You can bet this wood-burning fireplace gets hot!

"Fireclay is perhaps the most fire-resistant mortar you can use in a fireplace combustion chamber."

This column was SO GOOD that I shared with the 31,000 subscribers who read my December 15, 2019 AsktheBuilder Newsletter.

DEAR TIM: The fireplace in my new home is a mess. Water is leaking into the fireplace and then into the basement. The interior firebricks are not level, some have gaps between them and where there is mortar the joints are one-half inch wide and sloppy. The builder says I should not be concerned as when a fire is built, the brick gets covered with soot and the defects will not be visible. What is an acceptable level of quality? How should the firebrick have been installed? Diane McC., St. Leon, IN

DEAR DIANE: Hypothetically speaking, I wonder what your builder would say to a truck salesman who sold him a shiny, new truck and then said to him, "Don't worry about all those scratches and dents in the body. After a week at the job site, mud and dirt will hide them."

You are looking at real firebrick in a real wood-burning fireplace. But at this time, I use a vented set of natural gas logs in the fireplace. Look at how thin the mortar joints are. PHOTO CREDIT: Tim Carter

Fireplaces are one of many places in a new home where quality can't be compromised. Cracks in granite countertops, dents in new drywall, blemishes in ceramic tile do not present life safety issues. But a gap or crack in firebrick that's poorly installed can cause a house to burn to the ground. Death and serious injury are real possibilities.

How Hot Are Wood-Burning Fires?

The temperature inside a fireplace that contains a roaring fire might surprise you. It's not uncommon for temperatures to fluctuate between 1,200 - 1,500 F. The gases swirling inside the fireplace can seep through the tiniest cracks searching for another way to the atmosphere other than the chimney. If the mason was sloppy with the critical firebrick, imagine the level of quality in the body of the chimney masonry that you can't see. If other gaps and cracks exist, the blistering gases can ignite nearby wood framing in a matter of minutes.

How Wide Should Fireplace Mortar Joints Be?

Mortar joints between firebrick in a combustion chamber should never exceed 1/4 inch in width. In fact, a smaller joint is preferred. If traditional sandy mortar is used, it should be a high-lime content mortar that does contain some Portland cement. High-lime content mortars perform well when subjected to repeated episodes of high temperatures.

What is the Best Mortar to Use?

I prefer to use fireclay as the mortar between the high-temperature white refractory firebrick in my fireplaces. Fireclay is a powder that's mixed with water until you have a consistency like pancake batter. The edge of a firebrick is dipped in the fireclay batter and then the brick is set and tapped in place. The resulting joint is usually one-eighth inch or less in width. Fireclay is perhaps the most fire-resistant mortar you can use in a fireplace combustion chamber.

But keep in mind that the firebrick you see is but one part of a system. Behind the firebrick, one should find a minimum of 8 inches of solid masonry material. This concrete block or brick should be laid with precision making sure that all horizontal and vertical mortar joints are completely filled and solid. The smallest gap in the masonry can create a pathway for rogue, searing flue gases that are constantly trying to escape from the fireplace combustion chamber.

This is a close-up shot of the same firebrick. The joints average a little less than 1/8th inch in thickness. There is no sand in the mortar. It's fireclay. PHOTO CREDIT: Tim Carter

The water leaks you are experiencing may be from two sources. Rain falling down the chimney may roll off a poorly constructed smoke shelf that is not constructed to capture it. Or rainwater could enter the exterior of the fireplace masonry that is outside your home. The leaks often happen during moderate to severe wind-driven rainstorms. These exterior brick can be sealed with a clear brush-on sealant that can bridge cracks up to one-eighth inch. The best part is the sealant is breathable and allows water vapor produced by burning fires to escape from the brick back to the atmosphere.

Fireplace construction is a strict science. If you think you just start laying brick and everything will be fine, you're dreaming. The shape of the firebox, its internal dimensions, and the height and width of the opening are all extremely critical. If the fireplace is not shaped and sized right, the fire will not burn properly and smoke can easily roll out into the room.

The part of the fireplace you can't see unless you poke your head up into the chimney is even more important. The dynamics of the combustion gases as they leave the firebox and make their way to the actual chimney are just as important. This area of a fireplace/chimney is called the throat, and it must also be shaped and sized in accordance with the visible part of the fireplace you see from your family room. Professionals need to build masonry fireplaces. This is not a place for building technology experimentation.

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This is a combustion air intake for a fireplace of my own. I had just enough room to install it but was forced to rotate the intake sideways. I check the intake regularly to ensure it is not clogged by leaves and other debris.

DEAR TIM: I have a house built in the 1950's that has two fireplaces, one in the living room and one directly underneath it in the finished basement. They both share the same chimney. My problem is when I light a fire in the living room, smoke comes out the fireplace opening in the basement, even if the basement damper is closed. The reverse also happens. The only way I've found to stop this is to have fires going in both locations, but that's pretty impractical. Is this a common problem with this kind of construction? Do you have any ideas how to fix this? Rich Wilson, Cincinnati, OH

DEAR RICH: Your description of events makes me believe the fireplaces are starved for combustion air. If you could go back in time and talk with the original owners of the house I would be willing to bet you a dinner at The Precinct that the fireplaces didn't smoke. In the past 50 years, it is my guess that upgrades to the windows and doors of this house have blocked many of the air pathways that supplied the house with the needed combustion air.

Fireplaces consume vast amounts of air

The residential building codes back in the 1950's did not address combustion air for fireplaces or fuel burning appliances, such as furnaces or water heaters. But they do now, since houses are built so much tighter. Fireplaces, furnaces, water heaters and any other device that consumes air to burn a fuel can consume vast amounts of air. Just look at the top of a chimney that has a roaring fire in it. When you see a smoke plume chugging from the chimney top, an equal amount of air must be flowing into the house to offset what you see going up into the air.

What is back drafting?

Your situation is possibly quite dangerous. The back drafting you are experiencing may also happen when your furnace operates. The air needed by a fire or your furnace, or water heater, is going to come back into the home through the path of least resistance. When you start a fire in one or both of the fireplaces, the air needed for combustion may come back down the furnace or water heater flue as these appliances operate. If this happens, you are sucking vast amounts of carbon monoxide into your home.

How can you test for back drafting?

You can prove this back drafting theory of mine quite easily. I would start a fire in the living room fireplace and have someone watch the basement fireplace. As soon as smoke starts to issue from the basement fireplace, have a person open all of the windows in the living room. As soon as the windows are opened, the smoke should stop coming in the basement fireplace as the living room fireplace can obtain all of the air it needs from the nearby windows.

I would hire an experienced bricklayer, who builds masonry fireplaces, and have him see if he could safely install a combustion air kit from the back of the living room fireplace combustion chamber to the exterior wall of the fireplace. It may not be as hard as you might think. A single brick on the exterior of the house and one of the firebrick inside the combustion chamber will have to be sacrificed. But if you hire a pro, the cast iron or cast aluminum vents he installs in place of each brick will look like original equipment installed by the 1950's bricklayer.

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The basement fireplace is going to be the challenge. It may be nearly impossible to add combustion air inside the combustion chamber unless the back of this fireplace is visible. This is not uncommon as many basements in Cincinnati are built on hillsides and one wall and parts of others are partially exposed at grade level.

Your only option may be to provide the needed combustion air via a 4-inch metal pipe that extends from the outside of your home to a point very close to the basement fireplace combustion chamber. If this fireplace is in the center of your basement, you may be able to pipe the air into the back of the fireplace. I would be sure to consult with the building department and your local fire inspectors to ensure the installations of the air vents for both fireplaces meet modern codes and are safe.

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