Calculating Truss Angles & Dimensions


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Old 04-20-04, 07:32 PM
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Question Calculating Truss Angles & Dimensions

I'm attempting to build a 3 season porch attached to my garage this summer, but right now I'm stuck drawing the plans. We want to use scissor trusses, so I contacted Home Depot to get an estimate and a drawing of the truss to submit to the township for my building permit.

I've been there three times now, and each time I leave more frustrated than the last time. On Sunday, they couldn't tell me anything because the company they order the trusses from wasn't open, but they could fax them to me on Monday. On Monday, the "Pro" at the desk had no clue what I was talking about, and just gave me a catalog with the different styles of trusses they can order. Today, a different clerk told me they don't have drawings.

The guy on Monday said all trusses are pretty much the same, so I was wondering if any of you here might be able to tell me the angles and dimensions so I can just draw something myself and get the plans submitted. The trusses in the catalog look like this:

http://www.byronsworld.com/images/scissor.jpg

The room is 24' wide, with a 4/12 pitch. The catalog I got is from MiTek, if that makes any difference. Thanks in advance for any help.
 
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Old 04-21-04, 06:53 AM
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here is some help

the angle that is produced by a roof can be calculated by comparing the rise to the run. In this example you take 4 / 12 which equals .33333 now to get the angle associated with that tangent you need to find the arch tangent which is 18.4349 degrees. The 'normal' scissor truss has a bottom pitch of 1/2 of the top pitch so the bottom will be at 2/12 thus producing a 9.4623 degree angle on the bottom.
Now if the span is 24' then it is 12' to the peak so the height is 4' to the bottom of the top chord. You must also add the heel height to that to come up with the overall height. IF this is a 2 x 4 heel truss then 3.5 / cosign of 18.4349 degrees plus the butt cut plus the 4' will yeild the overall height.

However that really has nothing to do with how a truss apears in plan view. Assuming that all of the pitches are the same on the house, all angles from plan view will be drawn at a 45 degree angle.

Tell the guy at home depot since all trusses are "pretty much the same" then ship me what ever you have and come fix any problems caused by them. I would be willing to be at that point he starts back tracking cause trusses are usually uniquely constructed for each house. Many factors go into why the shape and configuration is what it is. Not only do you need to design for geometric contrainsts, you must also design for snow, wind, hurricanes, etc etc.. so depending on where you are located and the code that your area follows the truss configuration maybe different then the catalog.

If HD can't tell you what is going on maybe they could let you talk directly to the supplier to get the needed info.

If this is a very simple porch as you described then it should take under an hour for a designer to do the truss layout and required engineering for submittal purposes.

Do you know if you need just a truss layout or do you also need engineering for submital?
If you don't get any results in a couple more days shoot me a private message containing your email address. I might know some lurkers on this board that have connections at the plate manufacturer that you mentioned. They might be able to whip you out something.
 
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Old 04-21-04, 05:56 PM
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Thanks for the info. It's a bit more complicated than I imagined, so I think I'll try my luck one more time at HD rather than trying to draw them myself. I'm not 100% sure what all I need for the building permit, but I'm pretty sure they require an engineered drawing if you're using prefab trusses. I haven't gone in yet to submit my plans, I'm just going by what my friend told me and what I've found online about local building codes. I helped him build a similar porch last year. Maybe I should just build my own trusses and just buy the raw materials at HD. They just require a sketch with dimensions for self-built trusses. Thanks again.
 
 

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