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[PatrikLundell]

It's the second time I've been subjected to this. A fire spewing FB is "safely" kept out of a cavern by a wall with a wall on top of the single tile wide strip the FB is confined to with two raised stone drawbridges in the wall and Up staircases on top of the bridges. The FB then finds something to blast with fire, and fire somehow spreads into the cavern and eliminates my stocks of logs (that's only a temporary nuisance, though). Since dorfs don't really understand fire, the whole cavern will have to be placed off limits while the fire rages.

Any ideas of why fire jumps through walls and, potentially, how to block it?

z+0:

w..b......w
WWWBWWWBWWW
......hbh..

z+1:

wWWWWWWWWWw
...u...u...

where:
w = natural wall
. = floor (natural on Z+0, top of wall on Z+1)
b = lifted part of raising drawbridge (i.e. the one "disappearing")
W = constructed wall
B = "hinge" part of raising drawbridge (i.e. the one remaining visible when raised)
h = ramp down (and the rightmost bridge has a ramp under the lifted part)
u = constructed up stair

[Fleeting Frames]

Happened in Breadbowl too. Dragon burned wooden cage traps behind a wall. Best guess is that the fire heats up surrounding tiles enough for tiles near those to catch fire. Check the temperature of those walls?

Prevent it like you prevent a forest fire from spreading, I guess. For same 1-wide footprint, perhaps nether-cap barrier would work.

Still, you'd expect grass to not catch fire from fire behind wall if it is unharmed by magma under it. Alas.

[Melting Sky]

Still, you'd expect grass to not catch fire from fire behind wall if it is unharmed by magma under it. Alas.

Dragon fire is MUCH hotter than magma. Magma is only 12,000 U where as dragon fire is 50,000 U.

[PatrikLundell]

Nether-cap is a good idea. Unfortunately, I don't have any on my embark, and even if I did, I doubt I'd be able to get enough to build all the cavern entrance holding areas with it (it would require in the order of 500-1000 logs for 3 zones per cavern).
But yes, it was the floor fungus that caught fire.

I can't check the wall temperature, though, since the fire burned out and I have finished and used my FB killing device, so that FB is now on its way to become lavish meals. Hm, come to think of it: can you check wall temperature without DFHack (0.43.05, so it's a while before it catches up)?

[Fleeting Frames]

Probably not, but you could see if the wall flashes hot trying to designate digging.

@Melting Sky: Whoops, I suppose that statement wasn't perfectly clear. "Happened in Breadbowl too." connoted that the burning grass made things on other side of walls catch fire even without dragon breathing on the wall, with the dragon comment bit as an example of how heat brings ruination.

[PatrikLundell]

I guess the digging designation idea can be used to get an idea if it's hot or not, but, but it's obviously not conclusive even if it is (as per the comment about magma below).

[Goatmaan]

Could this be yet another reason for 2x thick walls?

  Goatmaan

[PatrikLundell]

I assume double thick walls ought to do the trick.

Funnily enough, I did get another fire breathing FB in exactly the same place. It did start a fire (it found a Reacher to roast), but digging designation did not warn about heat anywhere. The fire didn't spread either, though.

[fearlesslittletoaster]

I too have had fired jump walls, on the surface though. It was... problematic given what I had stockpiled inside the keep. I'm not in a position to test this right now, but I did have an alternate idea. Maybe somebody else can do the !!SCIENCE!! on it?

So what if the floor fungus/grass under a constructed wall can still burn? That would also explain this, especially since grass fires don't burn very hot if I recall correctly.

[FantasticDorf]

Creatures and fire in exceptions can pass through walls if they only have a floor covering the top because they enter at a angle.

Place free hanging ramps along the edges, a bit like the overhang of a roof, alternatively it might be the localised heat that is causing the wooden objects to catch fire, not the actual fire itself piercing the walls (if its not going in at a angular direction like i mentioned)

[PatrikLundell]

If you remove a constructed wall the ground cover is gone, so I expect it to disappear as walls are built, not as they are removed. That would argue against fearlesslittletoaster's theory.

[Sanctume]

Do only grass adjacent to the dragon fired wall bursts into flames due to the heat initially? 

Perhaps a 1 wide dry moat inside + wall + 2 outside moat would be an optimum setup. 

Or a 1 wide road + wall + outside moat.

A double wall does not sound too much compared to a floor + wall setup either.

[Loci]

If you remove a constructed wall the ground cover is gone, so I expect it to disappear as walls are built, not as they are removed. That would argue against fearlesslittletoaster's theory.

Yes, but drawbridges do not clear the underlying tiles, so it is conceivable that the fire spread via grass growing under an access bridge.

[PatrikLundell]

Hm, that's a thought, Loci!

If that's the case the counter would be to simply place a floor underneath the bridge. That's at least worth a try on future constructions. I'm not sure where the fire broke through, but the bridge tile has the merit that it at least would follow some kind of logic.

The wall, by the way, is probably not dragon fired, as the FB victims are orthogonal to the FB (or on top if it) as the area is a single tile wide, so there's no reason to blast into the wall. Given all the smoke it's hard to see, of course, but I expect the escaping fire to come from the floor fungi set on fire by the FB, rather than the FB itself.

[lcy03406]

Could this be yet another reason for 2x thick walls?

  Goatmaan

What's the first reason?