Ask the Builder: What makes a chimney dangerous

Each week, I receive quite a few emails from readers of this column. I treasure each one because they provide a window to view the real problems and challenges you face with your homes.

Some messages are small, some are medium, and others are extra-large. One reader sent me an XXL one. It was a pleasure to read. There are several teaching moments in his email that I’m about to share.

This man lives in an old house in the upper Midwest. It’s a grand home with a tall exterior brick chimney. He’s convinced the fireplace and chimney were afterthoughts and not original to the construction. This homeowner has lived in the house for decades, and he told me he builds quite a few blazing wood fires in the fireplace each winter. The embers and hot flue gases from at least 500 fires have traveled up this chimney.

He also mentioned he felt the fireplace was intended for just gas logs. After looking at his photos, agree 100%. It would be somewhat easy to verify this theory by looking for a small black-iron gas pipe in the basement that terminates in or near the underside of the fireplace. Of course, that gas pipe may have been for a log lighter for a wood-burning fireplace, so you would have to look at how the chimney was constructed to make a final determination.

Another clue to support the gas fireplace theory is the actual size of the chimney. The exterior of the chimney measures about 20 inches by 20 inches. He sent a photo taken by a chimney contractor looking down from above. This photo showed the inside dimensions of the chimney to be about 12 inches by 12 inches. This means the chimney walls are just 4 inches thick on all sides.

I could see traces of a stucco finish on the inside of the chimney, but much of it has worn off in the 100 years or so the chimney has been in service. There was no visible clay flue liner when you look down the chimney from the top.

The homeowner decided to obtain some quotes to repair loose bricks at the top of the chimney and to tuckpoint the mortar above the roofline. He also wanted a new metal chimney cap with mesh sides to stop birds from nesting in the chimney.

This is where the story gets interesting. All of the contractors told this homeowner that his chimney required a significant amount of work to ensure the hot embers and flue gases would not set the house on fire. The man thinks the contractors were just blowing smoke. He felt his chimney was safe because of the number of fires he’s had in it over the 50 years he’s lived in the house.

I agree with the contractors. This man has been extremely fortunate that his local fire department hasn’t pulled up on a frosty winter’s night to extinguish a fire inside his exterior wall or attic. His current chimney is an extreme fire hazard, in my opinion.

For starters, the chimney lacks a flue liner. These vitrified clay products are designed to withstand the hot temperatures from wood-burning fires. These liners are thick and are designed to have minimal joints between the smoke chamber above the fireplace, extending to the top of the chimney. It’s best to use fire clay to seal the joints where one liner rests on top of another. Regular mortar used to bond bricks together is not rated for the hot flue temperatures and will fail in short order.

The photo taken by the contractor shows lots of missing brick mortar between the bricks. Keep in mind there’s only one brick thickness between the combustible wood framing of the house and the 400F flue gases, just 4 inches from bone-dry wall studs and sheathing.

Years ago, I was tasked with moving an interior fireplace to an exterior wall. The chimney above the fireplace did have a proper code-approved clay flue liner surrounded by solid 4-inch concrete block. What I saw when I removed the drywall shocked me.

There were black tongues of soot extending from tiny cracks in the mortar between the solid concrete blocks. This soot and the hot flue gas had escaped from the flue liners. Wood furring strips nailed to the concrete block were next to the hot gases! I then discovered the flue liners had been set using regular brick mortar, not fire clay.

Let’s get back to the reader's chimney. There could be brick joints where 3 inches of mortar has worn away over the years. Now, just 1/2 inch of normal mortar may be the only thing between the dry wood and glowing tiny embers that are floating up the chimney. There’s no doubt in my mind that any insurance company would cancel a fire policy if it were aware of an unsafe chimney.

I told this homeowner to rush to the Brick Industry Association (BIA) website. I suggested he download and read all of their free technical notes about fireplace and chimney construction. You should do the same. Pay close attention to the one technical note publication about chimney crowns. I’d venture to say 99.9% of all chimney crowns on residential homes are not built correctly.

Your takeaways should be the following:

If you doubt what a chimney contractor is telling you, call your local fire department. Most have a fire-prevention officer. Allow him to share true stories about chimney fires. Ask him about the minimum construction standards that one should follow to prevent a fire from a wood-burning fireplace.

Rely on the technical notes from the BIA. They show you how to build fireplaces and chimneys that exceed the minimum building code requirements.

Chimneys servicing wood-burning fireplaces should, in my opinion, have a minimum of eight inches of solid masonry surrounding the properly sized clay flue liner.

A suitable stainless-steel heavy-gauge pipe inside an existing brick chimney might prevent a fire. I’d run this by the fire department. The challenge is how to make a safe connection between the pipe and the smoke chamber above his damper. This is not easy and must be done to ensure no flue gases or embers travel up the chimney on the outside of the stainless-steel pipe.

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