First, you can find information that is helpful in the DMG on page 246.
Stone has a AC of 17, Iron/Steel 16, and Wood is 15. So that gives us a start.
From there we look at Object HP and we see that a Large object (10x10) has a HP pool of Fragile 1d10 to Resilient 5d10.
We assume a basic house of Resilient quality, made of wood, we get somewhere in the ballpark of 27hp per wall and half the roof for a total of roughly 162 hp. This would be a tiny house, 10x10x10, but it's a starting point to work from.
If we look at something like a tavern, we look at 30x20x20, giving us easily 270 hp.
The DMG has some guidelines for the hp an object might have but for things like buildings or castles, the things players would probably like to tear down with Earthquake, it's more up to the DM. The DMG does mention Damage Thresholds which is how much damage of an appropriate type an object or a structure has to take in a single attack for it to take any damage at all but for the most part this is designated by the DM. Usually logic should come into play here . A sword hitting a castle wall shouldn't really do much, nor fire on stone. But a sustained earthquake on that castle wall? Eventually that thing is going to come down or at least break in places.
You would be surprised at how much damage sustained fire can do to stone. Even a stone castle tower uses wood for the floors and joists for everything above ground floor. And stone walls are rotten at heat insulation/retention, which is why anyone who could afford sturdy, defensive stone walls sprung for thick wool tapestries to hang on them to keep the heat in during winter and out during summer. When those burned, if the fire from that got hot enough and lasted long enough the stones would shatter or crack, or sometimes even explode. Basically it’s the same principle as popping corn, the tiny bit of moisture inside boils and evaporates into steam. Eventually it gets so hot and the steam generates so much pressure that it has to go somewhere. When enough of the stones crack under that pressure, eventually the whole structure would sometimes collapse under its own weight. If memory serves, that was how Nebuchadnezzar destroyed the First Temple In Jerusalem, since the interior was adorned in precious woods.
PS- Just so I don’t come off as some random loon here, I came across this thread in doing research on building HP.
FYI, the DMG has rules for the statline of a capital ship, which neglects to cover resistances or reductions for wood, but does cover AC, HP, and Threshold. That's functionally a wooden building. There's a bit more detail in Ghosts of Saltmarsh, but WOTC squandered the opportunity to cover what resistances wood should have - Saltmarsh's ships, RAW, don't have any damage resistances or vulnerabilities other objects don't, so e.g. fire doesn't burn it very easily, it's not a good insulator against lightning, etc. It's challenging to draw conclusions, as a result. The DMG specifically suggests wood should have Piercing resistance.
It's also challenging to draw conclusions about why boats and ships have variable AC. Wooden objects are supposed to always be AC 15, and cloth is AC 11; why is a rowboat, which is entirely wood, AC 11, while an Airship, which is depicted as being more than 50% cloth by surface area (and you'll reliably aim for the cloth when you attack it) AC 13? No explanation given.
The DMG also lists hit point values for objects independent of their volumes, which is so absurd Wall of Stone disregards instructions entirely and just tells you hit points per inch of thickness. On the other hand, both Wall of Stone and Wall of Ice violate the rules for the AC their material is supposed to have. Why? Who knows. Galleys have only 500 hit points, but should have well over 600 based on their size. Why? Who knows.
I'd say the easiest solution for stone buildings is to just assume Wall of Stone hit points and DMG AC for stone, while trusting Saltmarsh for condition immunities (i.e. the DMG never clarified if a building is functionally permanently paralyzed or not). For sane and sensible damage resistances, you'll probably have to do some googling and then engage in guesswork. For said guesswork, I recommend treating radiant damage as fire damage, necrotic as acid, and force as unresistable, based on the PHB descriptions, but also you need to disregard the PHB description of Thunder damage, which contradicts Shatter - I'd generally rely on Shatter as how to distinguish Thunder damage from Bludgeoning.
I think the way most buildings should be handled is with the "Huge and Gargantuan Objects" section, which recommends breaking the object in question (a building) into parts, each with its own HP, AC, etc...
This 1) makes the combat against the object more realistic (the wooden drawbridge shouldn't have to take a buildings total 2000 in damage before it collapses under sustained fire, but the tower in the back shouldn't be "damaged" as a result of attacking the drawbridge either. 2) this makes the earthquake spell much deadlier and IMO realistic. If a tiny house, by nature of it's collective walls and roof, has 162 HP as was suggested in a post above, then an Earthquake spell wouldn't collapse it or really any building on its own, but if earthquake affected all walls connected to the ground, and each had <50 hitpoints, that building is coming down, as would a lot of other, stronger structures too.
