Draw a 3d polyline 3dpoly along the edge of the walls to outline the shape.
How to create roof slab from lines.
Open the tool palette that contains the slab or roof slab tool you want to use.
Click duplicate to create a new type.
Use the thicken command to convert the surface into a 3d solid with the desired roof thickness.
Select a closed polyline and press enter.
You want to create a custom roof slab with 3 different corner heights.
Use the edgesurf command to select all 4 lines and create a surface that stretches between them.
Select element properties type properties.
You may have to scroll to display the desired tool right click the tool and click apply tool properties to linework and walls or linework walls and roof in the case of roof slab tools.
If you have a traditionally designed roof created from a two dimensional 2d plan showing ridge hip and valley lines you can trace over that plan with roof slabs specifying the desired height and slope.
Give it name roof slab edge.