Roof with odd angles

[zasanil]

I'm trying to connect the slanted roof section to the flat roof part with a 1 foot "boarder" to fill in the gap. It seems that I cant change the elevation of just 1 corner to make the right slant to it. See the red line for what I want. Could someone please explain an easy way to draw a roof like this?


After I fix the roof I plan to add a dome for a telescope to the center of the flat part. Is there any easy way of adding a hole for this?
THanks for any replies.

Dan