Whether used for heating or ambiance, fireplaces and wood-burning stoves are great amenities, but they also carry risk for the homeowner if they are not properly installed.
More than 10,000 residential fires each year in the U.S. can be traced to fireplaces, chimneys and wood-burning stoves, according to data from the Consumer Product Safety Commission and the National Fire Protection Agency. That’s least 40 fires per day during burning season. These fires cause property damage, injury and even death.
About 20 years ago, I was tasked with moving a wood-burning fireplace within a home. The original chimney was installed to minimum 1980s code requirements, and I was astounded by what I found as I worked on the project. As I removed the drywall screwed to the wood furring strips that were then nailed to 4-inch solid concrete blocks comprising the chimney, I could see soot and scorch marks where hot flue gases had worked through tiny cracks in the mortar surrounding the concrete block.
I slowed my demolition efforts to assess how the clay flue liner tiles were installed. I could see that fresh mortar was used to connect them. The mortar was in great shape. But there were numerous cracked flue liner tiles!
I hypothesized that these cracks resulted from hot fires. I asked the homeowner about this, and he said he would get fires going as fast as possible. I assume the fires got too hot too fast, and flue liner tiles didn’t have time to adjust to rising temperatures. The thermal shock, especially on winter days and nights when the flue liner would be quite cold, most likely made tiles crack.
If flue gases had escaped from the chimney, they could have set the wood framing near the chimney and the wood furring strips on fire. Fortunately for this homeowner, no fire occurred.
Keep in mind that the chimney was built to satisfy minimum standards permissible under the building code. My takeaway — and it should be yours, too — is that wood-burning fireplace chimneys should be built to exceed minimum requirements.
What does solid masonry mean? To me, 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 voids and no pathways for hot flue gases to escape from the chimney.
Skilled masons use a thin coating of wet fire clay to create a fireproof joint between firebrick in fireplaces. They surround flue liners with 8 inches or more of solid masonry. Mortar, masonry rubble, solid concrete block, solid brick, solid rock, or other material can create this barrier. It takes more time to build this way, but peace of mind is worth every penny.
Keep in mind there’s much more you — and your contractor — needs to know about building fireplaces and chimneys. The firebox must 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’d say almost none have the type of chimney cap the Brick Industry Association recommends.
You can go to AsktheBuilder.com to learn more about how your firebox should be built and see tables with required dimensions, and also learn more about how chimney caps should be built.
Tim Carter has worked as a home improvement professional for more than 30 years. To submit a question or to learn more, visit AsktheBuilder.com.