Outer walls of tunnels- Fixing height?

Whenever I make a tunnel, the wall above the enterance and exit sticks waaay up to whatever the ceiling height is, while the rest of the walls remain at their set heights. In case you don't know what I'm talking about, follow the link to see an example screen.

http://s758.photobucket.com/albums/xx221/Blazefan1/Srb2/?action=view&current=Heightissue.png

Its that rocky wall to the right of Sonic, obviously. Why do the walls over all my tunnels do that, and is there anything I can do about it?

Just make a really thin sector right up against the wall and set that sector's ceiling height to the height you want the wall to clip off at. It's actually done like this in GFZ1.