The AD&D system provides a fairly detailed system for determining structural damage in a siege situation. As a DM, I was never quite happy with these rules; after all, what DM is ever happy with all the rules.
     I decided to create an alternate system for structural damage, one that could be applied not only to buildings, but to ships, wagons, and more. These rules can be adapted to fit anything reasonably definable as a structure.

Structure Points

     Every structure has a number of Structure Points (SP). Structure points are like hit points, and can be defined as the building or vessel's ability to resist physical damage. The following table lists the most common building materials, and provides the means for calculating the SP for a given structure.

     Take the number found in Table 1, and increase it by one for every 10 square feet of exterior wall surface the structure possesses. For example, a small house has wooden walls six inches thick. The exterior walls measure approximately 100 square feet. That gives the house 5 SP (1 x ½ x 10).
     In another example, we consider a small granite keep. This dwelling is 50' square, and has walls 1 foot thick. As such, it would have 100 SP. Note, if the walls vary in thickness, you must use only the thinnest section of wall in the SP calculation (while ignoring design fluctuations such as arrow slits and windows).
     After determining the structure's SP, the DM must then adjust those SP up or down, using his own discretion. For instance, if the small home from the first example had it's walls shorn by dirt embankments, he might raise it's SP from 5 to 12.

Characters and Monsters vs. Structures

     As anyone who has ever punched a brick wall knows, damaging structures is not easy. A building or vessel only takes significant damage from extreme forces. These forces can range from siege weaponry to magical effects to very strong monsters and characters.
     Some very strong characters and human-sized monsters are capable of delivering structural damage. The creatures must be very strong - a minimum Strength of 18/00 is required for damaging wooden structures, and a minimum Strength of 20 is required to damage structures made of stone or metal.
     To determine how much damage a strong character can deliver to a structure, simply divide it's Strength "to hit" score by the structure's base SP per construction material type, per foot (as per Table 1), rounding down. If the number is less then 1, than that character is incapable of damaging the structure.
     For instance, a character with 18/00 Strength tries to batter down a wall to escape a prison. The wall is made of wood. According to Table 1, wood has a base SP of 1 per foot of thickness.
     Characters with 18/00 Strength have a +3 "to hit" bonus. 3 divided by 1 is 3. This is a positive number, so the character is capable of damaging the wall. He can do 1d3 SP of damage to the wall per round. Note, this is regardless of how many attacks he might receive in a round. Also, the character is not required to roll a hit to damage the wall, unless the DM rules that one would be required.
     In the same example, the character is trying to break out of a stone-walled prison. Stone has a SP rating of 4 per foot. Three divided by four is .75, which rounds down to zero. No matter how hard he might pound, the character can never cause structural damage to a stone wall.
     Should a character be capable of harming a structure, then he may benefit from the use of certain items, such as sledgehammers. Having such devices available allows the character to add a bonus of +1 to his SP damage per round. However, he must now roll a successful to hit (against AC 10) every round to damage the wall.
     On the negative side, characters and monsters using their bare hands to attack a wall suffer1d4 hps of damage per round for every 1 SP of damage they do. For example, if a character did 2 SP of damage to a wall in one round, the DM would roll 2d4 to determine how many hps of damage that character suffered in turn.
     When in doubt, the DM should consider that 10 hps of normal damage will equal 1 SP of damage (round down - therefore, 9 hps of damage does no appreciable structural damage). This is total damage per attack - even if five characters did 30 hps of damage in a round, only those single attacks that did more than 10 hps in a shot are used for determining structural damage.

Siege Engines

     Logically enough, siege engines are much more capable of causing massive SP damage. How much damage is done varies between the various construction materials. Table 2 details how common mundane siege equipment damages different building materials.
     Note that there are most likely magical versions of common siege engines. Thus, a ballista +1 would do 1d6+1 SP of damage to a wooden structure.

When A Breach Occurs

     The structure is considered breached when it's SP are reduced to zero. At this time, a hole is opened in the side of the structure, in a place in keeping with the location of damage.
     To determine the size of the hole, look at the maximum damage the primary damaging factor was capable of dealing in that round. Double that number. That is the diameter, in feet, of the hole. Reduce the hole's size by half for every foot of thickness of the walls past 1.
     For example, a large catapult is being used to attack a small keep. The keep's walls are made of stone, and are 2 feet thick. Finally, the walls are breached. The catapult's maximum damage in one round is 6, doubled, then reduced by half for the extra foot of wall thickness. Therefore, the catapult has created a hole, some 2 feet deep and 6 feet in diameter.
     Once the wall has been breached, the structure is still standing, but weakened. To simulate this, considered the structure "healed", but only to 75% of it's maximum SP (round down). So, if a keep had 100 SP and was reduced to zero, it would be considered to now have 75 SP.
     Each time the structure is holed, it's maximum SP are reduced by 25%, until such a time as the maximum SP becomes zero. At this time, the structure is unstable, and continued attacks (doing half of the building's total maximum SP) will cause it to collapse.

