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ErinsMom
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# Posted: 30 Apr 2012 07:33pm
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If building a cabin 20 x 30, how many roof trusses would one need? What size?
local craigslist has the following: 24 ft long 3/12 pitch x 9 @ $50 each and 30 ft long 3/12 pitch x 8 @ $70 each.
What could be done with either set?
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MtnDon
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# Posted: 30 Apr 2012 07:42pm - Edited by: MtnDon
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Trusses are designed for a specific width building. That is, there is a specific point where they are supposed to be supported, rest on the wall tops. Each truss set should have an engineering drawing that shows those points and also what the design loads are, including the design snow load and what wind load the design is rated for. Hopefully the seller will have that information available. The design sheet will also specify the spacing, usually 16" or 24", for the calculated loads. There are usually different trusses for the gable ends. depends on the engineer who drew it up, or ran the numbers through the computer.
The drawing(s) are part of the package the purchaser is paying for. They are necessary to get the plans approved in any area that enforces building codes and does construction inspections.
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ErinsMom
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# Posted: 1 May 2012 07:16am
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I knew you would be able to answer. Thanks so much.
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nickname
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# Posted: 13 May 2012 12:10am
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Trusses are not designed just for a specific width of building, but always for a specific building. Using trusses designed for another building on one that they were not designed for could leave you with a very compromised design. I suggest you be very wary of using 'used' or 'extra' trusses unless you are willing to pay someone with the engineering expertise to see if the trusses you are considering are a match for your structure, in terms of the points that MtnDon raised, but also in terms of shear, or lateral movement design strength. You might be best off designing your building to match the trusses, which might be worth it if the trusses are cheap enough, since they make roof construction fast and easy. But don't mix apples and oranges, don't put a Ford carburetor on a Toyota, dont put sewing maching parts in your dishwasher. Trusses are designed differently for each specific building, taking into account all the design factors that specific building requires.
A truss design company, or experienced contractor, may be able to look at the trusses and 'guess' that they will work, so you don't have to pay an engineer to do a thorough analysis. But when the roof has a 2 foot snow load on it...... you'll be guessing about your safety
On the other hand, if you have no snow or severe wind or earthquakes, and you follow MtnDon's guidelines, the design mismatch may never matter, since all the roof has to do is shed rainwater.
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toyota_mdt_tech
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# Posted: 15 May 2012 09:15am
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I know an inspector wants to see the "paperwork" for the trusses also. All engineered trusses come with paperwork.
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ErinsMom
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# Posted: 15 May 2012 04:49pm
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thanks everyone.
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OwenChristensen
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# Posted: 28 May 2012 01:49pm
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It sounds like they are steep pitch and a vaulted cieling. Is that what you want? If they are made to span 24' that should work for you. Yes, you can get engineering specs. , but they are not always asked for and most often stay at the lumberyard and are available if needed. Although rafter construction is still used 16'' centers are required. Trusses on the other hand are useually rated for 24'' centers and a full span. End trusses are not necessary as the load is carried by the end wall. That's not saying you cannot use a truss on the end wall, some people do and just side over it. You can frame the end set to match the roof. So you need a set every two feet. They are made for a particular weight load. If they were made for your area, you should be fine. Pole barn trusses are made for a wider spacing and thus stronger, I have never heard of trusse made for less than 24'' centers.
Owen
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