If you wanted to use the roof and walls method, I would recommend using full height walls then using the roof as an operator with an SEO with upwards extrusion to trim them down.
When placing your roof in 3D don't be afraid to use 'z-distance+' to set your heights, if you know what they are relative to the top of the walls. For the first three clicks when defining your roof plane simply place your cursor on top of the walls (as shown by the tick cursor), then enter the distances, eg. z-3000+ for 3m down from the top of the wall. For the last three clicks simply click on the top corner of each wall to define the roof polygon shape.
This theory also applies to the bottom of the walls of course, except it will be a positive z distance.
Alternatively you can set your Z value relative to the user origin in the coordinate box (instead of Project zero), then relocate your user origin at each step, and enter relative distances, without the + at the end.
Or you can use the mesh tool....
Cheers,
Link.