Create a roof surface to surround the plan. (You don't need to draw right over the wall as in the roof drawing, you can draw randomly from the outside of the wall to cover the plan.) Go to the Roof surface setting, set the Level 2m Roof slope/0.00001 Percent, Go to the Layer Settings, lock the Library layer. (In this way, the library objects will not be affected by the cut with roof operation you will do later.) Go to the tools, use the cut with roof command. Say create new trim group, select the roof, then select the plan and you're done. Go to Layer Settings and turn off the roof surface layer. (The roof surface will become invisible.) Switch to perspective view, say Default View/Top View. Render or edit in painting programs by pressing the PrtScn SysRq(Print Screen) key on the keyboard (taking a screenshot).