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In my story, the land of Koyokuni has a Japanese-style castle, Akimachi Castle. The castle - and indeed the entire city of Akimachi - is uninhabited, though the land is being repopulated after the end of the war in which it became completely depopulated.

The current ruler of Koyokuni is effectively a reincarnation of the ruler at the time Koyokuni was defeated, and as a result has bad memories of the castle in which the former lord and his wife were tortured to death. The current lord's girlfriend suggests that if the castle holds such bad memories for him, perhaps they should just burn it down and have done with it, and he agrees.

So, the "Akimachi Castle" which is burned is the main tower of a Japanese-style castle similar to Osaka Castle, built with the traditional materials. It is in good repair and partly furnished and equipped in the Japanese style, though it has no food reserves (it had been repaired and maintained over the past 17 years by automata, but not resupplied). The bailey of the castle is large enough that the main tower will be far enough from the buildings of Akimachi that the few people present can easily prevent them from burning.

Nobody is trying to prevent Akimachi Castle itself from burning.

My question is: How long would it likely take for the main tower of this castle to burn to the ground after a determined attempt at arson in which its ground-floor timbers and other flammable items are set alight?

References to accounts of similar historical events such as the fall of Osaka Castle in answers would be helpful, but I haven't been able to find any which say how long a Japanese castle might burn before it collapsed or burned out.

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    $\begingroup$ Have you tried asking on History SE? This seems like a real-world historical and/or firefighting question? (I understand that no one will fight the fire, but firefighters are typically the most interested in studies on how long it types particular types of structures to burn.) $\endgroup$
    – KerrAvon2055
    Sep 23 at 5:17
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    $\begingroup$ @KerrAvon2055 Yes, I have asked on History SE... days ago. Nothing but crickets. $\endgroup$
    – Monty Wild ♦
    Sep 23 at 5:19
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    $\begingroup$ What state do you envision as "burned to the ground"? Will collapsing do? If it's like a common wooden house of Eastern Europe (where they build wooden houses), there would still be a lot to burn left, and actually a pretty big fire after a collapse. $\endgroup$
    – Vesper
    Sep 23 at 7:07
  • $\begingroup$ @Vesper I mean how long before it collapses, and how long before there's nothing left but hot ashes and coals. $\endgroup$
    – Monty Wild ♦
    Sep 23 at 8:16
  • $\begingroup$ You may get some answer from Kurosawa's notebooks. Cobweb Castle in 'Throne of Blood' (Macbeth) was a full-sized castle. The initial plan was just to build the facade, but it turned out to be easier to make a full-sized castle for the set. This was in 1957, and there was a large US Marine Corps base nearby, with plenty of equipment and men keen to help. In 1985, he repeated this in 'Ran' (King Lear), burning another full-sized castle. The burning of either castle may have been helped along somewhat to fit a shooting schedule, but it is a castle-size thing made of wood and not a special effect. $\endgroup$
    – Richard Kirk
    Sep 23 at 15:40

5 Answers 5

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My Answer: Probably over a period of 1–2 hours.

According to Googling: Cypress wood is the wood of choice for Japanese castles; these are its firewood properties.

The first thing is that, compared to other woods, it burns quickly. This doesn't bode well for your castle smoldering over a period of several days.

The next key thing is that this type of wood is very sappy and prone to popping and crackling—sending embers flying. This should accelerate the process—even better than arson.

What I expect is that the initial fire (wherever it was set) will probably take a few minutes to catch fully and develop into a roaring building fire. From there, it will spread over the next 20–30 minutes through each room. The thickness of the wooden beams will mean it will take a bit of time for them to lose enough structural integrity to collapse—but I suspect this will only be a matter of a few hours—once the first beam fails, you'll probably see a chain reaction, and the building will rapidly fall down.

Another incident that might give you a benchmark is the burning of Fantoft Church by Varg Vikernes. Since it's unlikely you are an avid Black Metal fan (like me), in the early 90s in the Norwegian Black Metal scene, there were a large number of church burnings done by musicians/fans of the genre. Fantoft was an all-wood church that was nearly a millennium old. Although smaller than a castle—if you look at the style, there's a degree of similarity between it and a Japanese castle (that is, the kinda tiered design)—that church took 30 minutes to burn down.

