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Old 10-07-2009, 10:09 AM   #1
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Which construction is stronger?


I'm about to have a barn built. I'm down to two different companies. I have a question about the difference between roof designs between the two. The trusses will be 60' long. Company #1 wants to run trussses every 9', with 2x12 upper and lower chords. Company #2 wants to run trusses every 4', with 2x6 upper and lower chords. Which style would support the greatest load?
As for nailers: company #1 wants to span the 9' with 2x4's on edge, spaced every 2'. Company #2 wants to span the 4' with 2x4's, laid flat, every 2'. Of these two nailer scenarios, which is stronger? I can upgrade the nailers of company #2 to 2x6's for 17 cents per foot. How would that upgrade rate in strength between the other two options?
And as a sum of each truss and nailer design together, which roof would be stronger? (This will be a metal roof.)

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Old 10-07-2009, 11:28 AM   #2
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Both companies should be able to provide you with the loads the building was designed for.

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Old 10-07-2009, 11:49 AM   #3
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Both truss companies should be able to provide you with per square foot design load capabilities of both the top and bottom cords if installed to their specs.
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Old 10-07-2009, 12:14 PM   #4
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What is the roof load requirement/specification, and the benefit of exceeding it - if upon investigation one does exceed the other? Is this a major vendor selection criteria?
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Old 10-07-2009, 02:24 PM   #5
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I don't know what the load requirement is. There will not be a loft or anything else supported by the trusses, other than the roof itself. Both companies say that the trusses they're planning are designed for the standard requirements for their buildings in this area (Ohio). My uncle has seven barns, and three of them are from one of the companies in question. He says that their roofs are stronger than the other roof I'm looking at. I would just like an unbiased opinion from someone who knows lumber/truss ratings.
As for whether or not it matters to me in picking a building, the answer is yes, even if they will both do the intended job. I'm a fan of overkill, and I'm willing to pay a little more for the added security. I just want to make sure which is actually better. And I would like to confirm this information independantly- I'm not one to fully trust salesman to be completely accurate.
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Old 10-07-2009, 11:04 PM   #6
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There is no way to tell which roof is stronger based on the info you provided. Trussed roofs are an engineered system. Find out what the loading requirements are for your area for barn roofs. Check both designs to make sure they meet those requirements.

If you don't want to get too involved, just ask each company what the max vertical deflection is for their trusses. The less deflection the better.

Lastly, I've seen hundreds of trussed roofs but few barns. I have never seen 9' on center trusses. A warehouse I worked on has double trusses at 8' on center with 2x6s at 2' on center spanning the 8'. 2x4s spanning 9' seems a bit light.

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Old 10-08-2009, 12:08 PM   #7
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Quote:
Originally Posted by ajlandis View Post
Both companies say that the trusses they're planning are designed for the standard requirements for their buildings in this area (Ohio).
That's salesman baloney for I don't know and I want this question to go away without me expending any effort.

Tell the salesman that it's great that it's built to their standard requirements and that since they are their standard requirements it shouldn't be any difficulty for him to get you an official hard copy of their standard requirements showing the snow load and wind load that the specific building you are looking at will be spec'd to.

Make it very clear that this question isn't going to go away and that he won't have a sale without the document. But also make it clear that you are wanting to go forward with getting a building so he doesn't think your telling him no.

After they give you the info and you decide who to go with make sure the contract specifies the design loads.
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Old 10-08-2009, 01:44 PM   #8
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ajlandis -

Are you lucky enough to have a building code where you are? That would eliminate the question of the manufacturers "standards" which may not be good enough if there was a code.

The building code is the worst way you can build and still be legal, but it may be better than the manufacturers "standards", plus you get an inspector to look out for you.

You may find out that both contractors use the same truss manufacturer, but just have different preferences in how they want to do things and put things together and still meet the same loads (wind, snow, etc.). If that is the case, you would really end up trying to split hairs. That could even get more complicated if the contractors want to use different grades of lumber, since trusses are designed for specific loads and lumber grades/species. Some types can take higher stresses, so the weights could be different and affect the construction or erection.

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Old 10-08-2009, 05:18 PM   #9
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The strength of the truss itself is only a part of the overall strength of the building. In my experience inspecting failed buildings, most of the failures involve the connections between elements. So even if one truss is "stronger" than another, if the trusses are not adequately connected to the walls, the entire roof can blow off, trusses and all. I have some interesting photos of just such a failure after Hurricane Katrina, the trusses lifted right off the walls (roof and all) because no hurricane clips had been installed.

So if you are going to take the trouble to investigate the strength of the trusses, you really need to take a hard look at the strength of the entire structure, starting from the foundation up. Ideally, all the elements of the structure are adequately designed to resist the design loads, which as previously noted are normally greater than the minimum required by code. The critical factor in survival of a building when subjected to large loads is often the integrity of the connections, which includes the size and spacing of nails or screws, selection of sheathing material, proper use of hangars and clips, alignment of structural elements, compatibility of the building systems, and stiffness of connections and pieces.

For an engineered building such as you are describing, it is likely that all of these factors were considered when the building was designed, so you should be able to get a written description of the loads the building was designed for (wind, snow, seismic, impact, live and dead loads). Just as important as the design is the quality of the construction, check out some already built structures to see how carefully they were built. A great design is worthless if half the nails are left out of the hurricane clips.
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Old 10-08-2009, 07:57 PM   #10
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When saying that they are built to "their" standards, I was meaning the building codes for this area. both claim to meet them. The company that runs 9' spans is Walters buildings, a popular company in this area, and very well thought of. I just can't get over the idea of 2x4's spanning 9', even on edge. I'll just have to get the specs and see what is really rated the heaviest. Thanks for the direction.

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