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Damage reduction reduces damage from each source:

Consider having DR2, if you were attacked by 6 sources each dealing 3 damage, you would take 6*(3-2)=6, however, if it was 1 source dealing 18 you would take 1*(18-2)=16.

My question is how to determine the number of sources of damage in regard to DR when dealing with "AOE-ish" situations. Consider the two following scenarios:

  • A classic trap of a moving wall that crushes you (to the wall in front of it).

    • If the wall is blunt, it deals all its damage as one object and can be considered a single source and so DR applies once.
    • If the wall is spiky, then you can argue that the damage is distributed by each spike, thus DR applies multiple times. In this case, you might also be able to argue that a creature of size tiny (or whatever is small enough) would not get hit at all since it can move between the spikes (and the wall can't close on it all the way). You could not make this argument if you treat the wall as a single damage source, or could you?
  • A classic (densely) spiked shallow pit (shallow enough you could stand inside it).

    • You stand in the pit with both feet and the trap can be considered a single damage source.
    • You partition the pit between your feet so that now you have 2 pits, with one foot in each. Is it now 2 damage sources so that DR applies twice?

I'm not saying you should treat a sword as 10^20 atoms each dealing its own damage, but would you just treat "a trap" as "a single source of damage" regardless of how complicated it is?

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4 Answers 4

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A crushing wall trap doesn't require an attack roll (2000 DMG 115) and neither does the compacting room (2003 & 2012 DMG 72)--and there's no provision in either DMG for stapling spikes on the wall to make either traps nastier--, and while the DM makes attack rolls for a spiked pit trap (20 ft. deep) (2000 DMG 115) and a spiked pit trap (2003 & 2012 DMG 72), these aren't normal attacks. Further examples are necessary even to have a question because...

DR Only Affects Damage from Weapons and Natural Attacks

A hero can kill a creature he can't damage with his weapon by luring it into a nasty enough trap, and a creature with up to 6 hp and DR 5/magic can be killed by an unlucky fall.

Damage reduction hasn't affected anything but damage from weapons and natural attacks in Dungeons and Dragons, 3rd Edition (2000) and Dungeons and Dragons 3.5 (2003). This means most traps (except those employing weapons, which some do) will automatically overcome a creature's damage reduction.

Therefore unless the trap's an actual weapon or natural attack, DR doesn't apply. Moving walls (unless your poor PC is being beaten by a creature using a wall as an improvised weapon) and falls will bypass DR. The spikes at the pit's bottom are the DM's call (cf. armor spikes/spiked armor), however. (Don't forget to totally swoop on them, Grayhawk style, if the DM is putting +1 spikes at the bottom of pit traps just to overcome a PC's damage reduction; you earned those!)

"Why Would Damage Reduction to Work This Way?"

I don't know. Seriously. I'll happily speculate, though. Although I'm unfamiliar with earlier editions of Dungeons and Dragons, in at least Advanced Dungeons and Dragons and Advanced Dungeons and Dragons, Second Edition some creatures were flat-out, straight-up immune to weapons if the weapon didn't have a sufficient magical plus (e.g. Advanced Dungeons and Dragons has in its Monster Manual the vampire, which has the entry Special Defenses: +1 or better weapons to hit (99); Advanced Dungeons and Dragons, 2nd Edition has in its Monstrous Manual the pit fiend, which has the entry Special Defenses: +3 or better weapons to hit (11)).

This led to the phrase You must be this tall to play; that is, a party must possess sufficient magical equipment (which was much rarer in earlier editions) to combat a particular monster else the party, upon encountering the monster, fought it with magic spells (which were much more constrained in earlier editions), parleyed (not usually to the party's benefit), fled, used their combined ingenuity, were granted an out by the DM, or just died. Since all but the first were... distasteful, and the first unavailable to many characters, the concept of +X or better weapons to hit was changed to damage reduction in Dungeons and Dragons, 3rd Edition, which let combat ensue even against monsters who would've formerly been invulnerable to the party's warriors' attacks, albeit at a disadvantage if the party's warriors lacked the proper gear.

With that in mind, damage reduction wasn't ever intended to be a panacea against damage in general and was instead developed as a legacy to evoke older editions which had monsters that could only be hit by +X or better weapons, and weapons only and specifically. So, yeah, my guess is that falls and traps kill monsters with damage reduction in Dungeons and Dragons, 3rd Edition and its update because those killed monsters in older editions, too.


