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The description of the breath (here from the adult red dragon, but apart from damage amounts and save DCs, they are all the same) says:

The dragon exhales fire in a 60-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 63 (18d6) fire damage on a failed save, or half as much damage on a successful one.

That's all of it. There is nothing about combustible, or even flammable objects being set aflame by it.

The breath is not a spell, and therefore not limited by "it only does what it says it does". It is not even magical, according to Sage Advice Compendium. But as an action it also is a very short burst, under six seconds.

(I rule in my game that for sure the dragon breath will set anything on fire that is obviously combustible, like wooden buildings, but am I overruling the rules here by falling back to rule 0?)

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Arguably, "exhales fire in a 60-foot cone" should imply all the consequences that a mundane 60-foot cone of fire (like, say, from a giant gas torch) would cause. The problem is that most people probably don't have a particularly good intuition for what those would be, and are likely to either over- or underestimate the flammability of various things under such circumstances.

In any case, your players will (probably) not have any better intuition for such things than you do. Thus, the real objective here isn't really to accurately simulate the results of firing a giant scaly flamethrower at a particular set of objects, but simply to narrate the events in a way that feels appropriately realistic, intuitive and exciting to everyone playing.

So, yes, if everyone at your table agrees that a blast of dragonfire should set a house on fire, then that's what it should do. And ultimately, as the DM, you have narrative authority on details like how hot dragonfire in your world is and how long a breath the dragon can sustain and just how much flammable stuff there happens to be lying around. If it makes sense (in terms of intuitive physics and/or narrative reasons) for something to catch fire from a dragon flame, let it. Just try not to go too badly against your players' expectations.


(So, what's my intuition on what would actually happen? Well, that's a truly massive flame any way you look at it, and it would emit a lot of radiant heat, which would quickly heat up the surface of anything nearby by hundreds of degrees (on any common scale). Wooden surfaces would char, wax would melt, flesh would burn like meat on a grill, leaves would dry and wither and maybe even burn to ash, glass and ceramics and even stone might shatter from the heat. Small metal objects might heat up until they'd glow and maybe even melt, while larger ones would only get hot to touch, possibly painfully so.

But if the flame only lasted a second or two, it wouldn't have time to actually transfer enough heat to most large solid (or liquid) objects to properly ignite them into a sustained flame. As soon as the dragonfire stopped, the hot surfaces would cool down as the heat would diffuse into their interior, and without enough additional heat input, most flames would die down. Materials that contain a lot of water, like wet or fresh wood, would be particularly resistant to ignition, as the water would boil off and cool the surface.

Instead, it's likely that only small or particularly flammable things — cloth, paper, wood shavings, dry sticks, maybe oil or wax if they're soaked into something that can act as a wick — would initially catch fire and continue burning. But with enough such stuff to act as kindling, the flames could easily spread and grow and heat their surroundings until even larger objects like wooden furniture and walls caught fire, especially as those would've already been somewhat preheated and dried by the initial blast of fire.

So, in the end, it depends. On a lot of things. If the dragonfire got into a house, it would most likely set something in there on fire, and there would be a very real risk of the whole house burning down if the fire wasn't quickly put out. But the time available to do that could be anything from minutes to seconds to "too late, there's no f***ing way anyone's going in there!". If it just hit the outside wall of the house? Well, it might still set parts of it on fire, especially if it hadn't rained recently, and that fire might still spread if not stopped. Out in a field or in a forest? Even more dependent on recent and current weather. After a long dry spell the whole place might go up like so much kindling, while in a prolonged rainstorm all you'd get might be lots of steam and some singed leaves.)

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  • \$\begingroup\$ Agree completely with your intuition for what would happen. As anyone who has ever started a fireplace will tell you, it's surprisingly hard even to get things we consider fuel (wooden logs) to burn if you don't have good kindling. A building with a thatched roof would likely catch fire from what is effectively a short burst of a giant flamethrower, while a stone or brick building would likely be almost completely unaffected. \$\endgroup\$ Aug 15 at 10:21
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Yes, when it’s important to the narrative.

The rules don’t spend much time talking about objects. The Dungeon Master’s Guide provides some guidance on damaging objects, but provides some important context for when to use those rules:

When characters need to saw through ropes, shatter a window, or smash a vampire’s coffin, the only hard and fast rule is this: given enough time and the right tools, characters can destroy any destructible object. […]

When time is a factor, you can assign an Armor Class and hit points to a destructible object. You can also give it immunities, resistances, and vulnerabilities to specific types of damage.

