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Warning this roof should only be attempted by master roof cutters and stackers.

This baywindow rafter script - calculator will return the baywindow geometry angles and trigonometry rafter measurements for the Joe Carola bay to point baywindow roof framing with offset bay hips. The script is limited to 45° baywindow calculations. The plate line must meet at 45° at the two intersecting corners.

  The side major common rafters meet in the center of the baywindow with no ridge. The front of the baywindow is calculated as a pitch to point from the rise of the two side major common rafters. The bay hips are not at 67.5° from the corners of the baywindow. The bay hips are offset from the corners of the baywindow to keep the overhang run equal on all sides of the baywindow. The front rafters will also have a different H.A.P. then the two side major common rafters.

These four geometry angles for the bay hip are greatly effected by the side pitch, bay projection length, face wall length and overhang run. Just by changing the overhang run from 12" to 18" will drastically change the geometry angles for the bay hips.

• Bay Hip Rafter Run Bisect Angle
• Bay Hip Rafter Run Top Bisect Angle
• Bay Hip Rafter Run Adjacent Bisect Angle
• Bay Hip Rafter Run Top Adjacent Bisect Angle

The Bay To Point baywindow rafter script calculates all trigonometry angles from the fascia line.The fascia height at the back wall, where the major common rafters are located, will determind the minor pitch angle and the 45 wall rafter angle. To calculate the overhang rise we use the angle of the major pitch and the hypotenuse of the overhang run.
diagonal overhang run = hypotenuse(overhang run,overhang run)
overhang rise = tan ( Major Pitch ) × diagonal overhang run
It would be a common mistake to use:
overhang rise = tan ( Major Pitch ) × overhang run
However, the major common rafters are at a 45° angle to the fascia line so we must use the diagonal overhang run to start all of our calculations with.

To calculate the rafter angle for the 45° wall rafters we can use the angle of the major pitch and the hypotenuse of a 24'' right triangle:
run = hypotenuse(24,24)
rise = tan ( Major Pitch ) × run
45° wall common rafter angle = arctan ( rise ÷ 24 )

 

bay-to-point-baywindow.dxf -- auto generated DXF CAD file of baywindow drawing each time script is run
bay-to-point-baywindow.pl source code for perl script that generates these baywindow calculations
dxf_header.txt DXF header text file used by this script