Dilemma
The feat Roll With It (Savage Species, p. 39) contains come text that is confusing1:
This applies in addition to any damage reduction you have from other sources.
This appears contrary to what the SRD states about Damage Reduction:
If a creature has damage reduction from more than one source, the two forms of damage reduction do not stack. Instead, the creature gets the benefit of the best damage reduction in a given situation.
Could this be an oversight since Savage Species is 3.0? Especially since it contains:
You may take this feat multiple times.
But it doesn't contain what, in D&D 3.5, is usually also stated after that bit of text:
It's effects stack.
Would the damage reduction of 2/- stack with existing damage reduction of x/-, or is it an exclusive damage reduction of 2/- that applies at the same time to x/magic (for example)?
If one took this feat multiple times, would it's effects stack?
Hypothetical (but very plausible) situation
7th Level Mineral Warrior Barbarian has DR 1/- from being a barbarian and DR 8/adamantine for being a Mineral Warrior. He takes the feat Roll With It. Does that simply improve his DR 1/- into DR 3/-? Is it an exclusive 2/- that also applies at the same time to DR 8/adamantine?
Actual situation
7th Level Dwarf Barbarian wants to take Roll With It, and plans on taking it every time he gains a feat.
1After searching with "roll with it" [dnd-3e] & [dnd-3.5e] and not finding anything.
Best Answer
The DR from the feat doesn't stack but is in addition
The feat Roll with It (Savage Species 39) says
Thus the damage reduction from each iteration of the feat is in addition to any other damage reduction the creature already possesses. Don't think of it as stacking (i.e. combining for a cumulative effect and not generally possible with damage reduction); instead, think of it as another layer of damage reduction (also usually not possible, but the feat's specific phrasing trumps the general rules for DR).
In other words, after damage has been dealt to the creature and the creature's other damage reduction has reduced damage appropriately, then the creature applies the damage reduction from the feat Roll with It, each iteration of the feat applying individually and sequentially.
For example, a dwarf barbarian 7 with the feat Roll with It that would have been dealt 10 points of damage by a longsword applies his barbarian level's DR 1/— to that damage first then the DR 2/— from the feat Roll with It. The dwarf is dealt 7 points of damage. Were the dwarf to have taken the feat Roll with It twice, he would apply his barbarian DR 1/— then his first Roll with It DR 2/— then his second Roll with It DR 2/— so that the longsword would deal 5 points of damage.
The feat's phrasing is unique, and it (rather pointlessly, in my opinion) breaks some rules, but feats are allowed to do that, so it's no big deal. It would have been much clearer were the feat's Benefit simply to have read that it reduced the damage from manufactured and natural weapons by 2 each time the feat's picked, but that's not what we got.
If you still don't like it, have the DM fix it
The DM is allowed to revise to 3.5 standards items from Dungeons and Dragons, Third Edition that weren't upgraded to 3.5. The Dungeon Master's Guide (2003) says
The DM, presumably, makes these minor adjustments. This, for your group, may be one.