Replies

1. 
   dovetail97128 | Feb 27, 2009 03:03am | #1

   Courtesy of "framer" here.

   http://www.josephfusco.org/Articles/Roof_Cutting/split_pitch/PDF_Previews/Octagon%20Preview.pdf

   They can't get your Goat if you don't tell them where it is hidden.
1. 
   numbnuts | Feb 27, 2009 03:20am | #3

   Thanks, I will try this calculation when I get to my scientific calculator tomorrow...

2. 
   alias | Feb 27, 2009 03:14am | #2

   unit rise divided by 13 x .5x thickness of hip = hip drop
1. 
   numbnuts | Feb 27, 2009 03:22am | #4

   How did you get 13 in your equation?

1. 
   dovetail97128 | Feb 27, 2009 03:26am | #5

   12.96" is the run of an octagon hip rafter.
   12" standard run unit, 17" ( Rounded up ) run of 45 deg hip..
   He rounded it up to 13"
   They can't get your Goat if you don't tell them where it is hidden.

3. 
   MDFContracting | Feb 27, 2009 05:46am | #6

   I dunno if it helps, but in carpenter's school, I was taught to calculate the length of a non 45 hip rafter by simply using the pythagorean theorem.  (A squared + b squared = C squared.) Use the run of a common rafter on the minor roof and the run of a common rafter on the major roof and the hypoteneuse will be the run of your hip.  The next triangle will be the run of the hip and the total rise of the roof, with the hypoteneuse being the length of the hip rafter.  Sorry if i confused.

   
   

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   Edited 2/26/2009 9:48 pm ET by MDFContracting

1. 
   Piffin | Feb 27, 2009 06:16am | #7

   all true but it doesn't answer the Q about hip drop - only about rafter lengths and hip lengths 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

4. 
   JoeBartok | Feb 27, 2009 05:46pm | #8

   For roofs where the two intersecting slopes are equal …

   Hip Drop = ½ Hip Rafter Width × tan Plumb Backing Angle

   Or since the tangent of the Plumb Backing Angle is given by any of the following formulas you can use …

   ½ Hip Rafter Width × sin Backing Angle ÷ cos Common Slope Angle

   ½ Hip Rafter Width × tan Backing Angle ÷ cos Hip Slope Angle

   ½ Hip Rafter Width × tan Hip Slope Angle ÷ tan Plan Angle

   ½ Hip Rafter Width × tan Common Slope Angle × cos Plan Angle

   If the roof pitches are unequal and at irregular corner angles I find it’s easiest to offset the Hip rafter to produce equal shoulder heights, then apply the Hip drop calculation …

   Example Hip Shift Drop Calculations

   Joe Bartok

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   Edited 2/27/2009 9:49 am ET by JoeBartok

   
   

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   Edited 2/27/2009 10:33 am ET by JoeBartok

5. 
   cliffy | Feb 28, 2009 07:46am | #9

   You are asking more than one question. Do you need to calculate the length of the hip, the horizontal cut of the birdsmouth or the drop required to ensure the sheathing does not sit high?

   Havea good day

   Cliffy

6. 
   JoeBartok | Feb 28, 2009 07:11pm | #10

   "I know a regular hip can be figured by using the run factor of 16.97, but how do I figure this run factor for irregular angled hip/valleys?"

   Yeah, there is more than one question here. There are many ways of finding the "run factor". You haven't given the pitches or the corner angle at the eaves, so, I'll use the numbers in this image from the page linked to in my last post.

   If you let the 10/12 Common run be 12, the Hip run is ...

   12 ÷ sin 30.96376° = 23.32381

   Check: Hip Rise = 12 × tan 23.20706° = 10

   So you could call your Hip pitch 10 over 23.32381

   If make the 6/12 Common Run equal 12, the Hip run is ...

   12 ÷ sin 59.03624° = 13.99429

   Check: Hip Rise = 12 × tan 23.20706° = 6

   So you could call your Hip pitch 6 over 13.99429

   To find the Hip rise in terms of "over 12" for a framing square:

   10 × sin 30.96376° = 5.14496

   6 × sin 59.03624° = 5.14496

   Check: Hip Run = 5.14496 ÷ tan 23.20706° = 12

   Or you can make your Hip run any value you wish and use the following formula to find the rise ...

   Hip Run × tan Hip Pitch Angle

   

    

   Joe Bartok

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   Edited 2/28/2009 11:14 am ET by JoeBartok

   
   

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   Edited 2/28/2009 11:15 am ET by JoeBartok