Everything in the game is flammable, except for stone blocks.
There is a scale to flammability. It’s a scale of 0 – 1 of a chance to burn.
The flammability affects the chance of catching fire as well as how fast it burns once it does catch fire. So materials with a low flammability rating will spread fire less and burn away slower. Fire has a randomly assigned duration, and if the duration is less than the amount of time it takes for the block to burn away, then the block will stop burning before being destroyed.
Ethrel Quote: Brick is like a 0.1, whereas full timber is like a 1.0. Most materials are somewhere between, about as logically as we can (Timbered brick is, for example, 0.4 because of the timber portion).