watsermetjou:
1. Select the Wall
2. Right-click and select "Connect > Trim Elements to Roof Shell"
3. Click on the Roof
4. Select the lower part of the Wall
But then I get the Wall Edge Material on the upper surface of the roof (area not shown in your screen shot), so maybe it is easier to use another roof above the model roof to do the SEO, then put that upper roof on a hidden layer.
David