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  • $\begingroup$ I'd expect that a castle would take a bit longer to collapse than a church, since it'll be built to take abuse where a church just has to stay standing and weatherproof. $\endgroup$
    – Monty Wild ♦
    Sep 23 at 8:39
  • $\begingroup$ @MontyWild Japanese medieval castles were never subject to artillery-powered siege, only to a man-powered one, so their walls were not designed to withstand impacts and fire. I expect this estimate is pretty sound for a wooden part of castle, as the major set of fortifications (vs people) was still made from earth which won't burn up. And past collapse, ashes would linger for a day or two I think. $\endgroup$
    – Vesper
    Sep 23 at 13:11
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    $\begingroup$ In the good old days (at least 1975), the U.S. used to kiln-dry its lumber to avoid the very problems you're talking about. Working with wet wood is a pain in the neck. Would sap, therefore, be a significant issue? Or did the Japanese work with wet wood? $\endgroup$
    – JBH
    Sep 23 at 22:30
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    $\begingroup$ @JBH - good points, but given the year of construction - I'm not sure Kiln dried is/was an option. Quite possible though they may have aged the wood naturally before construction - or the process of just being stood there for decades/centuries may have the same effect. However, if it's completely bone dry, then it should burn easier - whereas if it's still sappy, it will pop and spread easier - so it may or may not make a huge amount of difference. $\endgroup$
    – TheDemonLord
    Sep 24 at 1:40
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    $\begingroup$ @MontyWild - The problem is that if you add more wood to make it more robust - that's just more stuff to burn, so I don't think it adds anything significantly in terms of reducing flammability. Once the Fire is going, without modern advancements like Sprinklers, Fireproof doors, Fire-resistant building materials etc. It's gonna go quick. And think about this - Fire spreads upwards and the Castle design is perfect for that. $\endgroup$
    – TheDemonLord
    Sep 24 at 1:43
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A few hours, most likely 3-5

Average house in the United states takes 45 mins to 2 hours to burn, the average modern house size is about 201 square meters. Of course we can't use that as proper reference, so using a source from @TheDemonLord's answer, we know that the structural supports of the castle were most likely made of cypress wood, a type of lumber that is great for kindling, but not for sustaining a fire. In this illustration from François Caron's book, the fire appears to be on the left of the castle: "The Burning of Osaka Castle" (shown in the Wikipedia page as well)

The fire staring from the base/left wing of the castle in this illustration

Using the fact that the approximate area of the castle in square meters is about 1702.25 meters. We can assume that the use of accelerants could take 45 minutes to an hour to burn down a main wooden support beam or two, assuming there is ten support beams and the fact that the fire would grow (although move slower due to the accellerants burning out). This means that it would take 3-5 hours to take down enough beams to have the entire 5 story (Correct me if I'm wrong) castle to begin to crumble

Sources

Wikipedia page

How why newer homes faster

How Big is a house

How to measure height with google maps

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    $\begingroup$ Without knowing the size of the "castle," you're guessing a bit about how long the burn-down would take. However, +1 for providing a list of sources for your observations. That's a great start. $\endgroup$
    – JBH
    Sep 23 at 22:27
  • $\begingroup$ Thank you! I'll try to improve upon this answer $\endgroup$
    – WellActually...
    Sep 24 at 22:59
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Japanese weren't stupid, they coated their Castles in fire retardant/resistant plaster and mud. So while you can get a local fire going, it's not going to spread easily.

Your arsonist needs to set fire everywhere and perhaps remain and help it.

I would think you'd get the same effect as a flash bush fire, the easily flammable stuff goes up very quickly but the rest doesn't. It either slowly smolders until it destroys itself, or goes out. It's not a matter of setting fire to a curtain and then running.

