If a wizard casts Fireball indoors, wouldn't that start a fire in most cases?

Question

If a wizard casts Fireball indoors, wouldn't that start a fire in most cases? Everything combustible nearby is engulfed in flames.

Fireball

The fire spreads around corners. It ignites flammable objects in the area that aren’t being worn or carried.

Considering that fire is usually hot and the reason it causes damage is because it is hot, it would make sense to me that it would cause combustible things nearby to catch fire - and that fire could spread to the effect that an entire city would catch fire, as happened in Chicago in real life, or the many wildfires that grace California every year.

This is significant since many D&D campaigns are set in worlds with medieval style building construction, for the most part made of wood or other combustible material and likely significant portions of a campaign may take place in woodlands.

Fireball is a 40-foot-diameter sphere of fire. Many devastating fires have been started from a single discarded cigarette butt, I think it is quite reasonable to think that a 40 foot fire ball would put a woodland at risk of a wildfire or a set an entire house ablaze in short order.

Answer 1

Combustible vs. Flammable

The fireball spell ignites flammable objects not worn or carried. Flammable objects are easily ignited such as dried grass, while combustible objects are more difficult to ignite such as a wooden door.

A DM will likely have to judge what, if anything, is flammable at the time the spell is cast.

Example:

Bob casts fireball in a large room. In the area where the fireball hits, some curtains, research papers, and a tablecloth are set on fire (flammable). The door, table, floor, and walls are blackened and possibly destroyed but not on fire (combustible).

The unattended fires burning on or near combustible objects may or may not ignite them.

As a note, 5e makes no mention of the term "combustible", only "flammable". I have added the term into my answer to provide a line between something that “could” ignite and something that “will” ignite. That line will always be at the discretion of the DM.

Tinderbox. This small container holds flint, fire steel, and tinder (usually dry cloth soaked in light oil) used to kindle a fire. Using it to light a torch—or anything else with abundant, exposed fuel—takes an action. Lighting any other fire takes 1 minute.

I believe the Tinderbox gives an example of both flammable and combustible objects. The flammable object takes one action, while the combustible takes 1 minute.

Answer 2

Fireball sets flammable objects on fire

The spell description for fireball contains the text:

The fire spreads around corners. It ignites flammable objects in the area that aren't being worn or carried.

The important part of this is that any 'flammable objects' are set alight. Whether or not an object is flammable is at the discretion of the DM. What counts as an object and whether a room is made of objects determine whether the spell sets the room alight.

In general: Yes, fireball can start fires if there is a flammable object within the area, but it is unlikely to burn down buildings as they are harder to light than you think.

Answer 3

A fireball ignites flammable objects, but what's flammable is up to the DM.

While there's no general rule that says magical fire spells actually set fire to surroundings, the spell description for fireball itself gives that specific property (PHB p.240 - 241):

The fire spreads around corners. It ignites flammable objects in the area that aren't being worn or carried.

This property is shared with several fire spells, including burning hands, delayed blast fireball, fire bolt, fire storm, flaming sphere, and meteor swarm. Interestingly, lightning bolt also has this property.

Anecdotally, in the earliest games run by D&D creator Gary Gygax, fireball would very specifically destroy flammable treasure in the room. Gygax reportedly would tell his players what the valuable items were that they had now recklessly destroyed. A big reason why cone of cold was two spell levels higher is that it did not damage treasure.

What's flammable?

Whether fireball starts a fire depends on the definition of "flammable", which isn't strictly defined in the rules, and therefore adjudicated by the DM according to the commonly understood definition of the word.

Some items have vulnerability to fire (PHB p.197, Damage Resistance and Vulnerability; DMG p.246 - 247, Objects). However, this is stil entirely up to the DM:

Paper or cloth objects might be vulnerable to fire and lightning damage. A pick can chip away stone but can't effectively cut down a tree. As always, use your best judgement.

There's also no general rule for what happens when something is on fire, so that's up to the DM too.

The DM may also choose to ignore collateral effects for efficiency.

Answer 4

Massive Fire is a Matter of How Much Dry "Kindling" There Is

I think you and others have got a sense for what sort of things are directly flammable. I will simply add three things.

Is Your Local Biomaterial Dry?

The cases of a single cigarette, candle, etc. causing a massive fire, or even a lightning bolt doing so, are much more common when biomatter, particularly brush, sticks, thatch roofs etc. are dried out, particularly in Summer. Seven major wildfires started in North America in 2018, all during the month of June and mostly late June, though most lasted into August. Of course in some environments the Summer is a period of particular humidity This is fairly common in major cities since they are often abutting a major body of water, especially in eras when water travel was more important. In many deserts summer is monsoon season.

To a D&D game this is mostly relevant in whether the DM can "realistically" say fire spreads or does not spread when they have already extensively discussed weather, environment, season, etc. If they have described days of heavy rain, for example, saying the whole town burns down is substantially less fair then if they have described days of the sun beating down.

Thick Pieces of Wood Usually Don't Directly Catch Fire, but Burn Long once They Do.

As anyone who has ever started a campfire can tell you, it takes a great deal more sustained fire to light up a log on the fire than kindling, which is the whole point of kindling. The wooden beams in many buildings as well as substantial trees generally require more flammable materials burning around them before the catch fire, at which point they then potentially burn for a long time. From a DM perspective, the more tapestries, furniture, and other biomatter based things around that can function as kindling at some level or other the more likely that a vast, general conflagration results.

And Let's Discuss Medieval Construction

Presuming one is using the typical D&D pseudo-Medieval, quasi-European setting, it is probably worth discussing the construction methods of the period and place generally for anyone interested. I am not an expert on historic construction but I do have a masters in Medieval and Renaissance history, which means lots of anecdotal knowledge.

So far as fire goes, the prevalence of thatched roofs was the most devastating part, as it was both most easily ignited, and the part closest to one's neighbor (in places where people lived near each other) Aside from the material being inherently flammable it likely has the sun beating down directly upon it every sunny day making it nice and dry in the right season. Wood is of course something that burns, though the thick wood supports don't usually ignite easily without a substantial fire near them. But, between the thatch and the small sticks often used to infill between wooden wall supports, your average medieval home lights up quite easy. Also be aware that in many places they were not primarily made of wood but rather what we would view as hovels made of sod, wattle and daub, etc. which I believe are all still more easily flammable material in a dry season, and these materials would often be the sorts of things infilling between beams in a wood framed structure as well. In a "nucleated" village where everyone has homes next to each other this was likely to be a catastrophe for the whole community, but keep in mind that as often as not the buildings of a village were spread out around the fields. A field of wheat, by the way, is also nice and flammable when dry.

As for stone (or brick) buildings (usually churches, fortresses, etc), they were still in danger of substantial destruction so long as there was sufficient flammable materials in the roof, supports, floors, furniture, decorative carvings, tapestries, etc. Between some stone and bricks melting to some degree in an intense fire, and the stresses of attached wooden support beams collapsing, many walls were toppleable. While stone structures are likely to survive a fire in a gutted form, other than thick fortress defensive walls no man made structures are immune.

So basically a typical urban, village, etc. scenario is that roofs and furnishings catch fire, if there is a lot of this to catch fire then after a while this spreads to wooden structural elements and neighboring buildings, and eventually (possibly hours later) there is no longer a building or a gutted stone structure with a few walls or towers collapsed.

There were no fire departments, but note that there was a firm tradition of the whole community dropping everything to do their best at trying to stop a fire by what limited means were available, something which actually may make for a more interesting element of rpg adventure than the actual fire.