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So one of my players just texted me and asked me, "Hey, how would you deal with me burning down a town"?

Honestly, I have no idea how to go about handling it during a game. And while I don't like the idea of it, I don't tell my players no. The player in question wishes to burn down a major city of an area I made up myself. Let us assume the entire city is to go up in flames. I understand this is a very broad question and is hard to answer.

I need help mechanizing how the fire plays out.

How do I deal with structure damage whether buildings are saved or completely burnt down?

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    \$\begingroup\$ @William I went ahead and stripped off the XP question, as it's entirely unrelated to the question of how to simulate/mechanize a fire in an urban area. But I do hope you'll consider posting it separately: there's nothing against posting multiple good questions! Something like "how do I award XP when my PCs cause a disaster" would be a good one, though it's probably a duplicate of this one \$\endgroup\$ – nitsua60 Jun 16 '17 at 15:12
  • \$\begingroup\$ What level of detail do you need? Do you have to determine for each building how much damage it took? \$\endgroup\$ – Szega Jun 16 '17 at 15:20
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    \$\begingroup\$ It sounds to me like the question your player is asking is: "is it acceptable behavior in this campaign for my character to commit some extremely villainous act?" Or, alternatively: "what sorts of consequences would my character face for trying this?" It sounds like this question deals with the mechanics of large-scale arson, but there might be a separate interesting question dealing with consequences. \$\endgroup\$ – Dan B Jun 16 '17 at 15:20
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    \$\begingroup\$ Should we understand from your question that you don't want to simply decide how the burning turns out at a large scale, and want uncertainty in what the success/outcome might be (via mechanics)? \$\endgroup\$ – SevenSidedDie Jun 16 '17 at 15:39
  • \$\begingroup\$ what class and level is this character? Has he already come up with his plan on how he's going to do it? While I am of the DM school that says you can say no to players, I appreciate that you take a different approach, so I am trying to be helpful here. His level and class matter in terms of the reaction to the act. \$\endgroup\$ – KorvinStarmast Jun 16 '17 at 16:03
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There are no existing first party rules of this sort. Rules about fire in D&D are limited to fairly small fire spells and/or setting an opponent on fire at most. Related: How does fire work in D&D?

It's possible someone's published some third party rules in a splatbook somewhere, but they would likely be more oriented towards someone else set the fire and the PCs are trying to escape/stop it.

So what you'll need to do is take both a) your knowledge of reality and b) an appreciation for what will be fun and challenging in a game and come up with some rules yourself. There's not a "right answer" here. Don't put more detail into it than you really need to - you can make it as simple as a DC check of some sort (Int?) to see how well you get the fire going (<10 - someone sees you trying to start it, 10-15 the fire's seen as soon as it's started and it only affects the one building, 15-20 it really catches and burns down the building and adjacent buildings...). Once it's all done if the question is "did building X get burned down" say "20% chance it's still standing. yes."

You can make more of a minigame out of it if you want - lay out a map and there's a X% chance a round of adjacent buildings catching but local people fighting the fire make a DC Y check to put out a house, etc - but consider where there's any value to that additional work.

Needless to say things like this are always harder than they sound, and especially even if it works culprits are often seen or found out, and you have every right to sic a bunch of trouble on the PC from a) the law/ruler, b) everyone whose stuff was damaged or relations killed. In an at-all-realistic campaign, this is the point of no return, because after this the campaign becomes about the "Burning of Kislev" or whatever.

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  • \$\begingroup\$ While a good answer in general, +1 for the last sentence. \$\endgroup\$ – KorvinStarmast Jun 17 '17 at 19:31
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In terms of structures that do or don't survive the fire, you might look into the Great Fire of London (mid 17th century) as an example. Large sections of London survived more or less unscathed, while other sections were practically burned to the ground. The same has been true of most whole-city fires (Chicago, San Francisco, Seattle).

Wholesale destruction, however, is the assumption, when wood frame structures are in direct contact or separated by only narrow streets over a wide area (typical medieval/renaissance construction). Once the fire is burning well, embers may jump even a broad avenue (or a river) and start the fire anew on the other side, especially if roofs are flammable (wood shingles or thatch, as opposed to tile, for instance). On the other hand, an area that's upwind of a burning area may be spared even in absence of effective means to fight the fire(s).

As far as simulating the fire, I'd start with knowing the wind direction. The fire will generally tend to move downwind, though if it gets large enough it may begin to create its own "weather" -- mainly inflowing winds from all directions leading to a firestorm similar to the ones that consumed Dresden and Nagasaki during the Second World War, though there have been documented cases of large forest fires generating their own rainstorms (this requires that the air drawn in be humid enough to precipitate when it rises sharply).

For fires that start on the outside of a building, it's generally structural damage that destroys the building -- the roof will burn and fall in, or the walls will collapse, both before the interior is gutted, in direct contrast with a fire that starts inside, and may completely gut the interior before the walls or roof come down.

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    \$\begingroup\$ OP seems to be more interested in the mechanical aspects of a fire. I think the history you provided gives an idea of what the end point should be, but the answer would be improved with either rule citations or recommendations to effect that result. \$\endgroup\$ – Pyrotechnical Jun 16 '17 at 18:00

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