1. Use the Opening tool to cut the desired window opening in the front wall of the dormer.
2. The last step is to cut out the main roof slab where the dormer space connects with the attic space. There are a couple of approaches to this.
A very quick, but approximate, approach is to select the main roof slab, and then use the Poly-Opening tool to cut around the inside of the dormer walls and roof to make the appropriate opening.
But this results in the faces of the opening in the roof being at the wrong angle for a dormer. (They'd be good for a skylight though.)
So we'll show you a more accurate way to get all the pieces to line up nicely.
3. Using the Trim tool on the main upper face of the main roof slab, make three parallel cuts, along the fall line of the roof pitch, aligned with the center of the dormer and with the inside of each of the dormer side walls.
4. This produces two strips in the main roof, which need to be trimmed at a compound angle. (This tricky cut is horizontal along a level line perpendicular to the eave line, but is angled relative to a level line parallel to the eave line, and it's also angled relative to the face of the main roof slabs.)
To make the correct trim, select one of the strips of main roof slab, and
Group it with itself. Then use the Trim tool on the front elevation face of the group bounding box, to slice aligned with the underside of the dormer roof. Repeat for the other strip of main roof slab.
5. Now there's just a little clean-up work. Delete the lower, scrap portion of one of the roof strips.
Then select the lower portion of the other roof strip, and again
Group it with itself. This time, use the Trim tool on the top or side face of the group bounding box, to cut a filler piece for where the main roof crosses below the dormer.
6. Finally, select and delete the inner scrap piece of roof slab, and then select the lower filler piece, space-jump to a corner handle, and stretch it parallel to the eave line of the main roof, until it matches the width of the front of the dormer, and fills the gap in the main roof.
That's it! Once you've made one dormer, you can group its parts, and duplicate the dormer around your project as required. To save time making the opening in the roof slab, include a section of the main roof as part of your dormer group as well.
In the next couple of screen shots we added a window frame, which you can easily do using the Paste Into Opening feature as discussed in the cottage tutorial.