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

[MechaPilot]

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!

In 5e, I think the way that DR which is not implemented as damage resistance would work best would be to apply it to total damage for the round. If you apply it to each individual attack, you run the risk of invalidating the threat of low level enemies even in huge mobs.

Alternately, I think that a pool of bonus HPs provided by the armor probably works best as an alternative to a granted AC bonus (but that's not what you asked for).

That said, if I were going to implement damage reduction on each attack I would do it this way:

Light armor: 1d4.
Medium armor: 1d6 or 1d8 depending on the armor.
Heavy armor: 1d10 or 1d12 (or 2d6 if you prefer) depending on the armor.
Shield: +1d4 to the die rolled from your armor.

The die rolling adds more time to each attack, but it reduces the invalidation of the threat of mobs of low level foes

 

[I'm A Banana]

Thanks a lot!

I think the percentage is a bit too fiddly for me, and I don't necessarily think that it's a problem that she'll be "immune to goblins." Especially taking into account the weakness of creatures who "do things on a hit," I think that's a reasonable trade-off. I'll probably rule that a hit that deals no damage counts as a miss for these purposes, but that won't save her from high-damage hits that carry effects on a hit, and since it seems that DR stays pretty low, higher levels will be full of this. It's a sandboxy kind of game, so her being able to decide which critters to run up against is kind of a feature, as is her having to occasionally choose to use bows when the T-Rex comes rampaging.

With the way damage escalates at higher levels, are low DR numbers a candidate for being "too weak"? Or might the fact that this comes from multiple "attacks" typically scale DR OK (with the exception of one-hit wonders)?

 

[I'm A Banana]

Alternately, I think that a pool of bonus HPs provided by the armor probably works best as an alternative to a granted AC bonus (but that's not what you asked for).

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

 

[Jester David]

Let's see...

Padded or leather armor: Soak 1 damage
Studded leather or hide armor: Soak 1d2 damage
Chain shirt: Soak 1d4 damage
Scale mail, breastplate, ring mail: Soak 1d4 damage, reroll 1s
Half plate: Soak 1d6 damage
Chain mail: Soak 1d6 damage, reroll 1s
Splint armor: Soak 1d8 damage
Plate armor: Soak 1d8, reroll 1s (or soak 2d4)

Magic armour can either increase AC or increase damage soaked by +1.

This would really need to be playtested. Especially in 5e, where having +3 AC is great at all levels but soaking 1d8 damage isn't amazing. +3 AC is a reducing of damage taken by 15%, while 4 damage each attack is only 15% if you were taking 27 damage (such as 4d8+9 or 3d12+8). If you're taking more damage than 27 then the AC would have been better, while if you're taking less, the soaking is better.

 

[MechaPilot]

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

I'm not sure on the exact numbers, but let's give this a shot.

Probably the easiest first step is to make a series of assumptions to start from:
#1) Let's say that combats last three rounds.
#2) Let's say that characters face three combats a day.
#3) Let's say the average attack does 1d8 damage, for an average of 4.5 damage per attack.
#4) Attackers have from one to four possible attacks per attack action, so let's say three attacks per attack action.

Assumption three gives us an average of 4.5 damage per attack.
Assumption four multiplies that 4.5 by the three possible attacks per round for an average of 13.5 damage per attack action.
Assumptions one and two result in an assumed total of facing 9 attack actions per day. Multiplying that by 13.5 damage per attack action gives 121 hit points.

Do not worry, this is NOT your bonus HPs! The 121 HPs assumes the character gets hit with every attack and takes average damage. These 121 HPs are multiplied by a fraction determined as follows:

The numerator is the portion of the static AC granted by the armor that exceeds 10. Studded leather would have a numerator of 2. The denominator is 20, the highest possible result on the d20 rolled to attack.

To complete the example, Studded leather would grant 121*2/20 bonus HPs, which works out to 12 bonus HPs per day as compensation for the 2 points of AC given up.