If your main support beams are incased in fire retardant plaster (which blocks oxygen as it would), it will take a long long time for them to burn through. Best option chop off the plaster in crucial places and make your fire there.

How long it would take is up to your story, but logically it would take hours or days depending on how much prep was done before starting the fire. Japanese castles are specifically built to withstand frequent earthquakes, and therefore would need multiple points of failure for the main supports to all collapse.

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  • $\begingroup$ "a determined attempt at arson in which its ground-floor timbers and other flammable items are set alight" isn't just a lit curtain. It means that pretty much everything on the ground floor is set alight and/or exposed to intense fire. Lots of accelerants are used. $\endgroup$
    – Monty Wild ♦
    Sep 23 at 11:15
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    $\begingroup$ Sure, that would work, make sure they're long lasting accelerants. When I was in forestry we used flame throwers on brush that had been cut down and allowed to dry, and while it would burn small branches and leaves we'd have to go through several times to burn anything substantial, it wasn't self sustaining in terms of a constant blaze. If your main support beams are incased in fire retardant plaster (which blocks oxygen as it would, it will take a long long time for them to burn through. Best option chop off the plaster in crucial places and make your fire there $\endgroup$
    – Kilisi
    Sep 23 at 12:09
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    $\begingroup$ Also, was the inside coated? Protecting against fire from the outside is a very different thing from protecting against prepared burning from inside. And even if the inside were protected, this guy has the time to have that removed. $\endgroup$
    – SoronelHaetir
    Sep 23 at 16:56
  • $\begingroup$ Flagged as not an answer because you haven't answered the OP's question. Can you provide some quotations and source information for this? It would be great insight that would help the OP, but as it is, it's only an observation with no practical application. Dried mud isn't much of a fire retardant. Plaster would be, if it covered everything. But how much was covered? Can you cite an example where a burning building had these things? $\endgroup$
    – JBH
    Sep 23 at 22:26
  • $\begingroup$ I've been in a few Japanese castles, and there's no coating on the beams inside, just bare wood everywhere. Also, everything hangs off a central pillar or two - Himeji castle has, according to Wikipedia: "The main keep has two pillars, with one standing in the east and one standing in the west. The east pillar, which has a base diameter of 97 cm (38 in), was originally a single fir tree, but it has since been mostly replaced. The base of the west pillar is 85 by 95 cm (33 by 37 in), and it is made of Japanese cypress."" $\endgroup$
    – Ken Y-N
    Sep 25 at 4:44
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I don't know much about Japanese castles, however, I do know Japanese homes are very very flammable. (Japanese castles do have stone- but also wood.) Windows made of paper and oil, built mainly of wood and bamboo, your castle will burn like- well a pile of wood and bamboo with oil. I suspect a day maximum, and likely even less then that, considering you are trying to burn it down. I think you could get away with a 2 hour complete burn without anyone caring.

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  • $\begingroup$ The thing is, it takes time for beams to burn through to the point where they can no longer support the weight above them. I doubt that it'll be as little as two hours. $\endgroup$
    – Monty Wild ♦
    Sep 23 at 6:16
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    $\begingroup$ Walls and windows are traditionally light weight. The main struture in homes is VERY solid timber - designed with the intention of being able to have the light material burn out leav ing the frames suitabl for tidying up and rebuilding. Castles no doubt oare of even sterner stuff framing wise. $\endgroup$
    – Russell McMahon
    Sep 24 at 9:34
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Japanese castles are made out of Cryptomeria and coated with some kind of fire retardant and usually mud, meaning it probably wouldn't happen in 1–2 hours as others answers point, it would at best maybe take a whole day, with multiple people setting different parts on fire ([several daimyō set the castle on fire][1]), and he still had time to catch the perpetrators and throw them off the castle, mind that he COULD NOT stop the flames and ended up suiciding.

So it would most likely take a whole day to burn, assuming that

  • 1 floor on fire (no one trying to stop its spread) == multiple fires/floors on fire (Multiple people trying to actively stop the spread and the fire)

Is correct [1]: https://en.wikipedia.org/wiki/Siege_of_Osaka#Theory

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