Roofs might very often be a hard nut to crack, especially when you have a little more complicated floor plan. The trick is to break the whole roof to a smaller parts and also make the roof as an extrusion.
Above you can see a fairly simple floor plan and our aim is to make roof which has gable on the west, east and south wall. Our mission starts at East Elevation.
Here we can see the east wall of our project. Architecture-Build-Roof-Roof by extrusion.You will be asked to pick a working plane. Choose Pick a plane and click on the east wall.
Now you can sketch the desired pitch of the roof.Once you click on Finish edit mode, it should look similar to this.And in 3D view.Now go to South Elevation and repeat the process.In 3D view, it looks like this. But I guess we do not want to have the dormer on the other side, so for a final touch Modify-Geometry-Join/unjoin roof.And select the edge of a small gable first and plane of the bigger roof second.Final result.
Hope I helped