# Rafter math is giving me nightmares



## schnitz (Sep 5, 2009)

So most of you guys know I'm currently at A-School learning to be a Builder. Right now, we're learning rafter layout and the accompanying math calculations. Right now, this stuff is just kicking my tail. The instructors keep using terms like "tail line length", "rafter line length", "projection", collar tie length", and "bridge measure". 

I understand the actual layout stuff, it's just how to end up with the dimensions I need from the givens in simple terms that is above me. Suddenly I'm feeling more confused than a baby in a topless bar. Stair math was a cakewalk compared to this. 

In each problem, we are given: slope, span, projection, drop, top of birdsmouth, and stud spacing. Here's what I have understood so far....

1. I know and understand that the total run is one half the span. That is calculation #1.

2. To get the bridge measure, it's the square root of the unit of rise squared plus the unit of rise squared.

3. I'm lost from here out (mainly because the accompanying worksheets are poorly worded). 

From here, I'm flat lost, and I'll take any help you guys are willing to offer up. So long as this doesn't go the way of calculating board footage....


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## jstange2 (Dec 5, 2010)

A squared plus B squared equals C squared. That is all I need to know for rafters. I don't have the time to explain at this time. Get a framing square with the rafter tables on it. Nice shortcut.


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## mveach (Jul 3, 2010)

jstange2 said:


> A squared plus B squared equals C squared. That is all I need to know for rafters. I don't have the time to explain at this time. Get a framing square with the rafter tables on it. Nice shortcut.


1+on that and a speed square.


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## mdntrdr (Dec 22, 2009)

Rise and run. :smile:


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## BigJim (Sep 2, 2008)

The run and rise form a triangle as you know and ajstange2s said A squared + B squared = C squared. Say you have a 5 and 12 pitch, cut the plumb cut at the top end of the rafter, hook your tape on the top edge of the rafter at the top and measure down the length of what the rafter is and make a mark on the top edge of the rafter. This is called the line length.

Now using your framing square make another plumb mark from the mark you made. Now before you go any further you will need to take half the thickness of the ridge board off which is 3/4 inch provided your ridge board is 1 1/2 inch thick. Square off the plumb line you just made and make a mark at 3/4 inch, now make a plumb mark at the 3/4 inch mark you just made, that will be the back side or outside of your bird's mouth. 

Take a square and from the bottom of the plumb mark with the square inline with the plumb mark slide the square up the plumb mark from the bottom of the rafter to where you have 3 1/2 inches on the square, mark on top and this will be the bird's mouth. 

That can't be very clear, if you had a 2X4 block you could just place it so the 1 1/2 inch edge is square with the plumb line and slide it to where the top edge of the 2X4 is inline with the bottom edge of the rafter which will form the bird's mouth. I know I didn't explain this very well and hopefully someone else can but I will give you and example.

You have a building with a 26.5 foot span, the run will be 13 feet 3 inches. Lets say we have a 5 and 12 pitch. "A" being 13.25 feet (run) to get the rise multiply 13.25 X 5 (which is the pitch) this will = 66.25 inches, ("B") this is how tall the ridge board is from the top of the plate minus the thickness of the bird's mouth cut. 

To make this easier we will convert everything into inches. "A" 162 inches squared = 26,244 + "B" 66.25 squared = 4,389.0625 . Add these two together = 30,633.062 inches, now get the square root of this and you have 175.02303 inches. Take 3/4 inch from this = 174.27303 inches. This will be the line length of the rafter. 

Line length measurement is this, make a mark on the top edge of a blank rafter, measure down the top edge of the blank and make another mark on the top edge of what the rafter length is. You will make the plumb marks or 5 and 12 on these two marks. OK, I'm done now, I have confused you enough. Just one more thing, to get the rafter tail or overhang place the framing square on the bottom plumb mark and square out the amount you want for the tail.


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## BWSmith (Aug 24, 2010)

http://www.tpub.com/content/construction/14044/css/14044_55.htm

Dig around the above for awhile.It might clear a few things up.Best of luck,BW


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## cabinetman (Jul 5, 2007)

jiju1943 said:


> The run and rise form a triangle as you know and ajstange2s said A squared + B squared = C squared. Say you have a 5 and 12 pitch, cut the plumb cut at the top end of the rafter, hook your tape on the top edge of the rafter at the top and measure down the length of what the rafter is and make a mark on the top edge of the rafter. This is called the line length.
> 
> Now using your framing square make another plumb mark from the mark you made. Now before you go any further you will need to take half the thickness of the ridge board off which is 3/4 inch provided your ridge board is 1 1/2 inch thick. Square off the plumb line you just made and make a mark at 3/4 inch, now make a plumb mark at the 3/4 inch mark you just made, that will be the back side or outside of your bird's mouth.
> 
> ...


Can you be more specific?












 







.


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## BigJim (Sep 2, 2008)

cabinetman said:


> Can you be more specific?
> 
> 
> 
> ...


With this ole mind?:laughing:


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## schnitz (Sep 5, 2009)

I'm actually not quite as confused as you might think. I thank you for the lesson!


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## schnitz (Sep 5, 2009)

Now I would like to thank you guys one more time, as I scored 90.01 (3 wrong with the only math one wrong being the one I clicked the wrong answer even after circling the right answer on my scrap paper....) on today's test. Hat's off, you guys are the best!!!!


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