Damage Rolls

Each weapon, spell, and damaging monster ability specifies the damage it deals. You roll the damage dice, add any modifiers, and deal the damage to your target. If there's a penalty to the damage, it's possible to deal 0 damage but not negative damage.
When attacking with a weapon, you add your ability modifier-the same modifier used for the attack roll-to the damage roll. A spell tells you which dice to roll for damage and whether to add any modifiers. Unless a rule says otherwise, you don't add your ability modifier to a fixed damage amount that doesn't use a roll, such as the damage of a Blowgun. See chapter 6 for weapons' damage dice and chapter 7 for spells' damage dice.