Check the heights of the Ceilings are correct and trim the Interior Walls. Do this in 3D view.
Note: At this stage if the Elements have been grouped then all the Interior Walls can be selected with one click.
Select any Interior Walls that require trimming.
Right click, find "Connect" in the menu.
Click "Solid Element Operations".
Interior Walls will be selected as the target.
Deselect the "Interior Walls" and select the Ceilings in the project.
Click "Add as Operator".
Under "Choose Operation" change from the default "Subtraction" to "Subtraction with Upward Extrusion".
Click "Execute".
Repeat the process for the Exterior Walls. This time check the height of the Roof is correct and use the "Roof as the Operator".