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Does a fireball spell do any damage to window glass, for example?

It looks like that its wording damages creatures and only causes fire on flammable objects.

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You are correct that the fireball spell only ignites (does not damage) objects.

The fireball spell's description states:

Each creature in a 20-foot-radius sphere centered on that point must make a Dexterity saving throw. A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one.

In order for a target to be damaged it must make a save and the only things that make the save are creatures in the area - Because the objects are not making a save they are not targets and so do not take the 8d6 fire damage.

However the spell goes on to say:

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

The extent of what "igniting an object" means it not explicitly covered by the rules and would be left up the GM; however, it does not mean that the objects take the 8d6 damage for another reason. If objects were supposed to take the damage then the spell's description would say so.
This is the case with the spell shatter whose description states:

Each creature in a 10-foot-radius sphere centered on that point must make a Constitution saving throw. A creature takes 3d8 thunder damage on a failed save, or half as much damage on a successful one...

A nonmagical object that isn't being worn or carried also takes the damage if it's in the spell's area.

Again, objects do not make the save at all, but the spell then explicitly states that they take the damage regardless of this fact.

The Sage Advice Compendium potentially provides further evidence that fireball does not damage objects stating:

Q. Some spells (like eldritch blast) target a creature. Some others (like fire bolt) target objects too. Does this mean that I can’t attack the door with eldritch blast?
A. The target specifications (creature, object, or something else) in spells are intentional.

The fireball spell only ignites objects, it does not deal 8d6 fire damage to them

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Yes, Fireball can break windows.

Without going into too much detail, an NPC in Waterdeep Dragon Heist (pg. 45) can tell the players about how a fireball blast blew out some of her windows.

Based on that published example, a fireball’s “explosion of flame” can break windows, but because the spell's description doesn’t call out damage to objects (other than the ignition of flammable objects) it would be on the DM’s shoulders to decide that a fireball had this effect.

But... the NPC I referenced had quite a few windows and only some were blown out. This suggestss that even a well placed fireball would be mostly ineffective.

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In terms of the Rules as Written ("RAW"), Fireball ignites flammable objects, but does not damage them. There are some existing rules regarding ignited creatures from other spells such as Searing Smite, but there are no rules regarding igniting objects.

Each creature in a 20-foot-radius sphere centered on that point must make a Dexterity saving throw. A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one. The fire spreads around corners. It ignites flammable objects in the area that aren’t being worn or carried.

This is contrasted by spells such as Meteor Swarm, which explicitly states that it damages objects.

A creature takes 20d6 fire damage and 20d6 bludgeoning damage on a failed save, or half as much damage on a successful one. A creature in the area of more than one fiery burst is affected only once. The spell damages objects in the area and ignites flammable objects that aren’t being worn or carried.

Consider, also Cone of Cold which produces an effect which is so cold that it can freeze a creature solid in under 6 seconds, but it can't freeze or even cool a glass of water.

A blast of cold air erupts from your hands. Each creature in a 60-foot cone must make a Constitution saving throw. A creature takes 8d8 cold damage on a failed save, or half as much damage on a successful one. A creature killed by this spell becomes a frozen statue until it thaws.

A good rule of thumb in D&D 5e is that spells do only what they say they do.

As you have undoubtedly observed, this creates some truly bizarre circumstances where fireball can instantly kill any number of commoners in a 20-foot radius, but it can't hit a commoner though a centimeter-thick pane of decorative glass, even where they are leaning up against it.

The spell does not even state that only the part of the object inside of the area of effect is ignited, but simply states that the object is ignited. RAW, this spell ignite a wooden hill fort because an inch of it is within range.

This would be problematic, if we are not to consider the design objectives of 5e and the role of the DM. This is not 3.5e, where there are rules to adjudicate just about everything. This edition was meant to be streamlined and easy to play. The DM is expected to adjudicate the effect of spells within the fiction in interesting ways. 5e is not meant to be played RAW, and I would argue that it is simply impossible to play 5e RAW without heavily houseruling it.

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  • \$\begingroup\$ "The default rule in 5e is that spells do only what they say they do." Nope, still not a rule. \$\endgroup\$ – Mark Wells Aug 12 at 18:03
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    \$\begingroup\$ @MarkWells I believe he meant that as "Rule-of-Thumb". \$\endgroup\$ – Slagmoth Aug 12 at 18:44
  • \$\begingroup\$ @MarkWells Well, considering that the devs have consistently spouted this over at Sage Advice, it's safe to say that this is a rule of thumb in interpreting spells and abilities. That being said, this is a PnP RPG - you can houserule it as you want. \$\endgroup\$ – James Aug 12 at 19:13
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Normally not

Generally, spells only do what they say they do. As you note Fireball says it damages creatures and sets flammable things on fire,1 and a glass window is probably not flammable (though its frame may be). If you want some justification of that, imagine a fireball more like a literal spherical ball of fire rather than an explosion like you were thinking.

That being said, if you want it to work more like an explosion that throws loose objects around and shatters brittle objects, that is a not-unreasonable change to the spell - but it's a change. If you as a DM or a player want it to work like that then you should communicate amongst yourselves that you intend for it to work like that. Something not working exactly as RAW describes usually isn't a problem unless

  1. you're in an AL game, or
  2. you didn't properly communicate this in advance so people are unpleasantly surprised to have their expectations violated.

1 Note that being set on fire will of course damage the object, but I'm assuming that's not what you meant.

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