Structural Damage to Ships

     Obviously, this system must be altered slightly when it comes to damage to boats and ships. Any breach has the chance of being below the water line, and thus sinking the vessel.
     To begin, all large boats are considered to be made of metal bound wood. If the wood is actually metal bound (as is the case with some warships), then the base construction material is considered to be clay or brick.
     Secondly, due to the construction of a these vessels, their base SP is raised by 50% (round down). For example, a large warship is approximately 75 long and 50 feet high. This gives it a base SP of 112. To get this number, you perform the following calculations:

     (75 ¸ 10) x (50 ¸ 10) = 7.5 x 5 = 37.5
     37.5 x 2 = 75
     75 + (75 ¸ 2) = 75 + 37.5 = 112.5, rounding down to 112.

     Thus, the ship has 112 SP.

Structural Damage from Magic Spells

     Certain magical spells are quite useful in delivering structural damage. I've attempted to identify some of the more common effective spells, and list suggested damages. The DM should modify this list where he feels appropriate, adding in those spells I may have missed.

Mage Spells

Burning Hands: This spell does only minor damage to thick wood. Roll damage normally. Every 10 hps of damage equals 1 SP of damage (round normally). Thus, 13 hps of damage from this spell would do 1 SP, but 15 would do 2. This spell may cause thin wood to catch fire, at the DM's option.
     Note, only spellcasters of at least 7th level would be able to cause structural damage with this spell.

Mending: This spell does not cause structural damage, but may be used to "heal" it. Every casting of this spell on a damaged structure will "heal" 1d3 SP of damage.

Flaming sphere: This spell is incapable of causing structural damage, even to thin wooden walls.

Fireball: Like other fire based spells, this spell causes 1 SP of structural damage per 10 hps of damage rolled (rounding down) when used against wooden walls. See the section on Fire Damage to determine the chances of the wall catching fire.

Flame arrow: This spell is instantaneous, and cannot cause structural damage.

Lightning bolt: This spell is very effective in causing structural damage. As stated in the spell description, this spell can breach 1 inch of wood or ½ inch of stone per caster level, to a maximum of 12 inches of wood or six inches of stone. If the wall is thicker than this number, then the spell does no structural damage at all.

Melf's minute meteors: Although the flames created by this spell are hot, it is incapable of causing structural damage.

Protection from normal missiles: This spell is very effective at reducing structural damage. It subtracts one SP of damage from every attack (to a minimum of 1 SP of damage per attack).

Ice storm: This spell can cause structural damage, but only to thin wood structures. Any structure not made of thin wood (walls 3 inches thick or less) cannot be damaged by this spell. If the damage is applicable, divide the damage rolled by 10, rounding down, to determine the number of SPs of damage.

Wall of fire: This spell is incapable of causing structural damage, but may set wooden walls ablaze.

Wall of ice: The ice sheet variation of this spell is capable of causing structural damage (divide the damage rolled by 10, rounding down, to determine structural damage).

Cone of Cold: This spell, in and of itself, is incapable of causing structural damage. However, it will weaken wooden, and thin stone (2 inches thick or less) walls. Any structural damage done to a wall after this spell has been cast upon it is increased by 50% (round down), for 1d6 rounds. The amount of time may be modified up or down by the DM, as is appropriate to the local temperature (lasting longer in cold climes, and shorter in warmer areas).

Passwall: While this spell is capable of making a passageway, it causes no actual structural damage.

Wall of iron: This spell cannot be used to cause structural damage, but is incredibly useful when it is time to shore or strengthen the walls of a structure. This spell can increase the total number of SP a structure possesses.

Wall of force: While this spell will not increase a structure's SP, it will totally protect it from damage from any form of physical attack.

Wall of stone: This spell cannot be used to cause structural damage, but is incredibly useful when it is time to shore or strengthen the walls of a structure. This spell can increase the total number of SP a structure possesses

Disintegrate: Not only will this spell instantly create a breach in a wall, each casting of the spell effectively "weakens" the structure, creating a breach. See the section entitled When a Breach Occurs for information regarding SPs of breached structures.

Flesh to stone: This spell is very powerful when used against stone walled structures. A stone wall affected by this spell is considered breached, and the structure's maximum SP is adjusted accordingly. See the section entitled When a Breach Occurs for information regarding SPs of breached structures.

Limited wish: If used to directly damage a structure, it can cause an immediate breach (as per a disintegrate), or can be used to deliver 3d10 SPs of damage.

Bigby's clenched fist: This spell is capable of causing structural damage, but only if the damage rolled is 10 or more hps in one attack. This damage is then divided by 10 (rounding down) to determine the SP damage.

Crystalbrittle: This spell, in and of itself, will not cause any structural damage. However, if used against a metal wall, it will weaken that wall such that the next effective structural attack on the wall will cause an immediate breach. See the section entitled When a Breach Occurs for information regarding SPs of breached structures.

Meteor swarm: Like other fire based spells, this spell causes 1 SP of structural damage per 10 hps of damage rolled (rounding down) when used against wooden walls. See the section on Fire Damage to determine the chances of the wall catching fire.

