Seeing The Stages

To begin, a warning: I am not an expert in fire engineering. I have inspected a fair number of buildings after fires, but my inspections have been for structural damage, not to determine the cause or path of fire. So the discussion below is in some sense an amateur’s description.

This is from a 2019 walkthrough of a New Law tenement after a fire:

Overall, we’re looking at the floor framing above, and we see, below the circle of my flashlight, a wood-stud partition with wood lath and plaster on the far side, and two doors. There are a few remnants of the wood lath from the ceiling and you can see the light-gray stripes on the joist undersides where the plaster was touching the wood between the lath strips.

What’s striking (to me, at least) is that we have a series of different stages of damage from the fire. There’s no obvious fire damage to the partition, although it has suffered the mechanical damage of having the lath and plaster forcibly ripped off one side. Most likely that was by fire-fighters looking for hidden fire in the clean-up, but it might have been from some plaster collapse during the fire itself. The joist in the middle looks fine at its bottom but is blackened, most likely by smoke but maybe by the beginning of charring, at its top. The next joist further away, on the other side of the partition, is visibly charred at the top, as is the piece of diagonal wood (displaced blocking, maybe?) above the center joist.

In other words, it seems like the fire was extinguished just before it caused serious structural damage in this area, but after some damage had already taken place.