General Rule For Areas of Effect and Large+ Creatures?

Question

How much of a Large+ creature (any creature occupying more than a single 5' x 5' square) must be included in a spell's area of effect before it is, well, affected? If there is a simple statement of a general rule, I cannot find it.

The PF2E core rulebook gives the typical areas of effect on pp456-457:

• Burst is specific: Any creature with even one square in the effect is affected
• Cone and emanation say nothing either way
• Line is specific: Any creature whose space the area of effect overlaps is affected.

I see nothing relevant on p304, either.

Does the burst/line rule generalize to other areas of effect, or are cones and emanations playing by different rules?

Note: House rules and lived/played experiences of how this is commonly ruled are welcome, but what I'm really looking for is support one way or the other from the rules, or admissions by people more familiar with the system than I am that there isn't actually a rule. I.e., "Did I, the querent, miss something specific in the rules?"

Answer 1

There isn't one

There is no specific rule that states how Large+ creatures interact with area effects. Notably, Areas falls under Effects, and neither uses words that would lock-in a particular ruling such as "any portion of a creature's space" or similar.

Some effects occupy an area of a specified shape and size. [...] Many area effects describe only the effects on creatures in the area.

Anything you do in the game has an effect. [...] Many spells, magic items, and feats create specific effects, and your character will be subject to effects caused by monsters, hazards, the environment, and other characters.

Rules on Spell Areas don't make it any more clear, only provide some guidance on applying area effects that also have a range.

From the other side of things, Size, Space, and Reach provides no guidance regarding applying effects; neither the word area nor effect even show up in the section (which makes some sense because it was written under the Movement in Encounters header).

Rule of Thumb

I have always, and all of my groups have always, considered that an area effect affects Large+ creatures fully if even one square is shared unless circumstances dictate otherwise. For example, clipping the corner of a Large creature's space with fireball would deal full damage and not provide any bonus to the reflex save. However, dropping an obscuring mist onto their corner would not usually make them concealed, or conceal others from them, in the direction they are standing out of it.

This ruling makes sense to my groups for a couple reasons. Firstly, it is reliable; you don't have to ask the GM about every area spell you want to toss out. Secondly, it is most in line with the idea that the effect affects an "area" and that area has a creature in it (albeit not that whole creature); other rulings would cause further questions and confusion about "if the Large creature can be in its other 3 squares, why is that square part of it at all?" and "I have cover because my foot is behind the wall".

Answer 2

They All Follow the Same Rules

Areas are defined in a few sections of the rules as you've pointed out, namely here and here, but also under Targets.

Spells that affect multiple creatures in an area can have both an Area entry and a Targets entry. A spell that has an area but no targets listed usually affects all creatures in the area indiscriminately.

All creatures in the area are affected indiscriminately. The additional rules you've pointed out under bursts and lines describe that even being within only a single square counts as being in the area.

Partial Exposures

There is no rule suggesting that a creature only partially within an area would experience less of the effect, or that they wouldn't be considered 'in the area'. The only relevant rules are under bursts and lines defending that even a single square of inclusion means being in the area.

So with no rule suggesting otherwise, there's no reason to apply additional effects for being in multiple squares or fewer effects for also being in squares outside the area.