Monday, December 1, 2014

Hipped roof with canted center purlins

Hipped roof with canted center purlins
Walmdach mit verkanteten Mittelpfetten

If this task model didn't have the half lap joint at the canted center purlins, then it would have been an easy task model. Bernd's drawings of the witches cut on the pent roof hip rafter is easy to follow. As, well as the jack rafter claws. He shows two ways of developing the hip rafter bird's mouth on the canted purlin and two methods should be drawn to make sure each of the methods produce the same results.

The half lap joint on the canted purlins took some time to develop mentally. However, after a couple of instructions from Bernd the canted half lap joint geometry is a winner. The book Basiswissen Schiften has some canted purlin drawings, but there almost impossible to follow. Bernd's method is similar to developing the joints on  hoppers. There's really not much difference between a canted purlin and a hopper and the method in the book presented by Bernd makes the half lap canted purlin joint or  "french pressure joint" (franz√∂sisches Druckblatt) easy to develop geometrically.

This task model required a little thinking to draw out the base drawing. You have enough information in the drawing to get started, but the profile rafter takes a little work to draw. In the first drawing I used some math to find the angle tangent to a circle with a radius of 20mm for the rectangle hap of 20mm. However, in a WorldSkills competition you should use the method in the second drawing, by using a stick 20mm in width and swinging it until it touches the exterior purlin(wall plate) to develop the profile rafter with a 20mm hap.

Method # 1 for developing the bird's mouth on the hip rafter for the canted center purlin.

Method # 2 for developing the bird's mouth on the hip rafter for the canted center purlin.

Drawing with the jack rafter claws developed.

This is an easy way to develop the miter & bevel angle cuts on the canted purlin, if the joint was mitered only.

Drawings showing the method to develop the half lap joint on the canted purlin.


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