Whenever dormer roofs appear, it is usually best-practice to use Solid Element Operations (SEOs) to take care of the subsequent required roof trimming.
The reason behind this is due the fact that:

• a wall can only be trimmed by 'parallel' roofs using the Trim-To-Roof method
• a wall can be affected by any number of roofs using Solid Element Operations

The downside of SEOs is that the effect of the operation is only shown in 3D and Section/Elevation.
Floorplan is not affected in any way (which may in some cases cause an inconsistency between 3D and Floorplan)

A PDF outlining the steps for Solid Element Operations can be found here and also a link here shows a video on how to implement the operation.