Collapsing Roof

From Dungeon Master's Guide, page 122.

Mechanical trap

This trap uses a trip wire to collapse the supports keeping an unstable section of a ceiling in place.

The trip wire is 3 inches off the ground and stretches between two support beams. The DC to spot the trip wire is 10. A successful DC 15 Dexterity check using thieves' tools disables the trip wire harmlessly. A character without thieves' tools can attempt this check with disadvantage using any edged weapon or edged tool. On a failed check, the trap triggers.

Anyone who inspects the beams can easily determine that they are merely wedged in place. As an action, a character can knock over a beam, causing the trap to trigger.

The ceiling above the trip wire is in bad repair, and anyone who can see it can tell that it's in danger of collapse.

When the trap is triggered, the unstable ceiling collapses. Any creature in the area beneath the unstable section must succeed on a DC 15 Dexterity saving throw, taking 22 (4d10) bludgeoning damage on a failed save, or half as much damage on a successful one. Once the trap is triggered, the floor of the area is filled with rubble and becomes difficult terrain.