Pole barn roof question

[beppington]

A purlin sits on top of the rafter/truss and runs perpendicular (parallel to the ridge). If you have rafters 2' on center and sheathing, you don't use them, but if you have a metal roof, you don't need the sheathing so you need something else to fasten to. If the rafters/trusses are too far apart to use 1x or 2x on the flat, you can stand them on edge to make them stiffer. There are several ways to use them. Most common is simply on top of the rafters, notched and inset was used a lot in the old post-and-beam, or hung off the sides of the rafters with hangers which is what these guys are talking about. While you can't use longer stock and span several trusses that way, it does mean the roof is closer to the trusses so it makes a little better use of the internal space, especially if you have a loft or attic.

Or if you prefer, sort of like a girt except on the roof instead of the wall.

OK, got it, thank you! :thumbsup:

 

[Thick82558]

I have built many pole buildings and am a carpenter by trade. Yes you can span 12' with a 2x6 2' oc. You also put the perlin on edge not flat flat dose not carry load. On edge dose. Example place a 2x4 flat not on edge on a five gallon bucket then try to walk up incline the board would either bend significantly or bust. But on edge you could walk up it. On edge is where strength is. The double truss works great it is on either side of treated 6x6 as far as hangers we use scrap 2x6 material cut 10" long or so. Shove between double truss as it is same width. Nail 5" hanging above truss then the rest goes in between double truss nail off on each side of truss. Then put pearl ins on they rest on that scrap that sticks up 5" above truss then nail to scrap 2' oc repeat till done hangers work just more expense and more work.

 

[Duffster]

First off, I'm no engineer, but every pole barn I've been involved in building or seen around here is Michigan has 2x4 purlins laid flat with usually 4' OC spacing on the trusses. I've seen some with 2' OC truss spacing as well. 8 ft? I don't know about that. Would seem to create an awful springy roof and tough to compensate for to save a few bucks in trusses on such a small building. I know I wouldn't build a P.B with 8' spacing.

I would venture to say that 7 or 8 out of 10 post frame buildings are built with trusses 8' o.c. There are literally thousands of them around here. I would not build a building with trusses sitting on header instead of the post where it belongs unless necessary like a door opening.

 

[jml755]

Thick82558,
If I understand your post, you put a truss on either side of a 6x6 post at the wall. This creates a nominal space of 6" (actually 5.5 or so) between them.Then you put a scrap 2x6 every 2' going up the middle of the gap between the 2 trusses, correct? (Shove between double truss as it is same width)These "hangers" are perpendicular to the roof edge of the truss, i.e. they're not plumb, correct? And they stick up ABOVE the truss edge 5"? (Nail 5" hanging above truss then the rest goes in between double truss nail off on each side of truss)Then you nail the purlins to the TOP (or side) of those scraps so the roofing material is actually 5" or so away from the truss top edge? (Then put pearl ins on they rest on that scrap that sticks up 5" above truss then nail to scrap) Am I interpreting your words correctly?

If so, please explain the advantage of putting the roof surface so far away (5") from the structural part of the roof. I would think that heavy snow loads would tend to stress those scrap "hangers" in a down-roof-slope direction. Don't know at what point you'd get a roof failure, but without some clear-cut advantage, I can't see that it's worth the extra time and effort ($$) to do this.

Plus your statement "Yes you can span 12' with a 2x6 2' oc." may be technically true, but at what load rating? Seems that 12' span would be at very minimum live/dead load ratings. Would it pass code?

 

[jml755]

I would venture to say that 7 or 8 out of 10 post frame buildings are built with trusses 8' o.c. There are literally thousands of them around here. I would not build a building with trusses sitting on header instead of the post where it belongs unless necessary like a door opening.

You may be right on the percentages, won't dispute that. But the only ones I can think of where they've used 8' spaced trusses here in Michigan are very large buildings (160'-200' long) where the trusses span 60-80 ft and are therefore pricey. I'm going to check one tonite but I think they rest on the headers there. I've also attached a picture that is typical of pole barn designs I've seen. I'm building a barn addition right now with the rafters sitting on headers, per code.

And for a 50' long building you'd need 14 trusses 4' OC. For 8' OC you'd only need 7. Not much savings in my mind, probably less than $1000. And resting them on the headers means I don't care if the post spacing is not EXACT. Heck, my line posts can be off a couple of inches and my headers and girts will compensate for that. How many of you have set 16' long posts in the ground and got the tops to be precisely where you want them and then not move during construction? Sure, it can be done but it's a lot faster and easier to set them without worrying about that. And if you're sheathing the roof, it IS important when installing the trusses that you get a perfectly square roof . Using purlins you can be off some on the spacing or even squareness as long as your corner posts are perfect.

 

[Thick82558]

Yes it passes code I have built many and all have passed inspection. The 2x6 on edge sets on the truss the back of perlin rests on the 2x6 sticking up. There is is no air space between prelin and truss. It's hard to explain I will try to take pictures. Here in Oregon it is rated for 2' snow load I don't know what your load rating is in your neck of woods but a 2x6 on edge has and will pass building code here in clackamas county Oregon which gose up to 13,000' in elevation.

 

[Duffster]

rafters sitting on headers, per code.

Are you saying code says they have to set on a header? Because I can set them on the post per code.

And for a 50' long building you'd need 14 trusses 4' OC. For 8' OC you'd only need 7. Not much savings in my mind, probably less than $1000. And resting them on the headers means I don't care if the post spacing is not EXACT. Heck, my line posts can be off a couple of inches and my headers and girts will compensate for that. How many of you have set 16' long posts in the ground and got the tops to be precisely where you want them and then not move during construction? Sure, it can be done but it's a lot faster and easier to set them without worrying about that. And if you're sheathing the roof, it IS important when installing the trusses that you get a perfectly square roof . Using purlins you can be off some on the spacing or even squareness as long as your corner posts are perfect.

I just ordered 40' trusses @ $240 a piece for 8' spacing. Not only will the truss set be cheaper you will not need the header material, and labor to install.

As far as wild spacing it doesn't make any difference this way either when you set the purlins on top of the trusses. But then I don't find it the least bit difficult to get the trusses spaced accurately.

Would seem to create an awful springy roof

I don't find using 2x4s this way to be the least bit spongy or springy, especially when set over the top of the truss in a compound span.

 

[Thick82558]

This is what I am talking about building.

 

[jml755]

Are you saying code says they have to set on a header? Because I can set them on the post per code.

No, I'm not. Set them wherever you want. I'm just saying that a header carries the truss load TO the post AND will pass code. My response regarding headers was to address the OP's 12' opening "problem" and his seeming intention to have his trusses spaced at 12' at that point. I just wanted to give him a simple solution that is well documented as acceptable pole barn design. Using a 2x6 on end 2' O.C. across a 12' gap is marginal at best. I just took a quick look at a span table and even for light loading 10' or 11' seems to be the limit. Not trying to say YOUR method of building is wrong, but insisting that a truss be on a post doesn't help him with a 12' opening. Plus, he stated his trusses were only around $100 each.

 

[Duffster]

Using a 2x6 on end 2' O.C. across a 12' gap is marginal at best.

Marginal? Anything but. It is no better or worse then a 2x4 on 8' center. Both 2x4 on 8' and 2x6 on 12' are a "simple solution that is well documented as acceptable pole barn design".

[/quote] but insisting that a truss be on a post doesn't help him with a 12' opening. Plus, he stated his trusses were only around $100 each.[/QUOTE]

I never implied that his truss couldn't set on a header. Insisting that he can't have a 12' span is even less helpful. Must be one of these I have never seen or done it so it can't be done.