A Comparison of DR in Dungeons and Dragons, 3rd Edition (2000) and Dungeons and Dragons 3.5 (2003, 2012)

Although tagged as Dungeons and Dragons, 3rd Edition question, it's useful to know these rules aren't edition-specific.

The Player's Handbook

The two big deals for damage reduction in both editions of the Player's Handbook are the barbarian's damage reduction and the damage reduction granted by the spell stoneskin.

The Player's Handbook (2000) describes the barbarian's damage reduction as

the extraordinary ability to shrug off some amount of injury from each blow or attack. Subtract 1 from the damage the barbarian takes each time he is dealt damage. (25)

The Player's Handbook (2003, 2012) describes the barbarian's damage reduction almost identically, except it clarifies further: The barbarian

gains the ability to shrug off some amount of injury from each blow or attack. Subtract 1 from the damage the barbarian takes each time he is dealt damage from a weapon or natural attack. (26)

Further, the Player's Handbook (2000) contains the 4th-level Sor/Wiz spell stoneskin [abjur] (257), which reads

The warded creature gains resistance to blows, cuts, and slashes. The subject gains damage reduction 10/+5. (It ignores the the first 10 points of damage each time it takes damage, though a weapon with a +5 bonus or any magical attack bypasses the reduction.) (257)

And the Player's Handbook (2003, 2012) describes the spell stoneskin almost identically, except it clarifies further:

The warded creature gains resistance to blows, cuts, stabs, and slashes. The subject gains damage reduction 10/adamantine. (It ignores the first 10 points of damage each time it takes damage from a weapon, though an adamantine weapon bypasses the reduction.) (285)

Emphasis in quotations mine.

The difference here, then, between damage reduction in Player's Handbooks for Dungeons and Dragons, 3rd Edition and Dungeons and Dragons, 3.5 is that--y'know, obviously, apart from how damage reduction works--is that when using the Player's Handbook (2003, 2012) one no longer needs to look for confirmation in the Dungeon Master's Guide or Monster Manual to determine absolutely that damage reduction only works against weapons, even though that was very strongly implied by the Player's Handbook (2000), as shown by the emphasized text.

The Dungeon Master's Guide

Although it received the most serious overhaul during the Dungeons and Dragons, 3rd Edition transition to Dungeons and Dragons 3.5, the description of damage reduction in the Special Abilities section of the Dungeon Master's Guide (2000) reads

Some magic creature have the supernatural ability to instantly heal damage from weapons or to ignore blows altogether as though they were invulnerable. The number in a creature's damage reduction is the amount of hit points the creature ignores from normal attacks. (73)

and the Dungeon Master's Guide (2003, 2012) reads

Some magic creature have the supernatural ability to instantly heal damage from weapons or to ignore blows altogether as though they were invulnerable. The numerical part of a creature's damage reduction (DR) is the amount of hit points the creature ignores from normal attacks. (73)

Emphasis mine. The remainder of descriptions of damage reduction in the Dungeon Master's Guides is how damage reduction's overcome by weapons (or "normal attacks") and natural attacks.

The Monster Manual

The Monster Manual doesn't say damage reduction applies to anything but weapons and natural attack either.

According to the Monster Manual (2000) damage reduction means

The creature ignores damage from most weapons and natural attacks. Wounds heal immediately, or the weapon bounces off harmlessly (in either case, the opponent (in either case, the opponent knows the attack was ineffective). The creature takes normal damage from energy attacks (even nonmagical ones), spells, spell-like abilities, and supernatural abilities. A magic weapon or a creature with its own damage reduction can sometimes damage the creature normally, as noted below. (9)

The Monster Manual (2003, 2012) is almost identical, saying that damage reduction means

The creature ignores damage from most weapons and natural attacks. Wounds heal immediately, or the weapon bounces off harmlessly (in either case, the opponent (in either case, the opponent knows the attack was ineffective). The creature takes normal damage from energy attacks (even nonmagical ones), spells, spell-like abilities, and supernatural abilities. A certain kind of weapon can sometimes damage the creature normally, as noted below. (307).

Emphasis mine.

And then the Monster Manual (2000) describe how damage reduction is overcome by magic weapons with different bonuses, weapons composed of different materials, and certain types of creatures, while the Monster Manual (2003, 2012) describes how damage reduction is overcome by weapons composed of different materials and certain types of creatures. Because while what overcomes damage reduction changed between Dungeons and Dragons, 3rd Edition and Dungeons and Dragons 3.5, nothing changed about damage reduction itself.

