laying out and cutting jack rafters
I have been reading several books and articles regarding this topic. Some say to find the length of jack rafters you measure from the corner of the building (where the hip is) to the center of your layout mark for the jack and this is the run you’d use to multiply by the unit rise on the framing square. Then deduct half the diagonal thickness of the hip and this is the actual length to cut your stock. I have also read to do the same thing but to use the side cuts on jack scale on the framing square to mark the cut. I tried this on a test rafter and got two different lengths. What am i doing wrong, or which is correct?? Is there an easier way that has eluded me?
Thanks,
Craig
Replies
Old school.
Craig,
The way I learned was to step off the common with a framing square marking the rise and run. If you mark 12" runs every other rise will be the length of a jack, assuming 2' centers. Measure the diagonal on the square of your rise and run, this is the difference in length of the jacks.
These days I just plug the numbers into a Construction Master, after laying out a common.
http://www.josephfusco.org/Articles/Roof_Cutting/Basic_Roof_Framing/Documents/Basic_Roof_Framing.html
Another method of solving the
Ooops! Double post ...
Another method of solving the jack rafter lengths is to treat all the working points as if they lie on the surfaces of a backed Hip rafter and ridge board. Using the Sheathing angle and a different set of dimensions is also a good way of double checking calculations. This example uses two equal 7/12 slopes and a 90° corner angle but can easily be adapted to an irregular roof.
Common Rafter Slope = 7/12
Common Rafter Pitch Angle = 30.25644°
Plan Angle = 45°
Sheathing Angle = 49.18024°
Run (inches):
(19 × 12) / 2 = 114
Length on slope of roof to WP:
114 / cos 30.25644° = 131.97822
Hip rafter width correction for WP:
.75 / (cos 45° × cos 30.25644°) = 1.22793
Difference in length to long point of first jack rafter:
23.25 × tan 49.18024° = 26.91661
Length of first jack rafter:
131.97822 – (1.22793 + 26.91611) = 103.83368
Difference in length for balance of jack rafters:
24 × tan 49.18024° = 27.78489
Successive jack rafter lengths:
103.83368 – 1 × 27.78489 = 76.04879
103.83368 – 2 × 27.78489 = 48.26390
103.83368 – 3 × 27.78489 = 20.47901
If you have a calculator with a formula editor:
131.97822 – (1 × 24 + 23.25) × tan 49.18024° – 1.22793
Change the bolded value to 2, 3, etc … zero returns the first jack rafter length.
Basically what you do is cut five or six pieces wrong, then decide to screw it all and make it a gable.