2011 London Dormer #3

More theories on the sloping ridge dormers. The sloping ridge on the dormer is actually a hip rafter, so I developed the ground plan for the hip rafter and then used the standard folding roof surface development. With the folding roof surface it's easier to develop the lines for the jack rafter miter and bevel angles, as well as the claw lines.

Sloping Ridge Dormer Ground Plan

with the hip rafter plan angle, profile rafter slope angle and the hip rafter slope angle. Once you establish the hip rafter run line based on the slope of the sloping ridge you can draw the Eave Line / Ground Line / Gutter Line / Sewer Line / Égout / Hauptdachtraufe / Main Roof Eaves /Trauflinie / Eaves.

The profile rafter run is always perpendicular to the Eave Line.

You can use the rafter length of the profile rafter to establish the real roof surface of the dormer.

In this drawing I'm checking the geometric roof surface development on the real roof surface of the sloping ridge dormer.

Perspective view of the sloping ridge dormer developed as a hip rafter.

Here we have the hip rafter in elevation. The profile rafter drawn with the depth of the jack rafter perpendicular to the roof and the hip rafter development on the roof surface for:

1. Line of the Base of Hip Rafter
2. Line of the Face of the Hip Rafter
3. Internal Edge of Hip Rafter on Roof Surface
4. Claw Line at the Top of the Hip Rafter Claw Plane
5. Claw Line for the Bottom of the Hip Rafter Claw Plane 

I used the hip rafter backing triangle with the rafter profile block rotated for the dimension of the Internal Edge of Hip Rafter on Roof Surface.

Finally I developed the jack rafter perpendicular to the roof surface by placing the timber over the drawing on the roof surface for the miter line on the side of the jack rafter, back bevel angle on the top & bottom of the jack rafter and the claw lines.

I still need to work on the miter, back bevel and claw lines at the foot of the jack rafter.

The main problem with using the roof surface for the development is size. This technique would never fit on an 8' x 8' drawing board.

I did check the jack rafter perpendicular to the roof surface with my Rafter Bevel Angles app for Android and my geometric drawing was correct except for the claw angle. The drawing had a claw angle of 30.4° while the Rafter Bevel Angles app had a claw angle of 28.33130°. Not sure what that's all about.

Rafter Tools -- Jack Rafter Plumb To Earth Offset Bevel Angles

Profile Rafter Slope Angle
37.87752°
Hip Rafter Plan Angle
10.69718°
Hip Rafter Slope Angle
8.21576°
Hip Rafter Edge Bevel Backing Angle
37.10703°
Jack Rafter Offset Angle
30.53374°
Jack Rafter Foot
Miter Angle Side
36.76964°
Bevel Angle Top Edge
24.96761°
Jack Rafter Peak
Miter Angle Side
45.45514°
Bevel Angle Top Edge
129.77149°
Jack Rafter Claw Angle At Hip Rafter
28.33130°
Tail Slope Angle
90.00000°
Rafter Witch Cut Miter Angle
90.00000°