Possible Contentions & Other Sources

A bone can be picked with the phrase normal attack, which is used instead of weapon or natural attack in some discussions of damage reduction, and that is somewhat problematic (e.g. in the Dungeon Master's Guide (2003, 2012), the CR 2 trap spiked pit trap (71) has spikes that get an attack roll, the CR 4 trap collapsing column (DMG 72) gets an attack roll), but I think that it's possible to agree that falling into a spiked pit or being crushed by collapsing column, for example, isn't a normal attack.

Another can be picked--for several reasons--with the frequent, ill-advised, and formally undefined use of the word blow. Dungeons and Dragons, 3rd Edition and Dungeons and Dragons 3.5 commonly and most frequently uses blow to mean an attack from a weapon or natural attack. Restrain your inner 12-year-old while I list feats like Awesome Blow (2003, 2012 MM 303), Death Blow (SF 6), Low Blow (Rac 166), Telling Blow (PH2 83), and--undoubtedly present due to lax editorial oversight and renamed by any player whose character took it--Toothed Blow (Sto 94); the 9th-level Tiger Claw martial maneuver feral death blow [strike] (ToB 87) and the 5th-level Diamond Mind martial maneuver disrupting blow [strike] (ToB 63); and even the 3rd-level Sor/Wiz spell dolorous blow [trans] (SpC 70). All of these involve weapon attacks or attacks with natural weapons.

Saying the SRD is incomplete and lacks context about this issue is a little unfair. The SRD omits only the examples from the Monster Manual (2003, 2012) for damage reduction, and all of those Monster Manual examples mention weapons or natural attacks, and, in fact, seem to be very careful to do so.

Finally, damage reduction is far more vague in Pathfinder, with its change to damage reduction's language, but that's beyond this question's scope.

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This thorough look at the text's treatment of DR has changed my mind, +1! –  SevenSidedDie Apr 25 at 14:49
    
@SevenSidedDie You, sir, are a true gentleman. –  Hey I Can Chan Apr 25 at 15:00
    
I read again your awnser. While I agree that it is really complete, and it is valid for a raw approach, I must note that it don't awnser the OP's question. He asked "What" is a source, and not what bypass DR or not. Nevertheless, +1 by the completness. My traps won't bypass DR, anyway :P –  Thales Sarczuk Apr 25 at 15:12
    
@ThalesSarczuk That's at the top. The question doesn't provide examples of other "AOE-ish" effects except traps, which, by my answer, aren't a thing. The only sources that DR affects are weapons and natural attacks. The question needs clarification if this isn't the answer sought. –  Hey I Can Chan Apr 25 at 15:29
    
I can see your point. Thanks for the clarification. –  Thales Sarczuk Apr 25 at 15:31

By the book, DR applies once for every time you take damage from a source. What is a "Source" depends a lot, that's for sure.

Luckily, the authors got this thing right, in a rare case of good thinking. I don't really know if this entry of D&D Wiki is absolutelly correct, but I suppose it is. I don't have my DMG at hand on the moment, but I will check later. Look at this:

Spiked Pit Trap

CR 2; mechanical; location trigger; automatic reset; DC 20 Reflex Save avoids; 20 ft. deep (2d6, fall); multiple targets (first target in each of two adjacent 5-ft. squares); pit spikes (Atk +10 melee, 1d4 spikes per target for 1d4+2 each); Search DC 18; Disable Device DC 15. Market Price: 1,600 gp.

A Spiked Pit deals damage for each spike that hits the target, so the DR would be applied individually for each spike. The same would happen for your Spiked Wall - more spikes, more damage. Imagine that each spike is like a dagger, and everything will make sense.

If your trap have tons of small spikes and someone got hit by 50 of them at once, and each spike does something like 1 damage, them it would be 50 damage. If this person have at least DR 1, the spikes wouldn't be able to hurt because they individually don't have enough piercing power to do that.

The damage source is not "the trap", but each spike that hits the target.

However, this rulling don't go well with swarm rules. If a Swarm goes over you, would you take damage? There are tons of really tiny creatures damaging you at the same time.

By the book, a swarm is considered a single creature and a single source of damage, but I myself house-rule this - if a character have enough DR to overcome the bite of a single tiny creature, the whole swarm is harmless against it.


On a side note

There was some speculation about trap damage being overcome or not by damage Reduction, so I picked up my DMG and started to read about them.

Spikes at the bottom of a pit may impale unlucky characters. The spikes deal damage as daggers with a +10 attack bonus and a +1 bonus on damage for every 10 feet of the fall[..]

