1. Create Planes
a. Save into new sequential file
b. Ctrl+Right-click->Plane->Draw in Top viewport framing walls
c. Adjust Length or Width Segments->Won't require so much probably
d. Click 'Move' button then Shift+X axis to copy plane to ceiling level
e. Adjust in Left viewport for precision using Edge/Segment & Endpoint Snaps
f. Rename new plane as 'Ceiling' and original plane as 'Floor'
2. Place into Layers the Two New Planes
a. With Floor selected->Right-click on main toolbar->choose 'Layers'
b. Layer in 'Default'->Create New Layer 'Floor'
c. Tick 'Move selection to New Layer'->OK
d. Freeze 'Floor' layer as to not disturb selection in workflow->Will turn gray
e. Do the same with the 'Ceiling' layer but don't freeze or hide it yet
3. Vaulting the Ceiling
a. Apply 'Edit Poly' modifier to 'Ceiling
b. Select 'Vertex' sub-object
c. Left viewport window select vertices in center and move to beginning of vault
d. Adjust to approximate closeness by zooming in
e. Window select vertices near highest point in Left viewport and align
f. Now will be a hassle to work in the interior so hide the 'Ceiling' layer