My rules system for determining how a structure fire spreads was extrapolated from the existing 3.5 rules for catching fire, found on page 303 of the DMG.

Per the DMG

1. When a character is exposed to non-instant flame, he or she must make a DC15 reflex save or catch fire.
2. unattended non-magic items do not get saves to avoid catching fire.
3. Fire deals 1d6 damage.

I decided that for the purpose of a structure fire, any burning section has a chance to spread to each adjacent square. To determine if the fire catches in the adjacent square, roll 1d6 fire damage for each square. If the fire beats the material’s hardness (e.g. on a 6 for wood objects, doors, floors etc. w/ hardness 5) the fire deals 1 hp of damage and catches in that square, which is now considered burning and will make its own attack the following round.

Any time a section of flames grows to fill the footprint of a larger size category (Large = 10×10, Huge = 15×15) The heat grows more intense, thus raising the fire’s chance to spread. A large fire deals 1d8 hp to all adjacent squares, while a huge fire deals 2d6 (same as lava) This intense heat allows larger flames to spread to more resistant substances like masonry walls and causes surrounding wood squares to catch more quickly.