SYSTEM REFERENCE DOCUMENT 3.5
When a character tries to break something with sudden force rather than by dealing damage, use a Strength check (rather than an attack roll and damage roll, as with the sunder special attack) to see whether he or she succeeds. The DC depends more on the construction of the item than on the material.
If an item has lost half or more of its hit points, the DC to break it drops by 2.
Larger and smaller creatures get size bonuses and size penalties on Strength checks to break open doors as follows: Fine –16, Diminutive –12, Tiny –8, Small –4, Large +4, Huge +8, Gargantuan +12, Colossal +16.
A crowbar or portable ram improves a character’s chance of breaking open a door.