Stacking DR

[martixy]

In general DR does not stack.

What instances are there of DR stacking?

I am aware of a couple of feats that explicitly mention it(such as Roll with it), but there's bound to be more.

Second, what would be the impact of allowing DR stacking? If it makes things silly, as I suspect it might, given the likely number of sources of DR, what about stacking just Ex(e.g. racial/feats/class features) and magic DR(items, spells, powers)?

[ExLibrisMortis]

DR sort-of-stacks, in the sense that DR 10/magic and DR 10/silver effectively combine to form DR 10/magic and silver (not completely, because they can be individually targeted by ability rip or whatever, but close enough).

Allowing DR to stack would be pretty nice, but it would hurt TWF/natural attackers, who rely on smaller individual attacks. You could design a sort of compromise, where you say "each DR you have applies once per round only (oh, and it's also doubled)", so having lots of different sources of DR would affect multiple attacks in sequence.

[supersonic29]

DR doesn't stack to avoid absurdity, even though it's one of the most easily checked defenses in 3.5. "Check out my +1 enhancement bonus weapon!" for the very common DR/magic. "Check out my weapon/attack which does any type of energy damage!" for all DR. The beauty of it is of course that you never receive the damage, but I moreso enjoy fast healing and a solid health pool. It's just so indiscriminate in the source of damage, and with a solid AC and ideally SR to go with it, you give it plenty of time to work.

That said, if you are insistent on DR for sake of thought experiment rather than application to a campaign, you're looking at this:
Grab a template that gives the best DR for (ideally) +1 LA, I'd check out the handy-dandy LA 1 race list associated with the more famous LA 0 race list. Said template also needs to give you at least +2 con. Drop that on any variant of human that gets the bonus feat, if one of those has +2 con then you can ignore the template needing to do so. Pool 18 into con, then get that +2 for 20. Undying way monk gets toughness as a level 1 bonus feat, if you can think of a better dip for toughness at first level, go at it. All four of your first level feats (obligatory, human, flaw x2) can now be Roll With It (SaSp) which explicitly increases DR by 2, as martixy said. You can grab that again every 3 levels if you so choose, but that's at least +8 to your racial DR at class level 1, ECL 2. Can't really think of a way to squeeze more outta that.

[Fouredged Sword]

One major instance of stacking DR is the Roll with It feat. This provides DR2/- that expressly stacks with all other forms of DR and with itself when taken multiple times.

The requirements of con 20 and toughness are pretty harsh for low level characters but I love it on an incarnate dwarf going into ironsoul forgemaster for DR 6/magic that stacks with DR 4/- early on in the game.

Feats
- toughness
- roll with it
- roll with it
...
- roll with it

[darkdragoon]

There's an armor in Magic item Compendium that specifically stacks IIRC.