An interesting issue came up in a game we were playing recently: Our party was on an airship and attacked by flying young dragons. My sorcerer cast a spell that triggered a wild magic surge and rolled a self-centered fireball (at 20,000 feet, whee!).

Here's the issue. Fireball says:

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

I've noticed that certain monsters have the "Siege Monster" feature which says:

Siege Monster. The [monster] deals double damage to objects and structures.

This suggests that objects and structures are legally different things and therefore Fireball would ignite an object but not a structure.

But then, perhaps a structure is a type of object? What is the nature/relationship of these two classifications of entities?

(For what it's worth, we ruled that the rope rigging was flammable because rope appears on the gear item table).

  • \$\begingroup\$ Related to, but not a duplicate of, this question \$\endgroup\$ – Mwr247 Apr 8 '19 at 17:05

A structure is not an object, but is made of objects

From page 246 of the DMG and the basic rules:

For the purpose of these rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects.

Lacking a definition from the source books, a "structure" is defined by its common English definition, which would be:

the arrangement of and relations between the parts or elements of something complex.

Thus a structure might be considered to be "a building or a vehicle that is composed of many other objects".

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    \$\begingroup\$ You probably didn't even need to go with the common english. The object rules call out vehicle or building separately. But +1 anyway :) \$\endgroup\$ – NautArch Apr 8 '19 at 17:13
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    \$\begingroup\$ @NautArch Agreed, but the object rules don't specifically state buildings or vehicles as structures. I wanted to solidify the assertion that 5e itself doesn't define what a structure is, and so we can fall back on the common definition to reach that conclusion with certainty. \$\endgroup\$ – Mwr247 Apr 8 '19 at 17:16
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    \$\begingroup\$ It might be worth pointing out that common sense would indicate that a burning object could certainly eventually set fire to a structure (GM adjudication, of course). I don't think this sort of thing needs to be made explicit within the rules themselves, though. \$\endgroup\$ – keithcurtis Apr 8 '19 at 17:19
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    \$\begingroup\$ "Is made of objects" that jibes with our table-side ruling of the rope rigging being flammable but not the deck planking. \$\endgroup\$ – Rykara Apr 8 '19 at 17:59
  • \$\begingroup\$ A DM could rule the structures to be combustible, meaning that when exposed to enough heat and time then it would start to burn. So a fireball would not start an airship on fire, but flammable objects on the airship left to burn could eventually ignite the ship. \$\endgroup\$ – Alk Apr 17 '19 at 23:52

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