I would like to create a prow roof, where a gable-style wall juts out a few feet further than the rest of the structure. How can I use Home Designer's automatic roof tools to accomplish this task?
Creating a Prow style roof in Home Designer is easy by properly defining the correct walls as High Shed/Gable walls. To begin, start with a basic plan, such as the one shown in the image below.
Once you have your new plan created, you are ready to start defining the walls to create a prow style roof.
tool to select the back horizontal wall, then click on the Open Object 
tool to select one of the angled front walls, then click on the Open Object 
to open the Build Roof dialog, then click OK to generate a roof over the structure.
Now that you have a basic understanding of how to create this style of a roof, you can use invisible walls to create a room that will allow for extending the roof out further.

tool to select one of the angled front Invisible walls, then click on the Open Object
to open the Build Roof dialog, then click OK to regenerate a roof over the structure.
Now that you have regenerated your roof, you will need to modify this room created with room divider walls so that it does not generate a ceiling over the room, allowing your structure to appear as though just the roof juts out over the area.
The Home Designer Pro version of the software, which allows for manual editing of roof planes, does not require creating a temporary room unless you would like to utilize the automatic roof tools for this process.
tool to click inside of the exterior room to select it, then click on the Open Object

You are now ready to take the information you have learned in this article on how to create a prow style roof, and apply it to your own design.