Regarding saving throw spells, are there any that can target objects that don't have specific rules in their description for objects? There isn't much guidance, but if such a case exists (or for homebrew) I would rule as follows, based on common sense (THIS IS A HOUSE RULE):
Buildings and their parts have a STR and CON scores equal to their AC (based on material), and make saves without proficiency
Buildings and their parts auto-fail DEX Saves
Buildings and their parts can't be targeted or are immune to spells requiring WIS, INT, or CHA saves (most of those require targeting a creature anyway, so its moot)
I would like to add that as RAW, the Earthquake spell does 50 points bludgeoning to structures in contact with the ground ... *per turn*. That means, over the entire course of a minute (10 rounds), the spell deals 500 whopping points of damage to structures. Lots of things (walls, supports, etc) won't stand up that kind of damage, if sustained over the whole minute.
First, you can find information that is helpful in the DMG on page 246.
Stone has a AC of 17, Iron/Steel 16, and Wood is 15. So that gives us a start.
From there we look at Object HP and we see that a Large object (10x10) has a HP pool of Fragile 1d10 to Resilient 5d10.
We assume a basic house of Resilient quality, made of wood, we get somewhere in the ballpark of 27hp per wall and half the roof for a total of roughly 162 hp. This would be a tiny house, 10x10x10, but it's a starting point to work from.
If we look at something like a tavern, we look at 30x20x20, giving us easily 270 hp.
A nonmagical wall of solid stone springs into existence at a point you choose within range. The wall is 6 inches thick and is composed of ten 10-foot- by-10-foot panels. Each panel must be contiguous with at least one other panel. Alternatively, you can create 10-foot-by-20-foot panels that are only 3 inches thick.
If the wall cuts through a creature's space when it appears, the creature is pushed to one side of the wall (your choice). If a creature would be surrounded on all sides by the wall (or the wall and another solid surface), that creature can make a Dexterity saving throw. On a success, it can use its reaction to move up to its speed so that it is no longer enclosed by the wall.
The wall can have any shape you desire, though it can't occupy the same space as a creature or object. The wall doesn't need to be vertical or rest on any firm foundation. It must, however, merge with and be solidly supported by existing stone. Thus, you can use this spell to bridge a chasm or create a ramp.
If you create a span greater than 20 feet in length, you must halve the size of each panel to create supports. You can crudely shape the wall to create crenellations, battlements, and so on.
The wall is an object made of stone that can be damaged and thus breached. Each panel has AC 15 and 30 hit points per inch of thickness. Reducing a panel to 0 hit points destroys it and might cause connected panels to collapse at the GM's discretion.
If you maintain your concentration on this spell for its whole duration, the wall becomes permanent and can't be dispelled. Otherwise, the wall disappears when the spell ends.
* - (a small block of granite)
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I would say that these are good guidelines: HP by thickness, AC by guidelines, damage threshold and immunities / resistances. Damage treshold is tricky, but a wooden galley has 20. Let's say a conservative 150%: 30. A 6 inches wall 10x10 section should have:
<h2>Simple stone wall</h2>
AC 17, 180 HP, DT 30, immunity to poison, psychic and slashing damage, resistance to fire and piercing.
<b>Building. </b>The stone wall cannot be damaged by small weapons such as daggers, swords, or spears. If they hit the stone wall, they must succeed a Constitution Saving Throw with a DC equal to the attack roll, or receive the damage themselves.
--------------------------------
A castle curtain wall is composed by 2 panels, one on the exterior and one in the interior, with a passage between both. Such passage could be empty or filled with dirt or debris. This should deter ancient artillery such as cannons. Normally, a small castle wall should have at least 1.5 m (about 60 inches), but it could be a lot wider (6 m). The actual thickness of the walls could vary, but being conservative we should assume at least 0.5 m (20 inches): 17 AC, 600 HP, and at least DT 40. A solid 1.5 m wall should have 1800 HP and, again, a curtain wall has two walls, and an empty space in between: Filling it with dirt should grant resistance against siege engines (thus, receiving normal damage instead of double).
A tower should have higher AC: The roundness increases deflection, and the thickness is perhaps in the lower end (this, of course, is not always the case).
For a siege, you should divide the wall in sections. Each section should have separate stats, and not all sections are equal. Older sections could be weared off by time, decreasing the HP, whereas reinforced sections should increase the DT.