Wish: There is very little a properly worded wish spell cannot accomplish. To provide some guidelines, a wish is capable of instantly destroying any structure with 50 SP or less. Should the structure have more than 50 SP, this spell is capable of causing 1d4 breaches. See the section entitled When a Breach Occurs for information regarding SPs of breached structures.

Cleric Spells

Warp wood: This spell is obviously only effective against wooden walls.

Call lightning: See the mage spell lightning bolt.

Control winds: If this spell is used to create storm-force or hurricane winds, it may then do structural damage. Storm force winds may affect only wooden structures, while hurricane force winds may affect any structure. To determine SP damage per round of exposure, consult the following table.

Flame strike: As with other fire-based spells, only wooden structures can be harmed by this spell. Such structures suffer 1 SP of damage for every 10 hps of damage rolled (round down).

Transmute rock to mud: This spell is devastating when used against stone structures, but obviously does no damage to ones made of wood or metal. When used against a stone structure, the spell will most like destroy it, or at least cause 1 breach for every 60 cubic feet affected.

Stone tell: This spell cannot be used to damage stone walls, but if used by the protecting side, it may be used to aid in repair and maintenance of such walls under siege. See the section entitled Repairing Structural Damage for more details.

Earthquake: This spell has a devastating effect on any solid structure. Such structures are required to roll a save vs. crushing blow, with a +2 bonus for every 1 foot of wall thickness. Those that succeed take damage as per the following table. Those that fail are completely destroyed by the earthquake.

     After determining the number of breaches, apply the SP damage to the structure if it is still standing. Remember, each breach reduces the overall integrity of the structure by 25% (see the section entitled When a Breach Occurs for more detail).

Fire Damage

     Any fire damage to done to wooden walls has a chance of setting that wall ablaze. The chance is equal the amount of damage done in a round (in hit points), considered as a percentage.
     For example, a fireball is cast against the a wooden wall, and the player rolls 27 hps of damage. This does 2 SP of damage to the wall (27 ¸ 10, rounding down), and has a 27% chance of setting the wall on fire.
     If a wooden wall catches on fire, then it begins by doing 1d6 hps of damage per round, with a cumulative +1 modifier. When the modifier exceeds the denomination of the die rolled, the die is increased by one factor, and the modifier is reset to zero.
     Therefore, the burn damage would be 1d6+1 the first round, 1d6+2 the second, up to 1d6 + 7 on the seventh round. On the eighth round, the damage then becomes 1d8, ninth round it is 1d8 +1, then 1d8+2, etc.
     This flame hit point damage is tracked by the DM. Each time it adds up to 10 hps, 1 SP of damage is suffered.
     To extinguish a fire, one character acting alone can extinguish 1d4 hps of flame damage per round. If he has access to effective fire fighting materials, such as wet blankets, water, sand, etc., the amount he can extinguish is doubled. A fire is considered to be "out" in any round where the firefighters extinguish more flame damage than the fire did in that round.

Repairing Structural Damage

     In order to repair structural damage, there must be at least one person on hand with some level of skill in working with the structure's primary wall component. Thus, a carpenter is required for a wooden structure, a stonemason for a stone structure, and a metalsmith for a metal structure. An engineer is qualified to repair any type of structure.
     With such a person on hand, and assuming sufficient resources, 1d10 SP of structural damage can be repaired per hour. If pressed for time (i.e. during an attack), up to 1d3 SPs of damage can be "temporarily" repaired per turn. These repairs are slipshod, and must be strengthened when time permits.
     In order to effective protect a structure, one repair person is needed for every 10 square feet of exterior wall. One qualified repair person (as previously mentioned) is able to effectively supervise up to 10 men at once.
      Should the repair crew working on stone walls have the advantage of a stone tell spell, then the repairs are significantly more effective. For every turn the spell is in operation, 1d3+2 SP of damage is permanently repaired.

Damaging Doors

     Damaging doors uses a set of rules similar to those found for structures, but different enough to warrant it's own section. Doors are considered to have standard hit points.
     To open a stuck door, a character needs to make his open doors roll, as per the character's Strength. If the door is locked, that chance is reduced by half (each successive lock reduces the roll by half again). Should the door be wizard locked, the character must roll is open wizard locked doors roll, again, as per Strength.

     Should the character's attempts at forcing a door open fail, he may attempt to physically batter the door down. Should he do so using only his hands and feet, he will suffer 1d4 hps of damage himself every round (except on the round in which he actually succeeds in breaching the door).
     The different types of doors are detailed in Table 5.
     When using weapons against solid objects, there is a chance these weapons will be broken or damaged. This chance is nil for weapons such as warhammers or mauls, but can be significant for swords, axes, maces, and the like.
     Where a weapon runs the risk of being damaged, a save vs. crushing blow must be made. The table lists the frequency of the save, plus whatever bonuses or negatives to the save are necessary.

     If the save is failed, magical weapons lose one attack plus. Magical items reduced to zero pluses are rendered non-magical. Non-magical items which fail a save vs. crushing blow are destroyed.