The idea here is that the rules typically only worry about objects when they are important to the story. Sure, fireball sets objects on fire, but it doesn’t tell you what that means beyond “it’s on fire”, because it usually doesn’t matter. But when it does matter to the narrative, the rules tell the DM to “use your best judgment”.

In that spirit, the answer to your question is “Red Dragon breath sets things on fire when things being on fire is important to the story”. This is consistent with the guidance provided concerning objects, and it is consistent with the DM’s relationship with the rules, which is that…

The rules aren’t in charge.

The introduction to the Dungeon Master’s Guide states:

The D&D rules help you and the other players have a good time, but the rules aren’t in charge. You’re the DM, and you are in charge of the game. […]

The rules don’t account for every possible situation that might arise during a typical D&D session.

The thing about “spells only do what they say they do” is that spell descriptions are player-facing and set the expectations for the player’s actions when they use spells. The rules for spells tell us that all of the spell’s effect is contained in its description because the players need to know what happens when they use the spell.

In contrast, the Monster Manual is primarily a DM-facing resource:

The Monster Manual is one of three books that form the foundation of the Dungeons & Dragons game, the other two being the Player’s Handbook and the Dungeon Master’s Guide. The Monster Manual, like the Dungeon Master’s Guide, is a book for DMs. Use it to populate your D&D adventures with pesky goblins, stinky troglodytes, savage orcs, mighty dragons, and a veritable horde of creepy crawlies.

The stat blocks provided are tools for the DM to use to bring the game’s world to life. They are not, in general, going to be player-facing rules, so there is no need for them to be thorough in the same way spell descriptions are. Of course, there are ways for players to access and utilize the stat blocks of creatures found in the Monster Manual, but it is still primarily DM-facing material.

All of this is to say: the absence of a clause explaining flaming objects in the Red Dragon stat block does not in any way mean Red Dragons don’t burn down towns and fields of crops. You’re the DM and you aren’t limited to exactly what is written. You’re in charge. “Only does what it says it does” does not apply to the DM describing how NPC creatures interact with the game world.

Destruction’s Light: The statblock doesn’t tell the story. The DM tells the story.

The adventure Icewind Dale: Rime of the Frostmaiden has a chapter called “Destruction’s Light”, featuring a dragon construct that destroys the Ten Towns. The chardalyn dragon’s breath weapon states:

Radiant Breath (Recharge 5–6). The dragon exhales a ray of radiant energy in a 120-foot line that is 5 feet wide. Each creature in that line must make a DC 16 Dexterity saving throw, taking 31 (7d8) radiant damage on a failed save, or half as much damage on a successful one.

Clearly the rules say that Radiant Breath only affects creatures, right? Then why does the adventure include this artwork, showing the dragon razing buildings with its Radiant Breath:

enter image description here

The dragon can do this because the dragon’s stat block does not account for every possible outcome of the dragon’s actions. It provides what is necessary for the DM to adjudicate the dragon’s interactions with the player characters, and the rest is up to the DM as part of telling the story, which in this case, is that the dragon levels the Ten Towns with its Radiant Breath if the party does not stop it.

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    \$\begingroup\$ I know that and even acknowledge so in my question. The question is if dragonfire is not magical according to SAC and things should behave as we'd expect them to from the real world, would it ignite something? This answer unfortunately does not help to answer that question. \$\endgroup\$ Aug 14 at 12:16
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    \$\begingroup\$ @GroodytheHobgoblin Did you try turning it off and turning it back on? (Refresh, I added a section) \$\endgroup\$ Aug 14 at 12:31
  • \$\begingroup\$ yes this is better, thank you. I think it is an interesting point to differentiate between rules aimed at players, and those aimed at the DM. It's not what I was thinking of (Ilmari's answer is more along those lines), and that's totally fine, in fact the very point of posing questions here, to expand the horizon of how to think about the right answer \$\endgroup\$ Aug 14 at 18:30
  • \$\begingroup\$ @GroodytheHobgoblin The idea is that the DM relates to the rules in an entirely different way than the players do, so the purpose of DM-facing rules is quite naturally very different from the purpose of player-facing rules. The players need to have relatively thorough and complete rules so that their expectations for their own actions can be aligned with how the DM will narrate the outcome of those actions. The DM facing features don’t need to be as thorough because the DM is making the decisions and determining the outcome of the decisions. \$\endgroup\$ Aug 15 at 0:17
  • \$\begingroup\$ This is one of the best answers I have ever seen, especially given I sometimes disagree with you quite a bit, and this may be another question but is it codified anywhere that the DM facing rules are different? I agree they are, just curious. \$\endgroup\$
    – SeriousBri
    Aug 16 at 9:39
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As a point of historical legacy, I'll point out that most editions of the D&D rules shy away from delving into this issue for a specific practical point: the complication of PCs equipment getting hit by fire.

