Why does some rock produce a thick roof area when mined and other produces open

[sparr]

I haven't been able to discern a pattern.

[Jones-250]

Because mountains are not flat.

[sparr]

And since my characters don't live in flatland, they can surely SEE the slope of the mountain, so... how do I tell whether it will be thick roof or not before I mine it?

[Tynan]

And since my characters don't live in flatland, they can surely SEE the slope of the mountain, so... how do I tell whether it will be thick roof or not before I mine it?

Mouse over the square. If you've discovered it, you can see the roof type in the bottom left. You can't see undiscovered ones because you could find caves this way.