This is the 'advanced' image that the ADT/ACA help menu gives you on the roof object. No wonder most people will convert to slabs if that's all you can do with it! (It's not - see here). Little of the roof object's secrets are divulged in the help menu.
After I posted my roof object demonstration file showing the 'trim the roof object for an extra vertices' trick, Doug posted in the NG that he could simply recreate the whole roof quicker than trimming it to gain extra control points. I think this is important to note because of course 'just because you can do something ...... ' Doug I believe was at least partly referring to the ability to change from hip (slope) to gable as you are creating the roof by altering the settings in the properties pallete. I tend to forget and have to alter it afterwards anyway. Some with the grips, others by edge editing and others by trimming. It's just good to know that you have these options.
I hope that Doug will confess when he resorts to trimming the roof instead of recreating :-)
If my instructions on adding a vertice to the roof object are unclear to you, you can check out Odin's excellant tutorial here.