By comparison, here's plate armor and a shield.
121 HPs * (20-10) /20 = 60 bonus HPs per day.


If you opt to use this method, one damage alternative would be to allow critical hits to bypass the armor's bonus HPs (applying it directly to the character's HPs) instead of dealing extra damage to the pool of bonus HPs.

Edit: I just realized from my two examples that this appears to work out to 6 HPs per point of static AC above 10.

 

[kerbarian]

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

I like that idea, and it also gives an opportunity to scale things in a more nuanced way without adding more math to each attack. Perhaps something like:

Each point of AC gives you temporary HP equal to 0.5 + 0.2 per level at the start of each of your turns. If you want, these could be in a different pool from regular temporary HP and called armor HP.

Chain + shield at 1st level would give you 5.6 = 5 armor HP per round. At 10th level, plate + shield would be 25 HP per round. At 1st, studded leather would be 1 HP per round, or 5 HP per round at 10th.

Not sure if those are the right numbers, but it gives you some knobs to adjust that only require recalculation when the character's armor or level change.

 

[MechaPilot]

Chain + shield at 1st level would give you 5.6 = 5 armor HP per round.

I know you said you weren't sure of the numbers, so I don't mean this as a rigorous critique, but that's quite a bit of bonus HPs. Assuming three three-round combats per day, that's 45 bonus HPs in a day for chain and shield. Additionally, it also means that someone making one attack with a dagger a +1 or lower damage bonus (which is not an unreasonable opponent for a first level PC) will never hurt you on anything but a critical hit.

Instead of making the small number of HPs refresh per round, I would suggest inflating the numbers a little bit but have them refresh each combat.

 

[kerbarian]

I know you said you weren't sure of the numbers, so I don't mean this as a rigorous critique, but that's quite a bit of bonus HPs. Assuming three three-round combats per day, that's 45 bonus HPs in a day for chain and shield. Additionally, it also means that someone making one attack with a dagger a +1 or lower damage bonus (which is not an unreasonable opponent for a first level PC) will never hurt you on anything but a critical hit.

Instead of making the small number of HPs refresh per round, I would suggest inflating the numbers a little bit but have them refresh each combat.

HP per round is definitely strongest when you're fighting a single opponent. The main reason (for me) to have it per round rather than per combat (or per short/long rest) is to make it still feel like armor. Why would my armor completely stop working part way through a long fight? I can rationalize it at least somewhat per round such that if I get ganged up on and my armor isn't helping against the later attacks in the same round, it's because they were able to attack from multiple sides and eventually get at the openings in my armor.

For 1st level, I'll try a couple examples for chain + shield. First, a single orc attacker. Using normal rules, the orc will hit 40% of the time (+5 vs. AC 18) for 9.5 average damage (1d12 + 3), or 3.8 damage per attack. With armor HP, the orc will hit 80% of the time (+5 vs. AC 10) for 9.5 - 5 = 4.5 average damage, or 3.6 damage per attack.

Next, two goblins attacking. With normal rules, each hits 35% of the time (+4 vs. AC 18) for 5.5 average damage (1d6 + 2), or 1.925 damage per attack and 3.85 damage per round with two of them attacking. With armor HP they hit 75% of the time (+4 vs. AC 10), so the total incoming damage averages 8.25, or 3.25 damage after armor HP absorbs 5 HP of it.

Those happen to be examples where things work out about the same, but it definitely changes the nature of combat -- focus fire becomes even more effective than it already is, and attack bonuses matter much less.

 

[Dausuul]

I would be inclined to stick to a fairly simple implementation--armor as a flat DR value, or perhaps a die roll, Iron Heroes-style. Don't overcomplicate things in an effort to solve hypothetical problems; wait to see if the problems really are problems, and address them as they come up. Just be sure the player understands the armor-as-DR rules are a work in progress and you reserve the right to tinker with them.

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.

 

[Quartz]

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.

PCs already have DR anyway: it's called Hit Points.