[RPG] What are the implications of using AC rolls for defense

dnd-5ehouse-rules

I was thinking about AC, attacks, and how to make it slightly easier on a DM, and more interactive for players. The idea I came up with was to reverse AC rolls.

To do this, a player's AC now becomes the Armor/Magic Item mods plus DEX modifier, and a monster's attack would be the listed attack bonus plus 10. Essentially this moves the +10 from player to monster. Finally, a player rolls a d20 and adds their new AC to this. From here, a tie or going over is a successful dodge, going under is a successful attack.

I've seen two problems with this, namely that you could see this as being able to have things that affect ability checks affect this, but this would follow the rules set out in the Ability Checks article on WotC website. The article states (talking about attack rolls, saving throws and ability checks) "If something in the game, like the guidance spell, affects one of them, the other two aren't affected unless the rules specifically say so." The other problem I saw was Advantage/Disadvantage on the monsters Attack rolls. This is also easily solved by applying the reverse to the player. For instance if a monster has previously been given advantage against a PC, then in this new system the PC would have disadvantage vs. the monster's attack.

I haven't tried it, but have thought about it for a while. My questions are these: Has anyone tried this? If they have, does it work? If you haven't tried it, can anyone see problems with it?

Best Answer

Yes, this was tried in D&D 3.5e as a variant rule.

An officially published version of this rule existed in Unearthed Arcana, a 3rd edition D&D manual of variant rules. It can be used essentially unchanged with the 5th edition.

Unearthed Arcana p.133:

Players Roll All The Dice [...] Attacking and Defending

With this variant, PCs make their attacks just like they do in the standard rules. Their opponents, however, do not. Each time an enemy attacks a PC, the character's player rolls a defense check. If that defense check equals or exceeds the attack score of the enemy, the attack misses.

To determine the creature's attack score, add 11 to the creature's standard attack modifier (the number it would use, as either a bonus or a penalty to its attack roll, if it were attacking in any ordinary situation using the standard rules). For instance, an ogre has standard attack modifier of +8 with its greatclub. That means that its attack score is 19.

To make a defense check, roll 1d20 and add any modifiers that normally apply to your Armor Class (armor, size, deflection, and the like). This is effectively the same as rolling a d20, adding your total AC, and then subtracting 10.

  • Attack Score 11 + enemy's attack bonus
  • Defense Check 1d20 + character's AC modifiers

If a player rolls a natural 1 on a defense check, his character's opponent has scored a threat (just as if it had rolled a natural 20 on its attack roll).

Note that in D&D's core mechanic, a d20 roll that exactly matches its target DC is considered to succeed -- ties go to the person rolling the dice. So letting the defender roll actually gives them an advantage if you convert the numbers naïvely. This is why it's necessary to add 11 rather than 10 to the attack score to preserve the usual odds.

From the rules for Inspiration in the Player's Handbook, p.126:

If you have inspiration, you can expend it when you make an attack roll, saving throw, or ability check.

So you can't use inspiration to give yourself advantage on a defense roll. This matches the usual case, in which you can't use inspiration to give an opponent disadvantage on an attack roll.

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