This is what happens when you dunno limits of Gzdoom with Zdoom
I fully understand what this guy means. He has a xx high outdoor sector with a house, which has a 3d floor as a roof. Though polyobjects are set so they'll be xx high, as their mother sector. So nope, there's no way you're gonna make a door like
that
I suggest you make a door texture as high as your door frame and use semi-3d poly objects using poly_explicit line and some trickery.
Refer to the wiki:
http://www.zdoom.org/wiki/index.php?tit ... plicitLine
There's a demo wad somewhere at zdoom.org that shows how to make explicitlined polys.
I'm also certain a vertically sliding grate in map01 of Rex's Darkest Hour is made in a similair way.
Go and explore, seek out the knowledge (not beg for it with doing nothing..)