The AC, hit points and damages threshold for objects, doors and walls.
D&D Core Info

Objects, Door & Walls

Object AC (by substance) | Object HP (by size)
Optional Rules: Doors | Walls

Given enough time, and the right tools, a party should be able to destroy most objects. However, a DM should use some common sense. For example, a fighter should not be able to use a sword to destroy a stone wall. If time is not an issue, but the party does have the correct tools (like an axe and a wooden door), then the DM could allow the party to just accomplish their task (unless there are other situational issues that could affect the task - like louding chopping at a wooden door may awaken a nearby sleeping enemy; or if the task takes time, an enemy patrol could wander onto the party). However, if time is an issue, you can assign an Armor Class and Hit Points to a destructible object. You can also give it immunities, resistances, and vulnerabilities to specific types of damage.

For the Purpose of these rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects.

Object Armor Class

Substance AC
Cloth, paper, rope AC:  11
Crystal, glass, ice AC:  13
Wood, bone AC:  15
Stone AC:  17
Iron, steel AC:  19
Mithral AC:  21
Adamantine AC:  23

Object Hit Points

Size Fragile Resilient
Tiny (bottle, lock) Fragile:  2 (1d4) Resilient:  5 (2d4)
Small (chest, lute) Fragile:  3 (1d6) Resilient:  10 (3d6)
Medium (barrel, chandelier) Fragile:  4 (1d8) Resilient:  18 (4d8)
Large (cart, 10'x10' window) Fragile:  5 (1d10) Resilient:  27 (5d10)

Huge and Gargantuan Objects: Normal Weapons are of little use against many Huge and Gargantuan objects, such as a colossal statue, towering column of stone, or massive boulder. That said, one torch can burn a Huge tapestry, and an Earthquake spell can reduce a Colossus to rubble. You can track a Huge or Gargantuan object's Hit Points if you like, or you can simply decide how long the object can withstand whatever weapon or force is Acting against it. If you track Hit Points for the object, divide it into Large or smaller sections, and track each section's Hit Points separately. Destroying one of those sections could ruin the entire object. For example, a Gargantuan statue of a human might topple over when one of its Large legs is reduced to 0 Hit Points.

Damage Threshold: Big objects such as castle walls often have extra resilience represented by a Damage Threshold. An object with 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 damage as normal. Any damage that fails to meet or exceed the object's Damage Threshold is considered superficial and doesn't reduce the object's Hit Points.



Optional Rules

Doors

Type Thickness Dmg Threshhold AC HP
Wooden, Simple Wooden, Simple Thickness:  1 in Dmg Threshhold -- AC 15 HP:  10 hpDetails
Wooden doors are the most common type. Simple doors are thin and often poorly made, and are not meant to keep out motivated attackers.
Wooden, Good Wooden, Good Thickness:  2 in Dmg Threshhold -- AC 15 HP:  15 hpDetails
Good wooden doors, while sturdy and long-lasting, are meant to block areas and keep out commoners, but are still not meant to take much punishment.
Wooden, Heavy Wooden, Heavy Thickness:  4 in Dmg Threshhold 10 AC 15 HP:  25 hpDetails
Heavy wooden doors are thick and hard to open. They are common as entrances to large halls, temples and smaller castles. They provide ample protection from intruders and the elements.
Wooden, Reinforced Wooden, Reinforced Thickness:  4 in Dmg Threshhold 15 AC 17 HP:  40 hpDetails
Reinforced wooden doors made of wood, but reinforced with lengths of iron. These doors are strong, sturdy and built to keep out intruders.
Stone Stone Thickness:  4 in Dmg Threshhold 25 AC 17 HP:  60 hpDetails
Carved from solid blocks of stone, these heavy, unwieldy doors are often built so that they pivot when opened (crafting hinges strong enough to hold up a stone door must be made by skilled craftsfolk, and dwarven smiths). Secret doors concealed within a stone wall are usually stone doors. Such doors stand as tough barriers protecting something important beyond.
Iron Iron Thickness:  2 in Dmg Threshhold 30 AC 19 HP:  100 hpDetails
Sturdy and protective. Iron doors typically protect secure areas or prisons.
Portcullis, Wooden Portcullis, Wooden Thickness:  3 in Dmg Threshhold 20 AC 16 HP:  30 hpDetails
Thick wooden shafts that descend from a recess in the ceiling above an archway. Wooden portcullises often have ironbound crossbars that create a grid. Typically raised by means of a winch or a capstan, a portcullis can be dropped quickly, and the shafts end in spikes to discourage anyone from standing underneath.
Portcullis, Iron Portcullis, Iron Thickness:  2 in Dmg Threshhold 30 AC 19 HP:  100 hpDetails
An iron porcullis has thick metal shafts that descend from a recess in the ceiling above an archway. Most, but not all porticullises, have ironbound crossbars that create a grid. Typically raised by means of a winch or a capstan, a portcullis can be dropped quickly, and the shafts end in spikes to discourage anyone from standing underneath.
Lock Lock Thickness:  -- Dmg Threshhold 5 AC 19 HP:  30 hpDetails
There are various kinds of locks, including crossbars, intricate key-locking mechanisms and magical spells. Many are made of iron or steal. If a keyhole is visible, an attempt to pick the lock can be made (the difficulty depends on the quality of the lock). However, destroying the lock is also an option. Its the DMs discretion if destroying the lock opens the door ... or keeps it locked in place.
Hinge Hinge Thickness:  -- Dmg Threshhold 5 AC 19 HP:  30 hpDetails
The hinges of the a door allow it to pivot and open. If the hinges are on the visible side of the door (they are usually on the 'inside', as most doors open 'inward'), then attacking or removing them can be a way to bypass the door.

