Damage Reduction and Sneak Attack...

[Ruvion]

It's extra dice of damage dealt by the weapon the rogue is using. For all intents and purposes, if the rogue is sneak attacking with a rapier and has 3d6 sneak attack, he's making a 4d6 piercing attack that does 5d6 on a critical hit (plus any applicable strength bonuses).

If the armor has, lets say, DR10 and the rogue rolls 14 points of damage, the opponent takes 4 points of damage. The armor still provided a *lot* of protection against the hit. The damage from Sneak Attack is not rolled and applied separately, bypassing the armor or DR.

I am partial to the idea that sneak attack is a special effect that occurs when you manage to get 'inside' the armor when attacking critical areas, so it shouldn't add to DR penetration:

It is common knowledge that you can not sneak attack a creature immune to critical hits. Creatures immune to critical hits are also immune to stunning. It is noted in the damage reduction entry that, unless you can get past the DR you can not stun the target. If you consider stunning a creature (an act of hitting a 'vulnerable area' according to PHB) similar to sneak attack (an act of hitting a 'vital area for extra damage' according to PHB), then by virtue of extrapolation, considering sneak attack as added dice of bonus damage (not multiplied when you critical), you need to deal at least 1 point of damage in order to apply sneak attack just like stunning blow.

I am not fully convinced that this is so, as the rules seem somewhat murky and silent on this issue, but at the moment it seems to me that what sneak attack does is very similar to what stunning does: that is you strike vulnerable areas for both effects but with sneak attack you deal extra damage instead of stunning your target.

 

[Primitive Screwhead]

Sneak Attack is categorized as 'Precision Damage'...
meaning when faced with an mostly armored opponent the attack does not strike dead center on the knights breastplate and 'magically' penetrate. It means a quick strike to the back of the knee where the greaves leave a small gap.. just enough to clip one of the very important ligaments or possibly strike the primary artery.

It is not a 'special effect' added to the weapon damage, nor is it weapon damage itself. Its additional damage gained against foes that have a weak spot or chink in thier protection, to be exploited by careful placement of the weapon/spell.

In games like Iron Might and Kenneth Hood's Grim n Gritty system, this makes Sneak Attack very nasty. Not sure about Iron Might, but GnG changes Sneak Attack to a flat +2 to damage.

Ruvion.. ".. when you manage to get 'inside' the armor when attacking critical areas, so it shouldn't add to DR penetration:"
When the Rogue is in place to make a Sneak Attack, a successful attack indicates that she got inside to opponents armor. Better armor [when armor=DR] makes it more difficult to get the strike all the way home and diminishes the strength of the attack, shown in D20 by reducing the damage taken.
Not to mention, occasionally the weak spot may be the center of the Knights chest where a crack in the armor has sprung up and the Rapier can finesse its way in

Anyway, trying to figure out exact details of a combat scenario when using D20 rules is never a good idea. The whole deal is a generalization anyway.

 

[Ovinomancer]

Trying to rule that DR can negate sneak damage is really putting a crimp on an ability that already has many restrictions. Even with sneak attack, a rouge cannot hope to deal the damage that a power attacking fighter or a blaster mage can deal against a foe with DR. The ability already has too many handicaps, why add more?

 

[Scion]

the damage of the sneak attack is the same as the weapon that made the sneak attack. It really is just extra damage tacked on.

I havent seen any reason yet for it to be negated by DR.