Welcome to How Buildings Burn. Fire is the same chemical reaction in every building on earth. Oxidation, heat, fuel, and the same physics governing how that heat moves. None of it varies from one address to the next. And yet two fires with identical ignition, identical fuel load and identical weather will produce completely different outcomes in two buildings on the same street, and will kill different people in a different order. That difference is the subject of this class. The structure is the variable. Over the next six lessons we will work through the framing methods, the assemblies, and the interior features that decide where a fire goes, how long the building holds, and what warning you get before it stops holding. We will finish with a method for reading a building you have never entered, from the outside, in about a minute. Everything before that lesson exists to make that minute worth something.

Three things by the end of this lesson, and all three are the vocabulary the rest of the class assumes. First, you should be able to name the three ways heat moves, and say which one does the travelling in a building fire, because that tells you where to look for the fire you have not found yet. Second, you should be able to describe the stages a compartment fire passes through, and in particular treat flashover as a threshold rather than a matter of degree. Third, and this is the one that will keep coming back, you should be able to explain why the same fire in the same room behaves differently depending on how much air can reach it. Get those three and the next five lessons are applications. Miss them and the rest is trivia about carpentry.

Here is the idea the whole class hangs on, and it is worth stating as plainly as possible. The fire does not decide where it goes. The building decides. Combustion needs fuel, heat and oxygen, and it will take whatever path supplies all three with the least resistance. What the building does is determine what those paths are. A continuous cavity inside a wall is a path. An unsealed penetration through a floor is a path. A soffit that runs from the kitchen ceiling into the attic is a path. None of them were designed as paths, and that is exactly the problem: they were designed for structure, or for plumbing, or for energy efficiency, and their behaviour under fire was a side effect nobody drew. So when you look at a building and try to predict a fire in it, you are not really predicting the fire. You are inventorying the paths the builder left behind.

Heat moves three ways and all three are always happening at once. Conduction is heat through solid material in contact with itself, which is how a fire on one side of a wall heats a steel beam and starts a second fire three rooms away where the other end of that beam sits in a joist pocket. Convection is heat carried by moving gas, and in a building fire convection is the one doing the travelling: the hot smoke layer rises, spreads across the ceiling, finds an opening, and moves up. If you want to know where the fire is going, follow the convection. Radiation is heat crossing open space as infrared energy, needing no contact and no medium, and radiation is what ignites the far side of a room that nothing has touched, and what preheats every surface before flashover. The useful discipline is to ask which one dominates in the situation in front of you, because each one points somewhere different.

Flashover is the moment a room stops having a fire in it and becomes a fire. Here is the mechanism, because the mechanism is what makes it predictable. As a fire burns, the hot smoke layer builds down from the ceiling, and that layer radiates heat onto everything below it: the carpet, the far sofa, the curtains, the paint, the plastic television housing. Every exposed surface in the compartment is being preheated at once. When those surfaces reach ignition temperature, they do not catch one after another. They catch together, in a second or two, and the entire volume of the room goes to flame. Ceiling temperatures at that point are in the region of eleven hundred degrees Fahrenheit, around six hundred Celsius. Nobody survives flashover in the compartment, and no protective equipment changes that. This is why flashover is a threshold and not a stage on a gradient: the room is survivable, and then it is not, and the transition takes about as long as this sentence.

One more constant before we start looking at buildings, and it is the one people find least intuitive. A fire is limited either by fuel or by ventilation. A fuel-limited fire has all the air it needs, so it burns as fast as the fuel allows and grows until the fuel runs out. A ventilation-limited fire has plenty of fuel left but not enough oxygen, so it slows, produces enormous quantities of unburnt fuel as thick smoke, and sits there hot and hungry. Here is why that matters. In a modern building, a fire behind closed doors in a tightly sealed envelope is almost always ventilation-limited. It looks like it is dying down. It is not. It is waiting for air. Introduce air, by opening a door, venting a window, or having a window fail on its own, and that stored fuel ignites and the fire transitions violently within seconds. Every experienced firefighter has a story about a door. This is the physics behind those stories.

Before we move on, take two minutes and think about a building you know well. Your own home, your station, the place you work. Now hold the three ideas from this lesson against it. Where would the convection go, meaning where does the ceiling of the room you are picturing connect to the rest of the building? Which room would reach flashover first, and what is in it? And would a fire in that building be fuel-limited or ventilation-limited at two in the morning with everything shut? Most people, doing this for the first time, describe the contents of the building. They think about the sofa, the mattress, the kitchen. That is a reasonable instinct and it is not what this class is about. The gap between what you just described and what the structure would do is the whole reason the next five lessons exist.

Three things to carry forward into the rest of the class. Heat moves three ways and convection is the one that does the travelling, so following the hot gas is how you find where a fire is going rather than where it is. Flashover is a threshold crossed in a second or two, not a fire gradually getting bigger, and it is driven by radiant preheating of every surface in the compartment at once. And a modern compartment fire is usually ventilation-limited, which means the quiet, dark, slow fire is the dangerous one and adding air is the event that changes everything. Each of the next four lessons takes one family of buildings and asks what that structure does to these three. The lesson after those gives you the method for reading a building cold.