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What happens with a spell when it is cast in a smaller space/area than the actual spell itself? Does it simply not work? Does it adapt to the size of the room etc?

For example: Ice Storm casts at a 20ft radius and 40ft high cylinder. If it is cast in a 30ft room but the ceiling is only 20ft high, what happens?

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The answer is implicit in the section of the spellcasting rules on areas of effect (Player's Handbook, Chapter 10):

A spell's effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area. To block one of these imaginary lines, an obstruction must provide total cover, as explained in chapter 9.

Obstructions that provide total cover, such as dungeon walls or floors, therefore limit the area of effect. This would mean that in your example, the cylinder goes from floor to ceiling, but not above, as the ceiling provides total cover to anything above it excluding the presence of holes.

As Anaximander's comment (below) puts it, you can think of the Area of Effect as light originating from the point of origin. If a target inside the Area is fully shadowed, it has total cover and is not affected, unless the effect is stated to spread across corners.

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    \$\begingroup\$ I'm having trouble actually understanding that paragraph. The first sentence is fine. But where it says "...If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area." I can't really get my head around, i'm not sure if i'm just being stupid or not? \$\endgroup\$
    – Mike44455
    Jan 30, 2017 at 11:15
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    \$\begingroup\$ Ah okay great! So the cylinder would form but would only be as high as the size the room if i have read that all correctly? \$\endgroup\$
    – Mike44455
    Jan 30, 2017 at 11:36
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    \$\begingroup\$ @Mike44455 Correct - as the ceiling would grant total cover to anyone on the floor above, it would likewise block area of effect. (assuming the ceiling is intact, of course) \$\endgroup\$
    – kviiri
    Jan 30, 2017 at 12:03
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    \$\begingroup\$ @Javelin Not like a fluid. A fluid will flow around pillars thick enough to provide provide total cover, and expand further in a small corridor than in a vast hall. The AoE is always bound by its stated dimensions, then cut down based on the "invisible lines" not unlike line of sight from the point of origin. \$\endgroup\$
    – kviiri
    Jan 30, 2017 at 14:17
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    \$\begingroup\$ Think of it as a light. If you put a lamp at the point of origin, anywhere that the lamp would cast a shadow is not affected by the spell, unless the spell specifically states that it spreads around corners (like fireball, for instance). \$\endgroup\$ Jan 30, 2017 at 14:19
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Area of effect spells radiate from the user and are blocked by walls, floors, and other bits of scenery.

From a mechanical standpoint, it ends there. One must also take into story telling, though.

How does the scenery react to the the spell? If a fireball (a 20x20ft area of effect) explodes in a small, 10x10ft cabin, are those walls staying in tact? Is the roof?

A spell will never pass through scenery, but it can shift and manipulate scenery in such a way that the area of effect expands beyond its initial bounds.

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    \$\begingroup\$ I believe a previous question covers that - fireball doesn't explode as such, it has no force and can't knock down walls etc. It's just a sudden burst of flames. (If it's a wooden cabin, it might catch fire, but won't blow apart) \$\endgroup\$
    – Adeptus
    Jan 30, 2017 at 23:10
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    \$\begingroup\$ Fireball may be a poor example, but there ARE spells with an area of effect that exerts force. Will have to check after work and get a good example :P \$\endgroup\$ Jan 31, 2017 at 17:09

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