Hi,
i just got around to start playing with SketchUp Free to see if i can model my house before renovations… like so many other 
it going ok, and i got most of 3 floors done i multiply levels and working fine with layers.
But i made the error of building my 1th floor first and then trying to make my basement.
The problem i’m running into seem like i have inverted the outer wall so its now hollow from the top but solid from the bottom.
can anyone assist in how i can fix this ?
Can you upload the .skp file here if it is less than about 3MB? If it’s bigger, upload it to a file sharing service like Dropbox or OneDrive or Google Drive, and post a link here.
That makes it easier for anyone to respond.
For what it’s worth, just looking at your images, you can close the top surface by drawing a line over an existing edge, while you are in the context of that edge - if it’s inside a component or group, open that for editing first.
This sort of thing happens when draw your walls on a face. If you delete the face representing the floor before using Push/Pull, this won’t happen.
