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What is a Fire Cut?

Mary McMahon
By
Updated May 17, 2024
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A fire cut is a cut made at the top of a joist used with a masonry wall such as a brick or stone wall. This cut is designed to act as a safety feature which will reduce the risk that the wall will collapse in the event that there is a fire which burns through the joist. If the wall does collapse, the nature of the fire cut will encourage it to collapse inwards, which will protect people who may be outside, such as firefighters and rescue workers. Fire cuts are one among many safety features built into the design of structures to reduce the risk of losing lives in a fire.

Joists are horizontal supportive beams which are used to distribute the weight of a structure to the walls so that it can be distributed down to the foundations. They can hold up flooring or rafters, depending on where in a building they are located, and they are usually large and arrayed somewhat redundantly so that if a single joist fails, the network of supportive joists can still function. As joists fail in a fire, they add stress to the extant joists, and joists which are burned through can add considerable horizontal pressure to the walls they are attached to.

The fire cut consists of a diagonal notch. The notch is not deep enough to compromise the integrity of the joist, but if the joint burns through, it will allow the joist to sag downwards, rather than putting high levels of horizontal pressure on the wall. While it might seem like a bad idea to have a joist tilting out of the wall, the fire cut is actually beneficial because it relieves horizontal pressure, which causes masonry walls to collapse during a fire.

The structure of the fire cut is also designed to encourage walls to collapse inwards, if they must collapse. Collapses can happen as the building becomes destabilized by an intense, widespread, or hot fire, and they pose a risk to people who are working outside. If walls collapse outwards, they can eject debris which may hurt emergency responders, and materials which may be on fire may also be distributed around the site of the collapse, putting neighboring structures at risk.

Fire cuts have to be carefully designed and placed to ensure that they are the right size. A skilled mason, contractor, or engineer can confirm that a cut is in the right depth and position.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a About Mechanics researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments

By browncoat — On Dec 06, 2011

It's a bit scary to realize that even a stone or brick house isn't really safe from fire.

I always kind of thought that, at the most, all you'd lose is the internal stuff, but the shell of the house wouldn't be affected really.

But, in reality everything inside the house is connected, so even with fire cuts, you could still end up with crumbling walls.

It's a good reason to double check that your smoke alarms and other fire and safety features are all working, no matter whether your house is made of brick or wood.

I've seen how fast a fire can spread and you would not believe how quickly it can take over an entire house.

By pleonasm — On Dec 06, 2011

@croydon - Well, I don't think they would have to actually set the practice building on fire all that often, to be honest. Maybe a few times in order to make sure they weren't missing anything, but in reality to test this kind of feature, all they'd have to do is simulate the damage it's intended to prevent.

So, for a fire cut, they'd just have to cut the beam in question. They could use a chainsaw, rather than a fire and it would still show the same effect, in theory.

Of course, they probably do just work it all out beforehand and then do it a few times in order to test reality, so I imagine it actually is more of a desk job than not.

By croydon — On Dec 05, 2011

This kind of thing always makes me wonder how they manage to test whether these architectural designs will work in an actual fire.

I mean, maybe now they just run a simulation on a computer, but I'm sure at some point they would build a building, put in the feature in question, like a fire cut, and then set the whole thing on fire to see what happens.

Wouldn't that be kind of an awesome job? I mean, it would have the same kind of general purpose as any sort of science or engineering job, where you do something over and over trying to see how it works and what works best, but in this case, you'd be setting a building on fire over and over.

I just think it would beat a desk job!

Mary McMahon

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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