Dormers follow a basic pattern, but edge lengths and angles are affected by the slope of the underlying roof. Also, dormers on opposite sides of a roof may differ if the roof slopes are different, which is not unusual.
The most prominent element of a dormer is the window. This will affect the scale of the dormer and the shape. Design the window (or re-purpose a window from elsewhere in the design if appropriate). Draw a bounding rectangle which will become the face of the dormer. Include the peak. Draw fold lines along the two sides and the bottom.
The next line of interest is the lower slope. This line will match the slope of the roof. Draw a line along the roof edge and drag it to meet the lower corner of the dormer window frame. Now draw a straight horizontal line (at right angle to the window) from the top corner of the bounding box to the slope line. Trim the slope line to meet the top line. This is the side of the dormer. The steeper the roof, the shorter the side of the dormer.
Reflect that process on the other side of the dormer. Make those lines into fold lines. Then draw an outer cut line that includes tabs at the top edges and a square tab at the bottom and along the slope edges.
The roof of a dormer is an odd shape since it must match the slope of the main roof at the back side, and the angle of the front peak.
You need two measurements from your design of the dormer:
- Length of Peak Edge (a)
- Length of Top Edge of Side (b)
Make a rectangle 2a tall and b wide. Place a fold line at least 2b long through the horizontal middle. Now copy the sloped line from your dormer design and place it from a bottom corner across the center fold line. Adjust your square with point editing to stretch one side into a peak ending where the sloped line crosses the center point. Clip the center fold line to size and discard guides.
Finally, you need to make a rectangular cutout on the roof where the dormer will be placed. The size of the rectangle is the width of the dormer by the length of the sloped line on the side. Make a rectangle that size, and place it over the main roof design where the dormer will be mounted. Remember that the roof of the dormer should be below the ridge line of the building roof, so place it closer to the edge of the roof peak rather than centering it.