How does the Shield spell interact with the Barkskin spell?

Question

I am wondering what the ruling would be on a creature under the effects of the Barkskin spell that then casts the Shield spell as a reaction.

Barkskin states:

Until the spell ends, the target’s skin has a rough, bark-like appearance, and the target’s AC can’t be less than 16, regardless of what kind of armor it is wearing.

Shield states:

Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack...

Which of the following results would occur?

A) The creature’s AC becomes 21, adding the +5 bonus from Shield to their AC from barkskin;

B) The creature applies the +5 bonus from Shield to their base AC and if it is higher then 16, it overrules Barkskin.

Answer 1

It depends on your original AC

Jeremy E. Crawford says in the Sage Advice Compendium (thanks to Daze 413 for the link):

How does barkskin work with shields, cover, and other modifiers to AC? Barkskin specifies that your AC can’t be lower than 16 while you are affected by the spell. This means you effectively ignore any modifiers to your AC—including your Dexterity modifier, your armor, a shield, and cover—unless your AC is higher than 16. For example, if your AC is normally 14, it’s 16 while barkskin is on you. If your AC is 15 and you have half cover, your AC is 17; barkskin isn’t relevant in this case.

So if your original AC is 12 or greater you add the +5 from shield to that. Otherwise it remains a 16.

Answer 2

They Don't Stack

Barkskin never stacks with anything; it simply never allows your AC to fall below 16. If your normal AC +5 is still lower than 16, your AC is now 16 with Barkskin active. If the +5 puts you over 16, Barkskin has no effect (regardless of when it was cast).

Answer 3

B) The creature applies the +5 bonus from Shield to their base AC and if it is higher then 16, it overrules Barkskin.

Barkskin only prevents your AC from being less than 16, but it does not otherwise affect your AC. If your AC is 13, Barkskin lets you act as though it were 16 until something raises your actual AC higher than that. Barkskin simply exists to help you get a little boost to your defenses until the spell ends or your AC of 13 gets raised higher than 16. If Shield activates, then your AC would be 13+5, not 16+5. As a result, Barkskin may still be active, but it becomes effectively moot until Shield ends or unless something else causes your AC to lower.

Answer 4

As with any Barkskin spell effect you perform the following two steps:

1. Calculate the current base AC of the character (ignoring Barkskin but including effects that modify your base AC such as from a Shield spell)
2. if the base AC < Barkskin AC 16 then the character's AC = Barkskin AC 16 otherwise the character's AC = base AC

That's it. Always. You never add or subtract anything from the Barkskin AC of 16.

For instance a Druid with 10 dexterity wearing leather armour and using a shield casts Barkskin on themself. Their AC is 16 due to the calculation below:

1. base AC = 11 (leather armour) + 2 (shield) = 13
2. Is a base AC of 13 < Barkskin AC 16? YES! Therefore the Druid's AC = Barkskin AC 16

The Druid then casts Shield (somehow), their AC is now 18 due to the calculation below:

1. base AC = 11 (leather armour) + 2 (shield) + 5 (Shield spell) = 18
2. Is base AC 18 < Barkskin AC 16? NO! Therefore AC = base AC 18

If the character has more then one base AC use the highest in the calculation above. E.g. a crazy Barbarian/Monk/Sorcerer who has cast Mage Armour on themselves you calculate the highest of the three base ACs they have, and plug the highest one into the calculation above.

Answer 5

Mike Mearls says Barkskin plus a shield (+2 AC) gives you AC 18:

Q: If I cast Barkskin and use a Shield, is my AC 16 or 18?

Mearls: I'd say 18

— 17 Sept 2014 on Twitter or on the Sage Advice blog