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[ManaUser]

Dang it, after carefully planning my initial hole-in-the-wall fort so that I could turn it into a defensive entryway later I discover a serious flaw: holes in the roof! Where did they come from? Something related to the trees up there seems most likely.

Does anyone know exactly how this happens?

[thefinn]

Trees have root systems, cut the tree, the roots disappear with the tree.

[ManaUser]

That might be related, I don't think that fully explains it, because normally the roots are replaced with dirt.

I did a little more experimentation and it seems the specific thing that triggers it is cutting a tree after digging under it. Cut then dig: No problem. Dig then cut: No roof. Almost tempted to call it a bug since I can't see a logical reason why the order should matter, but I guess as long as I know about it now it's not a big deal.

[mikekchar]

Holes in the roof when trees are cut down is well known.  I think it's intended to be a feature rather than a bug.  See the entry in the wiki: http://dwarffortresswiki.org/index.php/DF2014:Tree#Chopping_down_trees

[ManaUser]

I see. Live and learn I guess. I honestly haven't played that much since trees became multi-tile (quite a while back, I know). Come to think of it, the only major fort I did play in the modern era was on a site with an aquifer, which meant I had a partly above-ground wooden fort, and the underground portion was more than one layer below. So I guess that explains why I never encountered this before.

[anewaname]

I am fairly sure that if you cut down that tree while the dirt is still underneath it, and you give it some time, a new layer will grow (grass or something, possibly related to the biome's growth or rainfall), then you can dig out the z below and there will be no hole. My policy is just to avoid digging in the first soil layer.

[ManaUser]

Even digging it out immediately after cutting the tree seems to be fine actually. As long as there's a solid block under the tree when you cut, it seemingly leaves completely undisturbed ground behind.

Next time I'll just have to be more careful, although as you say avoiding the first layer entirely might be the smart way to go.

That fort's toast anyway, stupid wererhino.

[Mort Stroodle]

What's happening is that you're digging out the wall, leaving only a layer of floor. When a log falls onto a floor with no wall beneath it, it crashes through, destroying the floor.

[PatrikLundell]

When digging immediately below the soil (or under caverns) you need to protect the tunnel either by never cutting trees above it (which will probably fail time and again), or make sure trees can't grow above by placing a floor, road, or stockpile above it. You can also build dirt roads above every 2-3 years to get rid of any saplings (they take 3 years to mature).

[ManaUser]

What's happening is that you're digging out the wall, leaving only a layer of floor. When a log falls onto a floor with no wall beneath it, it crashes through, destroying the floor.

I'm not convinced by this explanation. I only ever got a single tile hole, where the main trunk was, and it's not reported as a cave-in. So this theory doesn't quite add up for me. Not that the exact game mechanics behind it really matter, as long as we all know what to avoid doing.

[Dozebôm Lolumzalìs]

That explanation is false. Falling items cannot cause cave-ins.