Most of the buildings people live in are wood frame, and there are two dominant ways of building one. They can look identical from the street, they can be the same size and the same shape with the same siding, and they behave completely differently once fire gets inside a wall. This is the clearest case in the whole class of structure deciding outcome, which is why it comes first. It is also the one where the era of construction gives you the most reliable read from outside, so it is where the exterior method we build in the last lesson gets most of its predictive power. If you take one distinction away from this class, take this one, because the difference between the two is the difference between a fire in a room and a fire in every floor of the building at once.

Three things by the end of this lesson. First, you should be able to describe both framing methods well enough to sketch them, because the geometry is the whole argument. Second, you should be able to explain why one of them gives fire a continuous vertical path inside the walls while the other interrupts it at every storey. Third, and this is the one you will actually use, you should be able to stand in front of an unfamiliar wood-frame house and make a defensible guess about which one you are looking at, from the age of the building and a handful of exterior details. That third objective is the point. The first two are how you earn the right to it.

In balloon framing, the wall studs are single continuous lengths of timber running from the foundation sill all the way up to the roof. The floors do not sit on top of the walls. They hang off the sides of the studs, carried on a ledger board, a horizontal strip let into the studs, sometimes called a ribbon. Picture what that leaves you. Inside every exterior wall there is a vertical cavity running the full height of the building, from the basement to the attic, uninterrupted, and in most of these buildings unfirestopped, because fire stopping was not standard practice when they were built. The method was popular from roughly the middle of the nineteenth century into the nineteen-thirties, because it was fast and it needed only long, cheap, machine-cut lumber. It built an enormous amount of the older housing stock in North America, which means a great many of these buildings are still occupied today.

Platform framing replaced it, and by the post-war period it was the standard. Here each storey is built as a complete platform. Studs run one floor high and no further. They are capped with a top plate, the floor deck of the next storey is laid across that, and the next set of studs starts on top of the new deck. Then the process repeats. The consequence for fire is structural rather than intentional: at every single floor level, the wall cavity is interrupted by the top plate and the floor deck. That horizontal timber is not a fire stop by design, and it is not rated, but it is a substantial obstruction of solid wood in the path of a fire trying to travel upward inside the wall. It has to burn through it, and that takes time. Time is the entire benefit, and it is a large one.

The practical difference comes down to this. In a platform frame, a fire inside a wall tends to stay roughly where it started until it burns through the plate above it, which buys you minutes and often keeps the problem on one floor. In a balloon frame, a fire that gets into a wall cavity at basement level has a clear vertical run to the attic, and convection will take it there. So the fire you are called to in the basement can be in the attic before anyone has been above the ground floor, with nothing burning in between that you can see. Understand what that means for searching a building: the floors between the two are not safe, they are simply not yet involved, and the fire behind those walls is above you while you are working below it. A balloon-frame wall is not a wall with a fire in it. It is a chimney with a wall around it.

Here are the assumptions that get people hurt in wood-frame buildings, and what is actually true instead. The first is that a fire in a wall is contained by the wall, which is only true if something interrupts the cavity. The second is that nothing is burning above you because nothing is visible above you, when in a balloon frame the absence of visible fire on the floor between is exactly what you would expect. The third is that a renovated older building has been brought up to standard, when renovations very often open cavities and rarely close them, and a retrofitted sprinkler or a new kitchen tells you nothing about fire stopping inside a wall. And the fourth is that the vintage of the exterior is the vintage of the frame, which fails on every building that has been re-sided, extended, or gutted. Each of these is a reasonable inference. Each of them has killed people.

You cannot see framing from the street, but you can narrow it down considerably, and a four-step read gets you most of the way. Start with the age of the building, because era is the strongest single signal: pre-nineteen-thirty leans strongly balloon, post-war leans strongly platform, and the decades in between are genuinely mixed. Then look at the proportions and the storey heights, since balloon-framed buildings often carry tall, narrow window openings and taller floor-to-floor dimensions than a modern equivalent. Third, look for evidence of alteration: mismatched siding, an obviously later addition, a roofline that changes pitch, all of which mean the frame may not be uniform through the building. Fourth, look at what the building sits on, since a full-height masonry foundation with a raised first floor is common in the balloon era. None of these four is proof. Together they give you a working assumption you can act on and revise.

Take two minutes with the buildings on your own streets. Which ones are almost certainly balloon framed? For most people the answer is a specific neighbourhood rather than a specific house: the older part of town, the streets that went up before the war, often the ones with the tall windows and the porches. Now ask the harder question. Of those buildings, which ones have been converted into apartments, which is extremely common, and what does that do to the picture? A conversion adds occupants, adds cooking, adds electrical load, adds interior walls that may or may not be fire stopped, and does nothing whatsoever to the original full-height cavities. That combination, an old balloon frame carrying modern occupancy, is one of the most dangerous residential arrangements there is.

Three things to remember. Balloon framing gives fire a continuous full-height path inside the exterior walls, so a basement fire is an attic fire sooner than anyone expects and the floors in between look untouched while it happens. Platform framing interrupts that path at every storey with a top plate and a floor deck, which is not a rated fire stop but is a real obstruction that buys time. And the era of the building is your best available read on which one you have, with pre-nineteen-thirty leaning balloon and post-war leaning platform, held as a working assumption rather than a fact, because re-siding and renovation break the correlation regularly.