† An object with 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 damage as normal. These numbers were based on the damage threshold for ships in the DMG p119. A wooden ship's damage threshold ranged from 0 (i.e. rowboat) to 20 (galley & warship). And, since most boats are made of wood, we made stone have a threshold of 25-30, and iron have a threshold of 30.
‡ The AC for these doors were based on the AC of objects made of certain materials (DMG p246).

Walls

Type Thickness Dmg Threshhold AC HP
Wood, Planks Wood, Planks Thickness:  6 in Dmg Threshhold -- AC 15 HP:  60 hpDetails
Wood walls consist of wooden planks, nailed together, often with crossbeams and supports. Wood is solid, but prone to fire and easy to chop.
Wood, Logs Wood, Logs Thickness:  1 ft Dmg Threshhold -- AC 15 HP:  70 hpDetails
Wood logs are thicker than wooden planks, and usually stacked using mud, cement or another sealant.
Masonry Masonry Thickness:  1 ft Dmg Threshhold 25 AC 17 HP:  90 hpDetails
Masonry walls are the most common type of dungeon and castle walls. These walls are made of stone bricks stacked ontop of each other. Worn masonry, and older ancient walls, often contains cracks and crevices.
Masonry, Superior Masonry, Superior Thickness:  1 ft Dmg Threshhold 25 AC 17 HP:  100 hpDetails
Superior masonary is well-maintained and has few gaps or crevices. These walls are made of stone bricks stacked ontop of each other, but due to their superior craftmanship the walls often look seamless. Some of these walls are engraved with intricate designs and patterns, hiding the crevices. Others are covered with plaster or stucco, or fine mosaics
Masonry, Reinforced Masonry, Reinforced Thickness:  1 ft Dmg Threshhold 30 AC 18 HP:  180 hpDetails
These stone walls are similar to standard masonry, but have iron bars on one or both sides of the wall (or placed within the wall) to strengthen it.
Hewn Stone Hewn Stone Thickness:  3 ft Dmg Threshhold 30 AC 17 HP:  340 hpDetails
Hewn walls are in chambers or passages that have been tunneled out from solid rock. Most hewn walls havee rough surfaces and uneven and jagged ledges, but others are smoothed and engraved.
Unworked Stone Unworked Stone Thickness:  5 ft Dmg Threshhold 30 AC 17 HP:  600 hpDetails
Unworked stone is typically a natural cavern wall. The surfaces are uneven and rarely flat. They often have deep indents, hidden alcoves, ledges at various heights and can be covered in moss, lichen or even dirt.
Iron Iron Thickness:  3 in Dmg Threshhold 30 AC 19 HP:  200 hpDetails
Iron walls are extremely sturdy. They line protected areas, such as treasure chambers and vaults.
Magically Treated Magically Treated Thickness:  -- Dmg Threshhold +20 AC -- HP:  x2Details
Magically treated walls are stronger, and typically immune to the material's vulnerabilities (i.e. a wood wall would noramlly burn because of fire. However, a magically treated wooden wall would take normal damage.)


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