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Ask the Builder: Building a safe fireplace and chimney

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Here’ s a roaring wood fire in my own fireplace.

It’s that time of year when you might be thinking of starting your first indoor fire — if you haven't already gotten your fireplace or wood-burning stove going. I happen to have two wood-burning fireplaces in my own home. Both are see-through, so four rooms in my house get the benefit of a roaring fire.

Whether used for heating or for ambiance, fireplaces and wood-burning stoves are great amenities, but they also carry risk for the homeowner if they are not properly installed. I got curious about how many residential fires are caused by fireplaces or wood-burning stoves, so I consulted the Consumer Product Safety Commission and the National Fire Protection Agency for some data.

The data both agencies offered is from a few years ago, but it reflects what I believe is a significant, ongoing risk. It turns out that more than 10,000 residential fires each year in the USA can be traced to fireplaces, chimneys and wood-burning stoves. That equates to at least 40 fires per day during burning season. These fires cause a lot of property damage, and even injury and death.

These numbers should put your head on a swivel. If not, this story should. About 20 years ago, I was tasked with moving a wood-burning fireplace from one place inside a home to another. The original chimney was installed per the minimum code requirement in place in the 1980s.

I was astounded by what I saw as I removed the drywall that was screwed to the wood furring strips that were nailed to the 4-inch solid concrete block that made the chimney. I could see soot and scorch marks where hot flu gases had worked their way through tiny cracks in the mortar surrounding some of the concrete block.

I decided to slow down my demolition efforts to carefully look at how the clay flue liner tiles were installed. I could see that fresh mortar was used to connect them. The mortar looked to be in great shape. But there were many cracked flue liner tiles!

I couldn’t see any evidence of house settlement and all the masonry in the house looked good. I assumed the foundations under the chimneys were in good shape and had never moved, causing stress on the flue liner. The cracks must have been caused by hot fires. I asked the homeowner about this and he said he would get fires going as fast as possible. I can’t prove it, but I assumed the fires got too hot too fast, and the flue liner tiles didn’t have a chance to slowly heat up. The thermal shock, especially on cold winter days and nights when the flue liner would be quite cold, most likely caused the tiles to crack.

The shocking thing is that the flue gases had escaped from the chimney and could have set the wood framing near the chimney and the wood furring strips on fire. Fortunately for this homeowner, a fire never occurred.

It’s important to realize this chimney had been built to the minimum standards allowed by the building code at the time the house was built. My takeaway — and it should be yours too — is that wood-burning fireplace chimneys should be built to a standard much higher than the code requires.

I’ve had the luxury in my career to employ a master mason, John Hoeh, who built all the wood-burning fireplaces and chimneys on my jobs. It’s mesmerizing to watch him skillfully use a thin coating of wet fire clay to create a fireproof joint between firebrick in fireplaces. He surrounds his flue liners with 8 inches or more of solid masonry.

What does solid masonry mean? To me, John and many other craftsmen, it means that the space between the outside surface of the flue liner and the outside surface of the chimney itself is completely solid. There are no air gaps, no void spaces and no pathways for hot flue gases to escape from the chimney.

Lots of mortar, masonry rubble, solid concrete block, solid brick, solid rock, or whatever solid masonry material available is used to create this barrier. It takes a bit more time to do this, but the long-term peace of mind is worth every penny.

I have written over and over in past columns that the building code is a set of minimum standards. I’ve never had a building inspector or building official disagree with me. Building anything merely to the code standard is like getting a 70 percent on a test. It means you just passed — barely. You can always build things better than what the code mandates. Never ever forget that.

Keep in mind there’s much more you need to know — and your contractor needs to know — about building fireplaces and chimneys. The actual firebox needs to be a very specific shape and size, or else it will smoke. Once the last brick or stone is laid at the top of the chimney, it needs the correct cap so the chimney lasts for hundreds of years. Based on my observations of chimneys I’ve inspected, I’d say that not one in 10,000 has the type of chimney cap the Brick Industry Association recommends.

You can go to AsktheBuilder.com and discover illustrations of how your firebox should be built. I have a table of all the required dimensions. In past columns I have gone into great detail about how chimney caps, the roof of your chimney, should be built. You can find that information for free at AsktheBuilder.com.

(Subscribe to Tim’s’ free newsletter and listen to his new podcasts. Go to: AsktheBuilder.com.)

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