2011 London Dormer

Tony McGartland sent me the 2011 London Dormer file several months ago asking me if I give the geometric construction lines of the sloping ridge dormer a go.  All of the WorldSkills Carpentry competitions have two things in common. A polygonal ground plan and a sloping ridge dormer. The polygonal ground plan isn't all that hard to construct, but the sloping ridge dormer is a tough one to get a handle on. Mainly the jack rafter perpendicular to the roof surface and skewed from the plate line.

Here I developed the ground plan, profile and elevation to develop the valley sleeper.

This drawing shows the relationship of the valley sleeper bevel to the ridge line. This drawing is just a reference drawing. It has nothing to do with the dormer. I needed to make sure the valley sleeper bevel was the same geometric construction with any type of ground plan.

I placed the valley sleeper material over the develop drawing and scribed the lines to construct the 3D model of the valley sleeper.

Next, I did the same thing for the sloping ridge and the front gable rafter.

 I couldn't reverse engineer the jack rafter perpendicular to the roof surface, so I decided to see if I could come up with any theories using a dormer with a ridge that is not sloping. 

Here's the ground plan.  

3D perspective of the dormer with a level ridge.

Here I developed the roof surface ignoring the valley sleeper. Using the standard folding roof surface development and developing the  jack rafter on the roof surface developed the correct miter and bevel angles at the ridge.

In the next post I'll develop the dormer with a sloping ridge.