If a creature has "damage resistance" to non-magical weapons how does the final damage number break down?
Is the weapon damage halved only? Is the total damage (weapon, STR/DEX dmg. bonus) halved? Would a rogue attacking with a non-magical weapon and using Sneak Attack get the Sneak Attack dmg. halved?
Same question applies to "damage vulnerable" when damage is doubled.
The roll and the appropriate modifier are added together to get total damage.
When attacking with a weapon, you add your ability modifier-the same modifier used for the attack roll-to the damage.
Let's say for example, a level 1 rogue with a +1 DEX mod attacks with a dagger(1d4) which does piercing damage. He would add the 1d4 + 1, but he also has Sneak Attack.
If a class feature increases the damage of an attack but doesn't specify a damage type, it is included in the total damage of the attack.
So, total piercing damage would come out to be 1d4 + 1(DEX) + 1d6(Sneak Attack) and if a creature had resistance/vulnerability to piercing damage that total would be halved/doubled respectively.
But if a class feature does specify a different damage type, you split the damage types and apply resistance/vulnerability separately.
Let's say we have a paladin with a +1 STR using a flail(1d8) with Improved Divine Smite(1d8).
Total bludgeoning damage would come out to 1d8 + 1(STR).
Total radiant damage would come out to 1d8.
If the target has resistance/vulnerability to either damage type, the damage of that type is halved/doubled independently of the other.
In each case, you halve or double the total amount of damage of the given type. Since the STR or DEX bonus to damage roll is still adding to the single damage type it is {halved|doubled} along with whatever was rolled. This applies in just about every case for every individual source of damage.
Any other modifiers are applied first, such as a magical aura that reduces damage within the area. Then resistance is applied, then vulnerability, in the event that both might apply.
Damage Resistance: If a creature or an object has resistance to a damage type, damage of that type is halved against it.
Damage Vulnerability: If a creature or an object has vulnerability to a damage type, damage of that type is doubled against it.
Resistance and then vulnerability are applied after all other modifiers to damage. For example, a creature has resistance to bludgeoning damage and is hit by an attack that deals 25 bludgeoning damage. The creature is also within a magical aura that reduces all damage by 5. The 25 damage is first reduced by 5 and then halved, so the creature takes 10 damage.
(D&D 5e SRD, Pg 97)
