Tuesday, August 27, 2013

How to Layout Jack Rafter Side Cut Angles using the Shiftungen Technique.

Jack Rafter Plumb Line Shiftungen.


The  Shiftungen Technique involves full geometric drawings of the rafters in plan view, elevation view and the roof surface view. However, you only need to know the plumb line shift to layout jack rafter side cut angles correctly.

How to Layout Jack Rafter Side Cut Angles on Equal Pitched Roofs using the Shiftungen Technique.
This method only works on eave angles of 90°.

Using 2x material, 1.5" thick, mark both sides of the framing square tongue, that is also 1.5" wide.

Next draw two perpendicular lines on the top & bottom edges of the jack rafter material from the 2 plumb lines.


Then draw the back bevel line as shown in this drawing.



Use a hand saw to cut along the plumb line & back bevel line or use a saw set to 45° to cut the compound angle on the face of the jack rafter that produces the correct Jack Rafter side Cut Angle.

The two plumb lines on the side of the jack rafter material are always parallel and they are drawn at the thickness of the jack rafter material. If you were cutting 3.5 x 5.5 jack rafters, then the 2 plumb lines would be 3.5" apart.

This method is only for equal pitched roofs with an eave angle of 90°.

Jack Rafter Plumb Line Shiftungen.


Jack Rafter Plumb Line Shiftungen for any roof eave angle or with unequal pitched roofs.

Jack Rafter Plumb Line Shiftungen = jack rafter material thickness x tan ( Plan Angle)

Example on an Equal Pitched Pentagon Roof
Jack Rafter Plumb Line Shiftungen = 1.5 x tan (54°) = 2.064573 = 2 1/16"

Example on an Equal Pitched Hexagon Roof
Jack Rafter Plumb Line Shiftungen = 1.5 x tan (60) = 2.598076 = 2 5/8"

Example on an Equal Pitched Octagon Roof
Jack Rafter Plumb Line Shiftungen = 1.5 x tan (67.5°) = 3.62132 = 3 5/8"


Example on an Unequal Pitched Roof
Jack Rafter Material Thickness = 1.5
Eave Angle = 90°
9:12 & 12:12
9:12 Plan Angle = 53.13010°
12:12 Plan Angle = 36.86990°

9:12 Jack Rafter Plumb Line Shiftungen = 1.5 x tan (53.13010°) = 1.99999" = 2"

12:12 Jack Rafter Plumb Line Shiftungen = 1.5 x tan (36.86990°) = 1.125" = 1 1/8"

Example on an Unequal Pitched Roof
Jack Rafter Material Thickness = 1.5
Eave Angle = 120°
8:12 & 10:12
8:12 Plan Angle = 70.89339°
10:12 Plan Angle = 49.10661°

8:12 Jack Rafter Plumb Line Shiftungen = 1.5 x tan (70.89339°) = 4.330126" = 4 5/16"

10:12 Jack Rafter Plumb Line Shiftungen = 1.5 x tan (49.10661°) = 1.732051" = 1 3/4"


Jack Rafter Plumb Line Shiftungen.

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2 comments:

  1. That was a very simple and ingenious way to make the bevel cut with just a simple square and not a framing square. Since you were using the Chappell Master Square 10 and 12.02 would have done the bevel cut also. The drawings were great and easy followed.

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    Replies
    1. John,
      Yes, your correct on using the tan of the roof sheathing angle x 10 on the Chappell Master Square.

      Take a look at this drawing. It shows the plumb line shift for
      Plumb Line Shift Dimension for:

      Square Pyramid (4 sides) = 90°
      Pentagon (5 sides) = 108°
      Hexagon (6 sides) = 120°
      Octagon (8 sides) = 135°
      Decagon (10 sides) = 144°
      Dodecagon (12 sides) = 150°
      Hexadecagon (16 sides) = 157.5°

      http://4.bp.blogspot.com/-pSjZPkoHxgU/UjHb87QfSdI/AAAAAAAAb5A/0gzYUMl1fNw/s1600/JackRafter1v_2-002.jpg

      Sim

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