## Tuesday, May 21, 2013

### Jack Rafter Plumb To Earth Offset/Skewed From Plate Line

Jack Rafter Plumb To Earth
Offset/Skewed From Plate Line
With Claw Tip
With Edge Bevel

This model is an unequal pitched roof, with a 9/12 & 12/12 pitch. This model is a study of the easiest way to draw out the jack rafter claws for a jack rafter plumb to earth and offset/skewed from the plate line. The geometry for this model uses the prism plane to develop the jack rafter claw angles. The geometry for the jack rafter claws can be developed using the folding roof surface or the draw down roof surface techniques, but using the prism plane seems to be easier and more accurate than the folding roof surface or the draw down roof surface method.

The points B1, B2, B3 and B4 develop the miter angles on each side of the jack rafter. The bevel angle on the top of the jack rafter is automatically developed from drawing a line across the top of the jack rafter material from points B1 to B2. The claw angle is developed from the H1 dimension off the hip rafter in elevation. Mark off the H1 dimension along the B1- B4 line. Then transfer the claw angle F1-C2-C1 to C3-H1A on the jack rafter to establish the claw angle on the jack rafter. The downhill side of the jack rafter will use the same claw angle, but drawn thru H1B that is the H1 dimension measured from the V2 edge bevel line on the downhill side of the jack rafter.

Jack Rafter Side Cut Angle
Horizontal Rotation Plane  = arctan(cos Vertical Plane ÷ tan Horizontal Plane )
Jack Rafter Angle = 33.00449
Jack Rafter Offset Angle = 60

Jack Rafter Side Cut Angle Foot = arctan ( cos 33.00449 ÷ tan 30) = 55.45467
Jack Rafter Side Cut Angle Peak = arctan ( cos 33.00449 ÷ tan 23.13010) = 63.00786
rotate_angle = 90 - (180 - jack_rafter_plan_angle - main_plan_angle)
rotate_angle = 90 - (180 - 60 - 53.13010) = 23.1301