When will the roof cave in on me?

How big can I make an underground room before it collapses?

It's a simple question that I haven't been able to get a good answer to. I assume that rooms made of stone can be larger than rooms made of clay or sand.

If I want to make a very larger room, can I place pillars in it to hold up the ceiling? (i.e. Can I just not dig out small sections of the stone, to leave small sections of wall to act as pillars?)

Solution 1:

In the Current Versions of DF (.34 as of this writing), he Earth, Stone, and Soil of Dwarf Fortress does not obey the laws of physics (yet!).

This means, you never have to worry about collapse, as long as the tiles above you are anchored to something other than simple air. Material has nothing to do with it; dunes of sand can be hollowed out just the same as loamy hills or dacite mountains.

Tiles in Dwarf Fortress have two parts: the tile body and the tile floor. If you mine out the tile body, the tile floor remains. If either of these pieces is wholly disconnected, a cavein will occur. Therefor, you might be causing caveins if you're hollowing out rooms via a combination of dig and channel. Instead, start from the top, and tell your dwarves to Carve Ramps into the layer below. This will hollow out the same area, but removes the risk of dropping floors that channeling has.

Solution 2:

Currently, only totally disconnected material collapses. See the cave-in article at the Dwarf Fortress Wiki for lots of details.