The rules for cover state that half and 3/4 cover grant a bonus to both AC and (importantly for this question) Dexterity saving throws. However, some spells, like fireball, state that the area of effect spreads around corners. So, if a creature has half or 3/4 cover (or even full cover) between them and the center of the sphere of a fireball, do they get a bonus to their saving throw against the fireball's effect, or is that bonus negated by the fact that it spreads around corners? (In the case of full cover, assume that there is open space for the fireball to spread around/over the cover, e.g. a 6-foot tall wall.)
If the effect can spread around the cover to reach behind it, the target gains no benefit from the cover
How does Fireball and Cover interacts? Bonus to DEX saving throws and "fire spreads around corners"? What about full cover?
Your cover is foiled if an effect spreads around it and reaches you.
If you're in a solid concrete bunker with no openings, the enemy can't target you with such an effect because there's no way to reach you - but if there's just a short 5-foot wall, fireball can easily spread around that cover to reach you. If the fire is able to spread around the cover to reach you, that cover grants no benefit because you're still in the area of effect.
A spell's description specifies its area of effect, which typically has one of five different shapes: cone, cube, cylinder, line, or sphere. Every area of effect has a point of origin, a location from which the spell's energy erupts. The rules for each shape specify how you position its point of origin. Typically, a point of origin is a point in space, but some spells have an area whose origin is a creature or an object.
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.
Thia is further clarified under the rules for targeting, specifically requiring a clear path to the target:
To target something, you must have a clear path to it, so it can't be behind total cover. If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.
As such, normally, spherical spell areas will be blocked by cover; a solid wall would mean that any area within the radius that is blocked by the wall would not be within the area of effect, even if the spell extends to the left and right of the creature behind the wall.
Fireball specifies that "The fire spreads around corners." because it ignores this restriction. If there is any part of the area behind cover within the radius, and that part is not totally blocked from the parts affected by the fireball, it is also in the area of effect. A single 5-foot wall, for instance, won't protect you from a fireball unless you're just outside the area of effect already.
As per the rules of full cover:
A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.
Full cover gives an explanation about spells being able to reach a target despite full cover.
Page 204 of the PHB under rule clear path to the target also states.
If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of the obstruction."
Under Area of effect also page 204
A spells point of origin extends out ward in straight lines from the point of origin. If no unblocked straight line extends from the the point of origin to the area of effect, that location isn't included in spells area. To block one of these imaginary lines, an obstruction must provide total cover, as explained in chapter 9.
P241 Fireball (cliff notes)
...to a point you choose within range and then blossoms with a low roar into an explosion of flame....The fire spreads around corners...
With the way fireball is worded it.
- Does not move in a straight line.
- even if it is placed on the near side of an obstruction will go around corners
- Even if there is full cover it will spread around corners.
In short. Take your pick.
The Spell does exactly as it says it does
Its Specific ruling beats General
The Developers won't hide Elephants in Mole hills