Replies

1. 
   Ken_Drake | May 14, 2001 04:37am | #1

   *
   Billy,

   Suppose the On Center measurement for the jack rafters is 16", and that the 2 roof pitches are 6/12 and 9/12.

   To find the common difference for the 6/12 jacks, multiply 16" x 9 then divide by 6

   = 16" x 9 / 6 = 24" That's the increase in RUN for each 6/12 jack. Then just find the length for a 6/12 jack with 24" run = 26 13/16"

   To find the common difference for the 9/12 jacks, multiply 16" x 6/9 = 10 11/16". That's the increase in RUN for each 9/12 jack, so the common difference for the length of the 9/12 jacks would be 13 5/16"

   You can use this method for ANY combination of roof pitches, for ANY On Center measurement, when the rafter plates are running at right angles to each other.

   Ken

1. 
   Ken_Drake | May 14, 2001 04:49am | #2

   *Billy,Here's a second example.On Center spacing of rafters = 24"Roof pitches = 7/12 and 10/12Increase in RUN for 7/12 jacks =24" x 10/7= 34 5/16". Increase in LENGTH at 7/12 pitch = 39 11/16"Increase in RUN for 10/12 jacks = 24" x 7/10 = 16 13/16". Increase in length of each jack at 10/12 pitch = 21 7/8"These measurements, as in my first post, are rounded to the nearest 1/16th of an inch. If you use a Construction Master Calculator, it will show the measurements of each succeeding rafter to the nearest 1/16th of an inch, so that no accumulation of error occurs.

1. 
   Allyson_Stiles | May 14, 2001 04:51am | #3

   *So where did you get the formula for the common difference. The formula posted was for the increase in runBilly

1. 
   Ken_Drake | May 14, 2001 04:58am | #4

   *Billy,Just take the increase in RUN, and find the increase in the length of the rafter as you would for ANY Run at a given pitch. If you have a Construction Master Calc., just enter your answer as RUN, enter the pitch, and press the diag key.Ken

1. 
   Allyson_Stiles | May 14, 2001 05:06am | #5

   *Ken,Thanks for the information. I don't use a Construction Master calculator. I was just trying to get the formula.. Thanks for your response though.Joe,Thank you for the answer. This is what I was looking for. All I have to do is figure the conventional common difference and use this formula. I can't believe it was that simple!!! Again, thanks again guys. I can easily get it by putting in the first jack, but I like to know the formulas to figure from the ground.Billy

2. 
   tomc5 | May 15, 2001 02:42am | #6

   *Ken and Joe -As a follow-up question to figuring CD, is my question on how to apply the CD. In the past I always applied CD starting from the tip of the common rafter pattern and marked each hip jack progressively shorter by the CD measurement. The last couple of jacks usually seem to run a little short, but I always attributed this to accumulated error and hasty/sloppy cutting on my part. I would normally just nail the lowest couple of jacks wherever they fit best at plate and hip even though they might stray from the 16" O.C.However, I recently noticed that the "Roof Framers Bible" has entries for "First Jack Deduction" and then the "Common Difference". The book doesn't have much on this, but I imagine it has to do with the perpendicular distance from the tip of the common rafter to the centerline of the ridge as compared to the angular distance from the tip of the jack rafters to the to the centerline of the hip. So getting to the question - since I haven't seen or heard much talk about this, to mark out the jacks do you guys typically apply the CD starting from the tip of the common rafter, or should you start the CD from the tip of the first hip jack; if so, how do you obtain length of the first jack?Does this hold true for the valley jacks as well?Regards,tomc

3. 
   blue_eyed_devil_ | May 15, 2001 05:33am | #7

   *Joe, you finally taught me something about roofs.Thanks.Too late for me to put in into practice however...blue

4. 
   Aaron_Smith | May 16, 2001 03:39am | #8

   *Wait a minute!Just who the hell you callin a bastard!

2. 
   Allyson_Stiles | May 16, 2001 03:39am | #9

   *
   What is the formula for figuring the common difference of jack rafters on a bastard hip or valley?

   Billy