D&D 5E Trading AC for DR in 5e

[Dausuul]

I'm really not sure that 5E is an appropriate system for this. The problem is that while the d20 roll is bounded, HP (and thus attack damage) are unbounded. You need to think about how this will work not only against the orc and the giant but the Tarrasque and the balor and the ancient dragon.

It isn't so clear-cut as all that. Look at the ancient red dragon: +17 to hit for 35 damage on the bite and 17 on the claws. Let's say your fighter would have AC 23 normally, which is assuming quite a lot of AC boosting (base AC for plate armor is 18). Then the dragon hits on 6 or better, for an average DPR (excluding legendary actions and breath weapon) of 51.75.

Now suppose you have AC 15 and DR 5. The dragon now hits on 2 or better, for a DPR of 51.3. You're a shade better off with DR. If you've got less AC-boosting gear, such that the "normal" fighter's AC is only 20-21, DR gains a noticeable advantage.

 

[CapnZapp]

So lets say I had a player who liked the idea of armor giving old-school DR. "Old school" because damage resistance in 5e is a half-damage binary state, rather than "subtract a bit off of every instance of damage."

She's aware of all the problems with using DR instead of AC (like, how it increases the fiddly math in the game to subtract a few HPs off of every attack rather than have a binary hit/miss system). She is fine with this. She wants armor to absorb damage, not make you harder to hit.

I'm not interested in changing the table rules for everyone - most of the players are fine with armor giving AC. This one prefers the fiddly bits, and I'd like to indulge her.

In 5e, how much DR should a point of AC be worth?

Let me know your thoughts!

I don't think it is easy to appreciate exactly how much of an headache armor-as-DR creates in D&D.

Based on my experiences with a homebrew d20 variant with this enabled (from level 1 til 16-17-something) even halving the "standard" suggestion (1 point of AC above ten equals 1 point of DR) meant serious play issues.

Lots of low-level foes do single digit damage. Shaving even four or five points off all those hits (for AC 18-20) becomes a huge deal eventually.

Based on this, my first recommendation is: "don't do it".

My second recommendation is to make the rules so it is reasonably easy to ignore the DR. A bit too tired right now to come up with something solid. It will not be trivial to balance this so "no armor" or "full plate" doesn't become a clear best option.

 

[Quartz]

Heavy Armor Mastery has already shown that DR can work within the 5E framework. This would be a step beyond that, but it's a difference of degree, not kind.

You could change that feat to give a DR equal to the proficiency bonus (or 1d4 / 1d6 / 1d8 / 1d10 / 1d12) so it scales.

 

[Dausuul]

Lots of low-level foes do single digit damage. Shaving even four or five points off all those hits (for AC 18-20) becomes a huge deal eventually.

I would be more concerned about this than about the high end, which is why it might be worth trying Iron Heroes-style "armor dice" (roll a die for each hit instead of using a flat value). The drawback to that, of course, is adding another die roll, especially since it's most important in the scenario where you're taking a lot of attacks per round.

Alternatively, you could have the armor be part DR and part conventional AC. How about having light armor grant DR 1, medium armor DR 2, and heavy armor DR 3, with ACs adjusted accordingly? That would keep things within the bounds of what has already been established for Heavy Armor Mastery. Something like:

Leather/padded armor: AC 10, DR 1.
Studded leather: AC 11, DR 1.
Hide armor: AC 9, DR 2.
Chain shirt: AC 10, DR 2.
Scale mail/breastplate: AC 11, DR 2.
Half plate: AC 12, DR 2.
Ring mail: AC 9, DR 3.
Chain mail: AC 11, DR 3.
Splint armor: AC 12, DR 3.
Full plate: AC 13, DR 3.

(Hide armor and ring mail actually come with an AC penalty in this model. You're trading off a slightly worse AC for 2-3 points of cheap DR.)

If you also go with the adjustment that Heavy Armor Mastery gives +5 AC instead of DR +3, then full plate and HAM get you to the same place they would normally; except that instead of magic weapons and spell attacks bypassing your DR, they bypass your bonus AC.

 

[Leatherhead]

I'm open to hearing what this might look like - how much "bonus hp" is a point of AC worth?

Approximately 20% of their total hp is a good place to start.

Keep in mind, you are going to have to be mindful of "repair" times on armor, making sure rider effects don't trigger on temp hp loss, and figuring out what to do about combat healing.

 

[I'm A Banana]

...the range of responses here is pretty fascinating.

I tried to do some back-of-envelope calculations (6 fights per day, 3 rounds per fight), but I don't have great values for the amount of damage a successful hit is meant to do against a given PC. One idea might be to look at the monster damage/round in the DMG and use that as a starting line, but I don't know what hit rate or PC HP total that number presumes. And I don't know that the number that would come out of that would be crazy divergent from some of the suggestions posted here...

 

[Greenstone.Walker]

Damage Resistance has an important side effect - it makes DoT area affects much, much less valuable and, in some cases, actually detrimental to the caster.

For example, in a game I ran a couple of weeks ago, the Bard cast cloud of daggers. The monster stood in the middle of the cloud and just took it. Now the cloud of daggers was stopping the party's fighters from getting close to the monster - pretty much the opposite of its desired effect.

 

[CapnZapp]

Problem is, of course, that if DR is low enough to work for the game, it's probably too low to entice the player. DR 1-3 would probably not wreck things, but as a player, would you sacrifice your armor as AC for that?

Presumably you wouldn't - a few piddly points of DR would do nothing against a Big Bad, and getting great results when swarmed is small consolation when you're almost defenceless against single big hitters.

I am not confident there is a happy medium here. Either DR is too good against many small attacks or it is worthless against few big ones.

Choosing armor as DR means specializing in a way few fighters can afford, I think.

 

[CapnZapp]

In the end, you gotta ask yourself: is it worth it?

At first blush it seems easily workable.

But I suspect it is far from worth the hassle.

Armor as temp HP otoh; there you might be onto something...!

 

[Twiggly the Gnome]

How about this...

Armor soaks the bonus damage of a successful hit, equivalent to the AC bonus. If the AC bonus exceeds the damage bonus, the damage roll is made at disadvantage.