How do these stack/do they stack?

[Yogibear41]

Say I am a warforged with the adamantine body feat which gives me dr 2/adamantine, I then take the Roll with it feat which gives me dr 2/- which normally would not stack with the existing DR but the feat specifically says it does stack so I effectively have DR 2/adamantine + DR 2/- or a total DR of 4. Now my question is what happens if I then take the Thick-Skinned Feat from SS giving me DR4/adamantine and DR4/- do they stack together as previously mentioned giving DR4/ adamantine + DR4/- giving an effective DR of 8 or would it result in only an effective DR of 6 because while the thickskinned increased both kinds of DR only the DR 2/- from the original Roll with it feat would stack with the DR4/adamantine?

[Otodetu]

Say I am a warforged with the adamantine body feat which gives me dr 2/adamantine, I then take the Roll with it feat which gives me dr 2/- which normally would not stack with the existing DR but the feat specifically says it does stack so I effectively have DR 2/adamantine + DR 2/- or a total DR of 4. Now my question is what happens if I then take the Thick-Skinned Feat from SS giving me DR4/adamantine and DR4/- do they stack together as previously mentioned giving DR4/ adamantine + DR4/- giving an effective DR of 8 or would it result in only an effective DR of 6 because while the thickskinned increased both kinds of DR only the DR 2/- from the original Roll with it feat would stack with the DR4/adamantine?

It's pretty vague; as SS is old 3.0 material. By RAI I would allow you to get Dr 4\adamantine and dr 4\-, you are after all spending 3 feats on this.

By RAW the thick skinned feat from SS makes no mention of Dr\adamantine or dr\- in the feat description, so it would do nothing for you if you have adamantine body and roll with it.

[OldTrees1]

You have DR 2/Adamantine

You gain Roll With it. This gives you a DR 2/- that explicitly stacks with other DR.
This resolves as DR 4/Adamantine & non stacking DR 2/-.

You gain Thick Skinned. This increases one of your DR by +2. You apply it to the DR from Roll with it.
This resolves as DR 6/Adamantine & non stacking DR 4/-.

Aka you have DR 6/Adamantine except you have a backup DR 4/- against Adamantine weapons.

[Yogibear41]

Thick-Skinned doesn't say pick one DR and increase it by 2, it just says your existing DR improves by 2 which sounds like it applies to all your DRs not just 1.

[OldTrees1]

Thick-Skinned doesn't say pick one DR and increase it by 2, it just says your existing DR improves by 2 which sounds like it applies to all your DRs not just 1.

Likewise it doesn't say it increases your existing DRs. It just says it increases your DR. Thus it sounds like it applies to 1 of your DRs not all.

[Yogibear41]

"your existing damage reduction improves by 2"

[OldTrees1]

"your existing damage reduction improves by 2"

Notice the singular?