Why can't a cracked chimney be used?

First and foremost, this question is purely for my curiosity only; I am not about to question what the fireman told my brother (even if I did, my brother is several hundred miles away).

My brother’s house has a chimney sticking up from the side of it (i.e. it doesn’t go through the roof, it goes up the side of the house and beside the roof). He had a chimney fire a while back requiring the services of the local fire department to put it out, but there was no house damage. The chimney is now badly cracked.

The fire department was quite definite that he must not use the chimney until it is replaced (no big deal - he just has to use only his natural gas heating for now instead of heating by wood much of the time as he likes to).

Why can’t he use the cracked chimney? Wouldn’t it just mean smoke would leak out the sides a bit rather than just come out the top? Or is the fear that it would catch fire again?

heat of the fire damaged and cracked the mortar.

heat of some future fire could cause a collapse.

the chimney no longer safely takes all the smoke to well above his roof. some may leak into his house and kill him.

A chimney is not just a hole to let the smoke out of the house. It is an integral part of getting the fire to burn properly. The rising smoke creates air currents that draw combustion gasses out of the house. If the draw of the chimney is messed up then combustion gases can go into the house. How likely a few cracks are mess up the draw I don’t know.

A steel liner can be installed to fix the problem. It’s a easy fix for old chimneys.


Just lining the masonry may NOT be sufficient - in addition to causing leaks, the heat may have also destroyed the structural value of the grout. resulting in a stack of bricks with little holdiong them together. Additional thermal expansion may cause the stack to fail, resulting in several tons of brick crumbling down.

See “re-pointing mortar” for the routine to replace mortar in normal wear (it is a very expensive product to maintain, as well as build). Whether it would be an effective repair for fire-damage chimneys is on open question for me.

the smoke carriers with it soot that coats the chimney and can reignite causing a fire. This is what happened the first time. Only now the fire is going to go past the chimney and attack anything else that burns.

If he has a insurance policy and he uses the chimney he will not be covered at all by the policy.
TBS, if he is not insured he can in most places do as he wants regardless of what the Fire Department thinks.
An external chimney is very prone to creosote buildup because there is so much heat loss causing creosote and a chimney fire then becomes inevitable. If it were to happen and not be detected like in the night the heat can and many many times does transfer to the structure, having cracks will just make that more of a probability.
There is a system that does a very good job of repairing damaged chimneys and is recognized by insurance providers and that is the Ahrens system. HERE
I spent 21 years on a Fire Department and back in the early 8o’s many were burning wood because of the big jump in fuels prices.
During the cold Dec and Jan months the fire siren went off almost every night and 80- 90 % were chimney fires.

With a cracked chimney flu, hot gasses can escape from the flu and ignite adjacent combustibles such as wood framing resulting is a serious fire.

Makes sense. Thanks guys! Given the distance, I’ll let him get his own advice; like I said, I was simply curious. From what he told me, his insurance will cover replacing the chimney (he’s already confirmed that). So, when the weather gets a bit more decent he’ll get the work done.