This document gives a rough method of how to implement something I call "TROR scissoring". Scissoring involves the use of (sometimes multiple) angled TROR sectors to allow TROR in a localized area without extending the entire zone.

The steps below refer to an example map called trorscissoring.map, which has examples of various stages of constuction of scissoring a small rectangular building.
There are 3 different methods shown in the map. The first one uses a single scissor sector to allow the mapper to carve out a doorway into a sector based building so that it can be entered. This result is in the top right corner of the map.
The other two ways involve surrounding the building with scissor sectors as to emulate normal TROR, allowing for an overhanging roof, and windows all around etc...
The difference between the second and third method is that in one case, the scissor sectors slope from the floor, and in the other, they slope from the ceiling. These results (which are visually and virtually identical) are on either side of the smiley face.

The following steps will refer to the general creation for both methods two and three, because method one should be relatively obvious. This guide also assumes you know how to use TROR.
1. Make 4 narrow sectors around the building, one on each side. These are the scissor sectors, and they all have one side connected to the building, two sides connected to adjacent scissor sectors, and one, outer side, exposed to the surroundings.
2. Make the outer side/wall the first wall for each sector, and slope the floor of each scissor sector until it touches the roof of the building. This step is the same regardless of whether you're sloping from the floor or ceiling. (This is step 1 from method 3, or step 2 from method 2.)
3. Extend each sector down one by one if you're sloping from the floor, (do NOT extend them all at once) or up if you're sloping from the ceiling.
4. Go into 2D mode, and make it so you're only editing the extended 'layer'. Press enter on all the walls connecting the scissor sectors to join them (making the 4 white walls turn red). Then press alt-s in the central white sector to make it a child sector.
5. Adjust the ceiling of the top 'layer' so it matches the height of the surrounding ceiling. Or the height of the floors if you extended down.
6. You should now have a regular floating island, just like you were using normal TROR. You can now edit the top and bottom as though they were your average, every day sectors. (this includes creating void space for solid walls.)

I encourage you to make a copy of the example map, and try messing around with the final product of each method to see what you can and can not do. I.e, try making a chimney on the roof, or a television hanging from the ceiling inside. Or maybe even a window in the roof that lets you jump into the building from above!
It can help to visualize the situation in mapster by pressing 'i' without quotation marks to reveal the TROR boundaries in-game.

This guide, along with the construction of steps 2 and 3 in the example map, were made by Micky C, who discovered TROR scissoring. Mike Norvak built step 1 in the example map.