Hypothetically, if it's hard-coded that dragon breath breaks stuff as does fire in the real world, then every time PCs get hit by dragon breath, every player has to go down all of the equipment lists (which can be quite extensive!) and evaluate the various effects of fire on different types of materials: cloaks, shields, swords, gold coins, magic staves, rings, etc., etc. This would grind the game to a halt and possibly take many minutes (or even hours?) to comprehensively adjudicate.

Therefore, most editions of D&D state that the preceding problem does not generally happen, and the side-effects of fire et. al. are hand-waved, at least for PCs and their equipment. Here's where it's first addressed in Original D&D (1974, in Volume 2, the monsters & treasures book):

Magical items will, during the course of play, be struck by various forms of weapons. For the sake of simplicity it is generally easier to assume they survive unharmed if their wearer/user is not killed (exception, Helms). If the wearer is killed, or the items are alone, throw for them on the following table if struck by Fire (Dragon or Ball) or Lightning (Dragon or Bolt)...

(This passage refers only to magical items, but there are several places in those rules that do similarly, and the consensus is that the intent is to cover nonmagical items as well.)

So granted that we're hand-waving the side-effects on victim PC equipment, it would present a difficult contradiction to also present a dogmatic rule that fire et. al. works completely normally on everything else in the world. And hence the whole issue probably goes unaddressed, even though almost all DMs have always assumed that a red dragon can set fire to a field, a homestead, etc., in any edition.

Caveats:

  • As of 1E, breath weapons were considered to be a magical effect, as explicated in the language for the anti-magic shell spell.

  • Early editions by Gygax very much did not hold to a "no hidden rules" principle (in fact, that guideline is largely in response/rejection to how Gygax and other early game-runners interpreted the game rules). Gygax would regularly write articles in the vein of, "we can deduce that these other magic items or spells must exist in the world..."

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  • \$\begingroup\$ I seem to recall that the original Dungeon Master's Guide had a table for items to roll for saves vs. various items (scrolls had to roll over 20 to survive a fireball, which kinda makes sense). However, if a character makes a saving throw against a magical attack, the rules stipulated that everything worn or carried by the character would also survive. \$\endgroup\$
    – EvilSnack
    Aug 14 at 17:22
  • \$\begingroup\$ @EvilSnack: Yeah, the 1E table is an expansion on "the following table" referenced in the OD&D quote I included. But I don't think the 1E text actually says one way or the other when to make item saves. By default that might be on any hit, or by implied inclusion of 0E rules it might be never while the user is still standing (numerous presumed prior rules like that got left out of the rewritten 1E text). \$\endgroup\$ Aug 14 at 20:09
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There are no hidden rules

The “it only does what it says it does” applies to everything, not just spells. So, RAW, red dragon breath does not start fires.

However, also RAW, you can change that if you want.

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    \$\begingroup\$ “There are no hidden rules” and “The rules only do what they say” are entirely different claims with different applications, but this answer seems to equate them. \$\endgroup\$ Aug 14 at 11:59
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    \$\begingroup\$ @ThomasMarkov that’s because they are equivalent statements \$\endgroup\$
    – Dale M
    Aug 14 at 12:19
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    \$\begingroup\$ There may not be hidden rules, but there are certainly some in unexpected places, such as dropping concentration and being unconscious. I don't think an answer this simple is ever helpful. \$\endgroup\$
    – SeriousBri
    Aug 14 at 12:58
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    \$\begingroup\$ The rules don’t say (for example) that a sack can be cut open with a knife, or that a spear could be used to knock an object off a high shelf; would you say that “RAW” those actions aren’t legal? \$\endgroup\$
    – Marq
    Aug 14 at 16:01
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    \$\begingroup\$ @Marq no, they are perfectly legal because the game incorporates the “rule” that things in the game work like things in real life. However, there are no real life dragons. \$\endgroup\$
    – Dale M
    Aug 14 at 21:15

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