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Well, WOTC went out of their way to create a spell that specifically damages a building: https://www.dndbeyond.com/spells/earthquake
So, how many HP does a building have?
After the game, the king and the pawn go into the same box. – Proverb
First, you can find information that is helpful in the DMG on page 246.
Stone has a AC of 17, Iron/Steel 16, and Wood is 15. So that gives us a start.
From there we look at Object HP and we see that a Large object (10x10) has a HP pool of Fragile 1d10 to Resilient 5d10.
We assume a basic house of Resilient quality, made of wood, we get somewhere in the ballpark of 27hp per wall and half the roof for a total of roughly 162 hp. This would be a tiny house, 10x10x10, but it's a starting point to work from.
If we look at something like a tavern, we look at 30x20x20, giving us easily 270 hp.
The DMG has some guidelines for the hp an object might have but for things like buildings or castles, the things players would probably like to tear down with Earthquake, it's more up to the DM. The DMG does mention Damage Thresholds which is how much damage of an appropriate type an object or a structure has to take in a single attack for it to take any damage at all but for the most part this is designated by the DM. Usually logic should come into play here . A sword hitting a castle wall shouldn't really do much, nor fire on stone. But a sustained earthquake on that castle wall? Eventually that thing is going to come down or at least break in places.
You would be surprised at how much damage sustained fire can do to stone. Even a stone castle tower uses wood for the floors and joists for everything above ground floor. And stone walls are rotten at heat insulation/retention, which is why anyone who could afford sturdy, defensive stone walls sprung for thick wool tapestries to hang on them to keep the heat in during winter and out during summer. When those burned, if the fire from that got hot enough and lasted long enough the stones would shatter or crack, or sometimes even explode. Basically it’s the same principle as popping corn, the tiny bit of moisture inside boils and evaporates into steam. Eventually it gets so hot and the steam generates so much pressure that it has to go somewhere. When enough of the stones crack under that pressure, eventually the whole structure would sometimes collapse under its own weight. If memory serves, that was how Nebuchadnezzar destroyed the First Temple In Jerusalem, since the interior was adorned in precious woods.
PS- Just so I don’t come off as some random loon here, I came across this thread in doing research on building HP.
DDB Buyers' Guide
Hardcovers, DDB, & You
DDB CONTENT TROUBLESHOOTING
FYI, the DMG has rules for the statline of a capital ship, which neglects to cover resistances or reductions for wood, but does cover AC, HP, and Threshold. That's functionally a wooden building. There's a bit more detail in Ghosts of Saltmarsh, but WOTC squandered the opportunity to cover what resistances wood should have - Saltmarsh's ships, RAW, don't have any damage resistances or vulnerabilities other objects don't, so e.g. fire doesn't burn it very easily, it's not a good insulator against lightning, etc. It's challenging to draw conclusions, as a result. The DMG specifically suggests wood should have Piercing resistance.
It's also challenging to draw conclusions about why boats and ships have variable AC. Wooden objects are supposed to always be AC 15, and cloth is AC 11; why is a rowboat, which is entirely wood, AC 11, while an Airship, which is depicted as being more than 50% cloth by surface area (and you'll reliably aim for the cloth when you attack it) AC 13? No explanation given.
The DMG also lists hit point values for objects independent of their volumes, which is so absurd Wall of Stone disregards instructions entirely and just tells you hit points per inch of thickness. On the other hand, both Wall of Stone and Wall of Ice violate the rules for the AC their material is supposed to have. Why? Who knows. Galleys have only 500 hit points, but should have well over 600 based on their size. Why? Who knows.
I'd say the easiest solution for stone buildings is to just assume Wall of Stone hit points and DMG AC for stone, while trusting Saltmarsh for condition immunities (i.e. the DMG never clarified if a building is functionally permanently paralyzed or not). For sane and sensible damage resistances, you'll probably have to do some googling and then engage in guesswork. For said guesswork, I recommend treating radiant damage as fire damage, necrotic as acid, and force as unresistable, based on the PHB descriptions, but also you need to disregard the PHB description of Thunder damage, which contradicts Shatter - I'd generally rely on Shatter as how to distinguish Thunder damage from Bludgeoning.
Lol, now I want to calculate how many "houses of damage" the smiting paladin in our party does. "Wow, you did at least a rowboat with that hit!"
I think the way most buildings should be handled is with the "Huge and Gargantuan Objects" section, which recommends breaking the object in question (a building) into parts, each with its own HP, AC, etc...
