This is an excerpt from a 2011 article written by Marcello Sgambelluri. Even though Marcello has figured out a way to do this with Dynamo, this technique is still quite simple and valid. You can find the entire article here: https://www.augi.com/library/methods-for-modeling-complex-geometry
1. Make a Wall which intersects the Roof.
2. Join the Wall to the Roof
2. Use the 'Pick Lines' tool in the Beam command. Turn on 3D snapping. Select the intersecting line of the wall and roof.