Sunday, May 11, 2014

Verkanteter Gratsparren Axiom # 9

After looking at a 37 page solution to the jack rafter claw angles in a PDF file designed by a German Trainer for the World Skills Carpentry Competition, I'm adding a new Roof Framing Propositions – Axiom that applies to hip rafters rotated into the roof surface. This Axiom should eliminate a couple of the geometric steps to draw out the lower claw lines for jack rafters that are plumb to the earth. The  German Trainer's task model has the Verkanteter Gratsparren, canted hip rafter, with the jack rafters lower claw geometric development. His geometric development for the lower claw lines wasn't understandable, maybe because I don't understand some of the German Zimmerman drawing techniques yet. However, this Axiom should make it easier to draw out the lower claws lines for jack rafters no matter what  language you speak or read.

Roof Framing Geometry Proposition – Axioms # 9:
If the hip rafter foot print line is parallel to the jack rafter run line, then the lower claw line for the jack rafter is parallel to the jack rafter run line.

This Axiom will apply to roofs with an eave angle of 90°, with equal or unequal profile roof slope angles.

If you have Tréteaux Angles for RafterTools+ app for iPhone or Rafter Bevel Angles app with the Tréteaux Angles , you can check your drawings using P17a-DP for the upper claw angle, P16a-DP for the lower claw angle and P18a-DP for the back bevel angle on the top edge of the jack rafter. In this example I entered an Eave Angle of 90°, Main Slope Angle of 55.00798° and an Adjacent Slope Angle of 48.01279°. The hip rafter width does not affect the lower or upper claw angles in the app.

Some of the drawings to develop the DP line of the hip rafter rotated into the roof surface.

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