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

  The span for the common rafters is not the typical side projection * 2 + the face wall length. The span for the common rafters is taken from the fascia line when the two back bay hips and valleys intersect. The front face wall common rafter run is then calculated as: face wall common rafter run = (fascia line span ÷ 2) - overhang run. The front pitch of the baywindow is calculated at the same pitch of the side 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. With this type of bay window roof framing the 4 bay hips are all at the same pitch and the same length. The valleys are running at a 45° to the main wall lines. The 45° side wall king common is in not in the center of the 45° wall, but is in the center of the overhang fascia.

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 hip.

• 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

 

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