This 1) makes the combat against the object more realistic (the wooden drawbridge shouldn't have to take a buildings total 2000 in damage before it collapses under sustained fire, but the tower in the back shouldn't be "damaged" as a result of attacking the drawbridge either. 2) this makes the earthquake spell much deadlier and IMO realistic. If a tiny house, by nature of it's collective walls and roof, has 162 HP as was suggested in a post above, then an Earthquake spell wouldn't collapse it or really any building on its own, but if earthquake affected all walls connected to the ground, and each had <50 hitpoints, that building is coming down, as would a lot of other, stronger structures too.
Regarding saving throw spells, are there any that can target objects that don't have specific rules in their description for objects? There isn't much guidance, but if such a case exists (or for homebrew) I would rule as follows, based on common sense (THIS IS A HOUSE RULE):
I would like to add that as RAW, the Earthquake spell does 50 points bludgeoning to structures in contact with the ground ... *per turn*. That means, over the entire course of a minute (10 rounds), the spell deals 500 whopping points of damage to structures. Lots of things (walls, supports, etc) won't stand up that kind of damage, if sustained over the whole minute.
The URL for page 246
https://www.dndbeyond.com/sources/dmg/running-the-game#Objects
Discord: MasterWitch#2965
My World Anvil account if you're interested. Work in progress.
https://www.worldanvil.com/w/land-of-the-fallen-7Blandfall7D-masterwitch
Wall of Stone
A nonmagical wall of solid stone springs into existence at a point you choose within range. The wall is 6 inches thick and is composed of ten 10-foot- by-10-foot panels. Each panel must be contiguous with at least one other panel. Alternatively, you can create 10-foot-by-20-foot panels that are only 3 inches thick.
If the wall cuts through a creature's space when it appears, the creature is pushed to one side of the wall (your choice). If a creature would be surrounded on all sides by the wall (or the wall and another solid surface), that creature can make a Dexterity saving throw. On a success, it can use its reaction to move up to its speed so that it is no longer enclosed by the wall.
The wall can have any shape you desire, though it can't occupy the same space as a creature or object. The wall doesn't need to be vertical or rest on any firm foundation. It must, however, merge with and be solidly supported by existing stone. Thus, you can use this spell to bridge a chasm or create a ramp.
If you create a span greater than 20 feet in length, you must halve the size of each panel to create supports. You can crudely shape the wall to create crenellations, battlements, and so on.
The wall is an object made of stone that can be damaged and thus breached. Each panel has AC 15 and 30 hit points per inch of thickness. Reducing a panel to 0 hit points destroys it and might cause connected panels to collapse at the GM's discretion.
If you maintain your concentration on this spell for its whole duration, the wall becomes permanent and can't be dispelled. Otherwise, the wall disappears when the spell ends.
* - (a small block of granite)Discord: MasterWitch#2965
My World Anvil account if you're interested. Work in progress.
https://www.worldanvil.com/w/land-of-the-fallen-7Blandfall7D-masterwitch
I would say that these are good guidelines: HP by thickness, AC by guidelines, damage threshold and immunities / resistances. Damage treshold is tricky, but a wooden galley has 20. Let's say a conservative 150%: 30. A 6 inches wall 10x10 section should have:
<h2>Simple stone wall</h2>
AC 17, 180 HP, DT 30, immunity to poison, psychic and slashing damage, resistance to fire and piercing.
<b>Building. </b>The stone wall cannot be damaged by small weapons such as daggers, swords, or spears. If they hit the stone wall, they must succeed a Constitution Saving Throw with a DC equal to the attack roll, or receive the damage themselves.
--------------------------------
A castle curtain wall is composed by 2 panels, one on the exterior and one in the interior, with a passage between both. Such passage could be empty or filled with dirt or debris. This should deter ancient artillery such as cannons. Normally, a small castle wall should have at least 1.5 m (about 60 inches), but it could be a lot wider (6 m). The actual thickness of the walls could vary, but being conservative we should assume at least 0.5 m (20 inches): 17 AC, 600 HP, and at least DT 40. A solid 1.5 m wall should have 1800 HP and, again, a curtain wall has two walls, and an empty space in between: Filling it with dirt should grant resistance against siege engines (thus, receiving normal damage instead of double).
A tower should have higher AC: The roundness increases deflection, and the thickness is perhaps in the lower end (this, of course, is not always the case).
For a siege, you should divide the wall in sections. Each section should have separate stats, and not all sections are equal. Older sections could be weared off by time, decreasing the HP, whereas reinforced sections should increase the DT.