Dungeon Masters Guide : Page 66
Damaging Objects
Like characters, objects have hit points and defense scores (except for Will defense; see Object Immunities and Vulnerabilities, below).
An object’s AC, Fortitude, and Reflex defense depend entirely on its size. (As you can tell from the following table, it’s pretty easy to hit an object; so easy, in fact, that many DMs just skip the attack roll unless the situation is particularly dramatic.) An object’s hit point total generally depends on two factors: its size and its material. As a rule, larger or thicker objects have more hit points than smaller or thinner ones. Objects made of stone or metal have more hit points than those made of wood or glass.
Exceptions to this general rule abound. An object that’s big but full of delicate moving parts might have fewer hit points than a smaller, more solid object, because it doesn't take as much damage to render that object functionally useless.

Object Immunities and Vulnerabilities
Usually, it doesn’t matter what kind of attack you make against an object: Damage is damage. However, there are a few exceptions.
All objects are immune to poison damage, psychic damage, and necrotic damage.
Objects don’t have a Will defense and are immune to attacks that target Will defense. Some unusual materials might be particularly resistant to some or all kinds of damage. In addition, you might rule that some kinds of damage are particularly effective against certain objects and grant the object vulnerability to that damage type. For example, a gauzy curtain or a pile of dry papers might have vulnerability 5 to fire because any spark is likely to destroy it.