I'm trying to extend an edge from the existing roof face axis. How can I do this? I want it to go all the way down to the lower part of the building. It must connect there (I want a edge there too so the remaining right part of that lower building will be cut off).
You can create new transform orientation for that edge.
Select side face of the roof. In the N panel create new transform orientation (you can create it pressing Ctrl + Alt + Space, as Gonzou mentioned):
The created orientation should be selected as active. If it isn't, in Alt+Space menu select this orientation (it will be called something like
Face). Switch to Edge selection mode, select your edge, extrude it, cancel moving by Esc and start grabbing pressing G twice.
- Select the face of the roof.
- Press ShiftNumPad 7 to move the view to that face.
- Change the Transform Orientation (in the 3D view header) to "View".
- Press E ShiftY (to exclude the Y axis from the movment)
A new edge perfectly aligned to the roof.
In the image above a loop cut was made. Next we viewed the angles as indicated. The vertex of the loop cut was moved until angles were within tolerance. You may may have zero tolerance capable hands. In this example the angle in question is about 44 degrees. Finish by adding and deleting faces.
The loop cut allows deletion of faces.