Melee Attack Traps: These traps deal the same damage as the melee weapons they “wield.” In the case of a falling stone block, you can assign any amount of bludgeoning damage you like, but remember that whoever resets the trap has to lift that stone back into place. A melee attack trap can be constructed with a built-in bonus on damage rolls, just as if the trap itself had a high Strength score.

Ranged Attack Traps: These traps deal whatever damage their ammunition normally would. A trap that fires longbow arrows, for example, deals 1d8 points of damage per hit. If a trap is constructed with a high strength rating, it has a corresponding bonus on damage. For example, a ranged attack trap (+4 Str bonus) that fires shortspears could deal up to 1d8+4 points of damage per successful hit.

Crushing Wall Trap: CR 10; mechanical; location trigger; automatic reset; no attack roll required (18d6, crush); Search DC 20; Disable Device DC 25. Market Price: 25,000 gp.

Emphasis mine. Those texts are exactly what appears on the DMG.

As far as I can see, traps in fact do damage as weapons in most cases. The rules are silent about fall damage but you can think as the floor as really big mace that only overcomes bludgeodging DR. For other traps, the damage type is explicity typed (Slashing, bludgeoging, etc), so I would consider DR working normally against them. The wall example does "Crushing" damage, that as far as I know, there's not an explicit damage type for it but it seens it don't overcome DR/-. I would rule "Crush" as bludgeoging damage, since it is analogue to falling rocks. Not that a barbarian could ever hold back 18d6 worth of damage, anyway...

Traps that do energy damage (Acid pits, Energy spells, etc) do overcome DR, since it doesn't work against energy damage.

Also, it's worth nothing that some traps can deal damage from multiple sources at once. A spiked pit deals damage for the fall and for each spike that hits the target:

20 ft. deep (2d6, fall); pit spikes (Atk +10 melee, 1d4 spikes per target for 1d4+2 each)

The same would happen if your crushing wall also had spikes. Each spike that hit the target will do it's own damage, in addition to any damage that the wall would inflict. It would be something like this:

Crushing Spiked Wall Trap: CR 12; mechanical; location trigger; automatic reset; Wall Crush (no attack roll required, 18d6, crush); Wall Spikes (Atk +10 melee, 1d4 spikes per target for 1d4+2 each); Search DC 20; Disable Device DC 25.


TL;DR:

Each "thing" that does non-energy damage (be it each spike that hits the target on a pit trap or each bolt on a volley shot by a group of crossbows) is subject to Damage Reduction. The "Fall" that comes from falling on a pit or the pressure from a wall that comes to crush you are also individual "things" so they damage is calculated by its own and also subject to DR. So, if you are crushed by a spiked wall and hit by 4 spikes at the same time, you would take 18d6 damage from the wall, plus 1d4 for each spike, and all of those damage rolls would be subject to DR if applicable.

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2  
@ThalesSarczuk Cool it on the comments. They are for requests for clarification, not discussion. –  Brian Ballsun-Stanton Apr 25 at 10:51
    
@BrianBallsun-Stanton I really thought that what I wrote on the comment was clear on my awnser already (since it is basically what my example says), but I will try to clarify. –  Thales Sarczuk Apr 25 at 13:04
    
If it's clear, then you don't need to respond to the comment. Just flag the prior comment as obsolete and move on. –  Brian Ballsun-Stanton Apr 25 at 13:11

Admittedly this is just a "my two cents" answer, but I'd rule that DR applies once per attack roll or saving throw.

Thus if you're crushed between two spears that pinch you together as one attack, apply DR once, to the whole damage. However, if two arrows shoot at you independently and might miss independently, apply DR twice.

While I agree that this doesn't make "real world" sense, it seems consistent to treat two different attacks as different for each mechanic that applies to attacks. Similarly, two "simultaneous" sources of damage would require two concentration checks by a caster taking attacks of Opportunity.

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As you point out, there are as many ways to slice damage as there are atoms, but that gets ridiculous fast.

The easiest rule of thumb is: how many damage rolls are being made? That's how many sources of damage there are.

  • A crushing wall that does 3d6 bludgeoning damage: 1 source.
  • A crushing wall covered in short spikelets that does as a whole 3d8 bludgeoning/piercing damage: 1 source (with half the damage being bludgeoning and half piercing, if that matters for the DR).
  • A crushing wall covered in long spikes that does 2d8 spikes worth of 1d3 piercing damage per spike: a number of sources equal to the 2d8 roll.
  • A trap that drops you into a 10' pit (1d6 fall damage) and then hits you with a gout of fire (3d6 fire damage): 2 sources.
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