By James Philip
When you create a roof Dormer in two parts at the roof’s edge, the dormer section will not join to the main roof,
using the roof join tool. The resulting error message is displayed. As shown in Fig 1
Fig 1
The Roof is created using the by footprint option and when defining the perimeter, the outline is as shown in Fig 2
Fig 2
Please note that side that you wish to be the dormer is divided into 4 segments with slope removed from the ones that will form the dormer.
You draw two converging slope arrows the same length as the two segments, directly on top of them as shown in Fig 3
Fig 3
To set a height for the apex, select both arrows and in the properties panel set arrow Height Offset at Head to amount you require. The resulting roof looks as shown in Fig 4 
Fig 4