Generally speaking, you roll once for damage against all targets and they all roll saving throws.

What about AoE effects that stay in place like wall of fire and such?

Do you roll once and that's the damage for the fight? Do you roll for each target as they enter or stay in the fire? If multiple targets stay in the fire over multiple rounds, do you roll once each round? If a new target enters do you roll for them or take the damage already rolled for the others?

I don't think it really matters and DM can have you do it however.

Probably less to worry about if the damage just stays consistent.

I'd say if the damage happens at the same time, roll once for all.  If the damage happens at different times (even from the same effect/spell), then roll separately.  At least that's what I do at my table.  Others can do different...DM's choice and all.

The thing I would avoid is rolling damage for the same spell several times consecutively.

• If a single baddy runs through the fire, using normal initiative, then I would roll fresh.
• If several baddies are all in the wall of fire and are using Block Initiative, then I would just roll once for all of them.

Yes, I think the key here is that rolling once for many creatures is supposed to be a shortcut to speed up the resolution of of many outcomes at once (without greatly affecting overall statistics). It is not supposed to be used to affect decisions a creature/player might make - "Its ok everyone, the Wall of Fire is surprisingly cool at the moment! Come on through!". As long as the effect is taking place at one time and the creatures involved can't make a choice that would affect the outcome then you can roll once for all. 

Yeah, I'd run it the way Icon and Memnosyne mentioned.

Quote from Memnosyne >>

The thing I would avoid is rolling damage for the same spell several times consecutively.

• If a single baddy runs through the fire, using normal initiative, then I would roll fresh.
• If several baddies are all in the wall of fire and are using Block Initiative, then I would just roll once for all of them.

 

 Right but what if they stay in the wall over multiple rounds? Do you roll for all of them every round or use the first damage roll?

I would roll every round because what if you roll poorly - that's your damage for the rest of that spell. Rolling every round at least averages out the damage over time.

I think the one roll for an AoE spell is purely to speed up play - so as long as it's not slowing the game down - roll as often as you like.

Quote from Wysperra >>

 

 Right but what if they stay in the wall over multiple rounds? Do you roll for all of them every round or use the first damage roll?

Rolling damage is fun, but bogging down the game is not.

If a baddie is just lounging in the AOE, then I would roll at least once per round. Though, I probably wouldn't want to roll more than 4 times per round, so I'd decide in the moment how to run it.

Generally, I will use Block Initiative for large numbers of enemies, with the option of adding a second block for a BBEG, or if a group arrives late to the party, so I would never have a reason to roll more than twice per round.

I usually roll for each time damage is triggered, so once on cast, and then every time something would be damaged by it, and I ask my players to do the same.

Largely because it's one less thing to remember how much damage we rolled for the wall of fire last round, plus rolling dice is fun!

It's up to the DM but I roll every round and I roll once per enemy "type." For instance if the players are fighting two Goblins and three Ghouls and all of the enemies are within the area of the Wall of Fire when it is cast I roll damage once for the Goblins and I roll damage once for the Ghouls. If all of the enemies end their turn within 10 feet of the bad side of the wall, I roll damage once for the Goblins and I roll damage once for the Ghouls. I do the same with initiative.

File this under the same heading as "roll twice for Critical Hits, or double your dice totals?" There's probably a "right" answer, but it has no particular balance implications either way, and probably comes down to the DM's and Player's ease and speed of play for the combat as a whole. Do whatever. 

Quote from Chicken_Champ >>

File this under the same heading as "roll twice for Critical Hits, or double your dice totals?" There's probably a "right" answer, but it has no particular balance implications either way, and probably comes down to the DM's and Player's ease and speed of play for the combat as a whole. Do whatever. 

Wait for it..... *technically*... the RAW rule for Criticals is roll double the number of dice. That's the rule in the PHB. As far as I can tell (with a very quick and lacklustre search) there isn't even an optional rule to say roll the normal number of dice and double the value.

I think it's why things like Brutal Critical confuse people - because they don't get doubled. Per RAW they would just be added to the pool of critical dice.

Also doubling the value makes spikey damage like very high or very low results equally likely - but maintains the average over time. More dice means more consistent damage short-term. So it's at least something to think about.

Right, that's what I mean. That's the "right" answer. Individual tables may or may not play it that way. Balance wise? Zero impact, same average damage either way.

The RAW on AOE damage rolls isn't as clear as crits, but the ruling significance is the same. "Doesn't matter what's right, do what flows."