6 x 6 spaced either 6′ or 8′ apart on the high and low load bearing side walls.
    Headers are 2 x 12 and the roof slope is 2 1/2 / 12.
    We don’t usually get a lot of snow in western NC.
    Do you think the 2 x 6 x 16 rafters are of to span 14′?
    Thanks,
    Nancy

    Reply
  2. Terry Bock

    I’m building a pole barn and want to shingle it. I’m spacing my trusses on 24″ Centers with 5/8 plywood sheeting, rafters will be spanning 40′ plus a 24″ overhang on both eave ends. My question is what kind or size of headers do i need on the rafter support end’s? Vertical 6″x 4-1/2″ support posts are every 8′ C/L.

    Reply
    1. admin Post author

      The dimension and attachment of the truss carriers should be clearly indicated on the engineer sealed plans for your building. If this is not the case, you will want to contact the engineer who designed the plans. If, by some chance, you do not have engineered plans – you need to consult with a Registered Professional Engineer, licensed in your state, who can do the appropriate calculations and provide a design solution. DO NOT MESS AROUND HERE and go with “what some body says” – as when your roof collapses, the some body isn’t going to pay to replace it.

      Reply
  3. Dave

    I’m looking at doing a extension car port with living space, 25×25, above onto my existing shop. I’m laminating 6 treated 2x10x12 on two 12’6”centre 8×8 piers (total length 25ft) as sill set. Then use 8×8 post 9’6” (8 total), for the upper level support. Two post will support a 6ft belcony overhang. Then for the upper level switch to two lams of 2×12 in order to tie in 25ft span engineered joist set on 16” centres in order to lay 2×6 pine t&g decking. Roof will be 2×10 rafters over living space with collar ties to accomodate R50 insulation. Interior ceiling 1/2” t&g pine, outer side of rafters strapped with 2×4 to attach metal roofing.
    My question – I’m not sure what would be the best way to prevent “racking” being the carport lineal section will be married to the existing building. Should I consider some type of bracing at each post to prevent side to side movement since the load above will be quite heavy. .
    The building is in a North – South setting and the roof surface is facing west-east which sees allot of winds from the west.

    Reply
    1. admin Post author

      You’ve gone off and attempted to do structural design without the skills and knowledge of a trained design professional. Before you get in too much further over your head either hire a competent Registered Design Professional (architect or engineer) to do your structural design, or find a post frame building kit package supplier who can provide the materials as well as an originally engineered design.

      Reply
    1. admin Post author

      In most instances fence posts are set every eight feet, due to the spanning capabilities of the materials between the posts. If setting fence posts in very soft soil – bury them deep and use premix concrete around them (also use posts pressure preservative treated to a UC-4B specification).

      Reply
  4. C. Millard

    lookling to build a pole barn for hay. Thinking i would like a 108 x 48 with an additional 12′ shedrow off the E 108 as a loafing shed for cattle. Thinking the posts on 12′ centers. so….Double trusses of 2x6s

    Would I be saving money on lumber to make the barn 144 x 35 with an additional 12 shedrow off the E (lets just make it only 108 for simplicity sake)

    going to go 14′ from ground to truss fyi. and each long side covered completely, and 12′ wide opening without door on each end.

    Reply
    1. admin Post author

      Thank you for your interest in a new Hansen Pole Building. One of our Building Designers will be reaching out to you shortly.

      Reply

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