generating a one-and-a-half story house?

[fliphouse]

I am remodelling a house with 4 foot walls on the second floor. The roof rafters start here and then proceed upwards in a 10/12 pitch. There are no knee walls on the back. front has two knee walls separated by a shed dormer.

When I try to draw the roof I either have it starting at the top of the second story - 96 inches or so or if I put in a knee wall the roof wants to start at the top of the first floor.

How do I get the roof to start hald way up the second floor so the bedrooms have a small sloped ceiling in the corners due to the roof line?

The only luck I had so far is with invisible walls but this seems cumbersome.

Any comments appreciated.

Regards,