[RPG] How does the Fireball spell’s area of effect work when cast in a corridor or small room


The fireball spell is clearly described as a sphere with a 20-foot radius.

In most cases, that is applied at ground level, since that's where most creatures are. (We can actually see an example of the area here:
20 foot square versus a 20 foot circle on a battle mat)

If I think in terms of the law of physics, I would assume that for a fireball cast with its center on the ground, all the energy which is expected to go down has to go somewhere else. Similarly, if casting that same fireball in a small room or a corridor, wouldn't that affect people further than 20' radius around?

How does fireball's AoE work when cast in a corridor or small room, according to the rules of the game?

Are we expected to just lose all the extra area of effect? Or does the fireball expand outward to fill its full normal volume?
So in a corridor, would you only affect a 40-foot diameter, and the extra energy that would create the rest of the sphere just never materializes (the effect is blocked by the walls of the corridor)? Or do I calculate the volume and fill that much space in the nearby corridors?

Specifically, I'm interested in understanding how to interpret the rules in this case.

Best Answer

Unlike some previous editions in which the fireball will adjust based on the space available and spread down corridors if contained, 5th ed simply states that the fireball will take up the amount of space listed as 20' radius sphere and spread around corners to fill that space, never expanding to exceed that distance from the point of origin. This specifically breaks the line of sight rule on page 204.

So if cast at the entrance of a 10x10 room with a 5' corridor leading into it, it shall fill that room and spread 20' down the corridor.

Fireball PHB PG 242
The fire spreads around corners.

Related Topic