Replies

1. 
   john7g | Apr 22, 2009 04:04pm | #1

   39.8

2. 
   JoeBartok | Apr 22, 2009 04:15pm | #2

   The 39.08° must be a typo? Using my online calculator, the Common Slope Angle for a 10/12 roof is 39.80557°. When I calc dimensions based on long spans the more decimal places, the better. Usually I take the arctan of the roof slope (arctan (10/12) in your case), save it to the calculator memory, use this angle which is accurate to thirteen decimal places for my calculations, and round off after.

   Joe Bartok
3. 
   BossHog | Apr 22, 2009 04:22pm | #3

   Like Joe said - The 39.08° muct be a typo.

   Or maybe it's some of that "new math".

   (-:

   The correct angle is less important than making the pieces fit. It might be worth doing some test cuts to double check everything.

   I don't have a license to kill. I have a learner's permit.

4. 
   MiCrazy | Apr 22, 2009 08:01pm | #4

   Another possibility is it's not a 10/12 pitch in the particular area you're looking at.  There could be an attempt to match the ridges of the building (same ridge height on elevations, but different spans).  

   It's typical on the plans that I see to only call out 1 pitch, if any, on the elevations.

1. 
   dovetail97128 | Apr 23, 2009 03:02am | #5

   That seems a definite possibility.
   Without exact run/rise dimensions it would be hard to tell.
   Someone may have calced the slope and when it came up 9 7/8 or 9 15/16 rounded up to 10 for printing purposes. I have seen that before.

   They can't get your Goat if you don't tell them where it is hidden.

1. 
   DanH | Apr 23, 2009 04:51am | #7

   Yeah, the exact rise/run should be checked. When adding a deck roof on our house we "assumed" that it was 4/12 without checking. Turned out to be closer to 4.125/12, enough to throw us off by about 1.5". Turned it into a "feature" by adding a piece of 2x on top of the center ridge, but had to toss a bunch of rafters we'd pre-cut wrong.
   The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith

5. 
   jimAKAblue | Apr 23, 2009 03:02am | #6

   Maybe you could just use a framing square and hold at 10 and 12 and you won't need to convert anything to degrees?

1. 
   ruffmike | Apr 23, 2009 05:28am | #8

    This roof is what we call a mansard, because it has walls at the ridge to create an area for mechanical.

   The structural steel is in and our rafter box beams are supposed to bear evenly on the 20 inch plate. If I build a true 10:12 it will bear on the bottom but be 5/16" off at the top of the plate. the plate is bent at 39.08 which I believe makes a 9.75:12

    This is a school job and every weld and screw will be inspected, so We gotta get it right. We actually have shop drawing in review at the dept of state architeture that eliminates 90% of the welding, but the schedule jumped up a month.

   If I change the pitch it and maintain the rise it moves the mansard wall back into the mechanical area.

    It seems that the structural got approved with some bad calcs. This is going to be a nice job and I am hoping to do a photo thread. Close to 2000 lin. ft of roof and 2 story exterior wall.                            Mike

       Small wheel turn by the fire and rod, big wheel turn by the grace of god.

1. 
   ruffmike | Apr 23, 2009 05:33am | #9

   Thanks to all the replies, I just wanted to confirm the angle calc at this point, there are a few other issues as well. I got an RFI back today that is laughable and I wish I could post it, but don't want to go there. I'll try to follow up with a summarizing post eventually                            Mike

       Small wheel turn by the fire and rod, big wheel turn by the grace of god.

6. 
   [email protected] | Apr 23, 2009 06:44pm | #10

   Trigonometry definitions for Carpenters:

   the rise divided by the run is the tangent of the angle;

   the rise divided by the slope, is the sine of the angle; 

   the run divided by the slope, is the cosine of the angle. 

   So, 10/12=tan theta,  =>  0.8333333333=tan theta,  =>  arctan (0.83333333)= theta, => theta = 39.805571-degrees = 39d48'20"

1. 
   DanH | Apr 23, 2009 09:00pm | #11

   Yeah, but what's the hyperbolic cotangent?
   The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith

1. 
   JoeBartok | Apr 23, 2009 10:11pm | #12

   Coth 39.805571° = 1.66385Joe Bartok

2. 
   [email protected] | Apr 24, 2009 03:39am | #13

   What would a carpenter ever need one for?I taught a math class for building trades once. 99% of the guys could learn trig functions when I simplified them to rise, run, and slope ratios. The the traditional, opposite, adjacent, and hypotenuse, drew a lot of blank stares, and a few scared looks, from the guys who had been exposed to trig in public school.

1. 
   DanH | Apr 24, 2009 04:28am | #14

   Well, if you're ever designing a structure supported by a catenary ...
   The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith