Paul:
You could try this sequence:
1. In the 2D window, select the roof, and use the pet palette to extend the roof edge to your desired overhang depth.
2. Cut a section through the model using the Section/Elevation tool, then select the roof to move it up or down as required to fit on the wall.
3. In the 3D window select the end wall and make its top higher than the top of the roof, then use Solid Element Operations or Trim to Roof to cut the wall with the roof.
You can search the ArchiCAD Reference Guide and this forum for more details about each of these operations.
David