Need help with math for roof pitch

• posted on August 30, 2006, 1:48 pm
I am making a homemade truss for an addition to an existing barn. I have never been any good at math. The barn roof was built to be a 4/12 pitch. The barn walls are 107 1/2 inches outside front to outside rear. I measured the existing rafters, and they are 113 3/8 inches long from the highest part of the rafter (at the peak). I used an angle tool (tool to set to the same angle and transfer it to a board), and I cut the boards at the same angle. OK, I transferred the angles to the boards and cut them the same. Then I loosely nailed a piece of plywood across these 2 planks where the angles meet (at the ridge). I left the lower ends (overhang) a little long to adjust the height, so it's right. That's where I get puzzled. I want to be sure the height is the same. How do I figure this out mathematically, or am I better off just stretching a string across the existing roof at the top of the walls, then measuring from the peak to the string, and moving the string to the homemade truss, and make sure the height from string to peak is the same, before I nail the horizontal board across the truss.
If this was an outside wall, it would be fairly simple to make a 2x4 from the top of the wall to the peak, and be sure its the same as the existing roof. However, this truss is being placed from pole to pole, in the middle of the barn.
If I am not exactly clear, here is what I am doing. I bought and moved an existing pole barn. To move it, I cut it in half because it was too large to pull down the road in one piece. I now have both halves. One half is in the ground, the other half is still on the wagon frame that I used to move it. I am adding on to the barn as I put it back together. But I am adding in the middle. I dont plan to put the half still on the wagon into the ground until the new portion in the middle is built. The reason for that is so I can shift around the other half in their (oversized) post holes until the steel roofing and siding sheets fit together properly.
The reason I am telling all of this, is because I can not just run a string from the roof of one half to the roof of the other. I wish it was that easy......
Yes, this has been a complicated ordeal, but it will work out in the end......
Thanks
Mark
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• posted on August 30, 2006, 4:39 am
snipped-for-privacy@UNLISTED.com wrote in news:9fbbf2tbm41dktccdunsku85jt8nfqtu60@ 4ax.com:

I'm not trying to be a wise guy. If you cannot figure this out mathematically, how did you design the truss based on the xyz loads and streses which requires significantly more complex math? Aren't permits for such work required in your area. To get one issued, you would have had to submit a truss design with specs signed off my a PE, Certified Professional Engineer.
The discussion on making your own trusses has been hashed over before here. Bottom line is it's not a good idea. The impact of truss failure. long or short term, can be fatal.
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• posted on August 31, 2006, 1:20 am
wrote:

Permits, Engineers ??????? NOT
This is very rural farm country, not New York City. A permit is needed for a new home, and all that amounts to is to fork over some money and give a brief description of the house and the size. Barns and sheds dont need any permits, you just notify the county so they can raise our property taxes.
These are not the kind of trusses you buy at Home Depot. This is actually stick built. Two 2x6 rafters with a 16 foot horizontal 2x6 nailed near the bottom and several small 2x4 uprights. These are also TRUE 2x6's made at a nearby Amish sawmill. I'm just matching the existing home made trusses, but I still got to get the angle right.
There wont be any truss failure the way I brace stuff.
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• posted on August 30, 2006, 11:36 am
snipped-for-privacy@UNLISTED.com wrote:

oh yeah, you're the guy putting two halves of a pole barn together! the height is staightforward: half the span times 4 divided by 12, plus add in the heel height. however, i would not rely on math to try to match existing--pull a sting and measure.
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• posted on August 30, 2006, 11:47 am
OK - I know nothing about roof trusses, but I believe what you're looking for is a trigonometry function - tangent. Google " trigonometry table" or use a scientific calculator. You already measured the angle. I have tables if you can't find them.
Cathi
wrote:

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• posted on August 31, 2006, 1:01 am

Thanks for the help. Now I can do some calculating. However, I did sort of think the string/measuring method was the solution. The only reason I got into the pitch math was because I have been a bit puzzled about this, and I stopped over by the guy I got the barn from. He built this barn about 2 years ago, so he remembered immediately the pitch. But he did say the measurements could be off a little. I already knew that, because the one end of the barn is about an inch and a half wider than the rear 2x6 under the roof is about an inch longer than the front one. When I told him that I noticed the slight difference in width he said they just put a string across the peak and cut the boards to match. So, I measured the trusses on either side of the poles where I put the new one, There's about 3/4 of an inch difference. So, I figure if I make the new one 3/8 of an inch higher than the shortest one, I should be good.
I already know someone is going to ask why he sold a 2 year old barn. It's because he put up a huge barn now, with a riding arena and all sorts of fancy stuff. The barn I got from him was right in front of the main door on the new barn, so he was going to salvage the steel siding and demolish the rest of it. I paid him for the steel and got the rest of the building for removing it. That was a good deal for me, and saved him the time and cost to demolish it.
Thanks for your help.
Mark
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• posted on August 30, 2006, 11:39 pm
snipped-for-privacy@UNLISTED.co wrote:

mark, since it is hand framed, to really dial it in, first string a horizontal string between the two wall plates. then hang a plumb bob off of the peak, and measure to this from the outside of your wall from both directions (since the peak isn't necessarily in the center. then measure up from the horizontal string to the peak. if you have a flat place, you can snap out this triangle on the floor and build the truss to it. it'd be nice if the walls were level side to side. good luck.
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