Damage Threshold: a gate, not damage subtraction

SRD 5.2.1 · Damage

At the table

1. Read the target’s threshold before resolving damage.
2. Compare one attack or effect’s damage with that threshold.
3. Below the threshold, no Hit Points are lost; at or above it, apply the whole instance.

Example decision

A gate has damage threshold 10. One attack deals 9 damage and a later attack deals 11. How much damage does the gate take from those two attacks under this rule?

11 total: none from the first, all 11 from the second.

The 9-damage attack does not meet the threshold and is superficial. The 11-damage attack exceeds it, so all 11 damage applies. These separate attacks do not combine into one 20-damage instance. An instance of exactly 10 would also qualify because equality meets the threshold.

Watch for this mistake

Do not turn the threshold into a permanent pool that earlier weak hits gradually deplete. The comparison is made for each single attack or effect.

Ask the table

What is one instance here, and does that instance meet the threshold?

Source rule

A creature or an object that has a damage threshold has Immunity to all damage unless it takes an amount of damage from a single attack or effect equal to or greater than its damage threshold, in which case it takes that entire instance of damage. Any damage that fails to meet or exceed the damage threshold is superficial and doesn’t reduce Hit Points. For example, if an object has a damage threshold of 10, the object takes no damage if 9 damage is dealt to it, since that damage fails to exceed the threshold. If the same object is dealt 11 damage, it takes all of that damage.

Source: SRD 5.2.1, 180. Online guide.