Have you ever looked up through your chimney’s damper and noticed smooth, angled concrete or “stair-stepped” brick? If so, you were looking at your chimney’s smoke chamber. When building your chimney, the masons first constructed your firebox, which you have probably noticed is a good amount larger than the flue running up through your roof. So, how do the flue tiles not fall down? The answer…your smoke chamber.
After completing the firebox, the masons start corbelling (making them stair step-like) the walls, making each row of brick have a smaller and smaller area until it is equal in size to the flue tiles, supporting them on all four sides. According to IRC code R1001.8, your smoke chamber should be constructed with solid masonry or hollow masonry that has been grout-filled, and the walls should be at least 8 inches thick. Here is the most important part. According to ASTM C199, the inside of the smoke chamber must be parged smooth. You may be wondering what parging is exactly.
Essentially, parging can be defined as covering a surface in plaster or mortar to make it smooth. Many homeowners may be thinking “But, like, why does this matter? I’m never going to look up in my chimney. Who cares if it’s smooth”. Picture your firebox like a funnel. While it is really there to hold up the flue tiles, as discussed earlier, it also plays a big role in how well your chimney drafts (a.k.a, how well it gets smoke out your roof and not into your house).
If you have corbelled brick in your smoke chamber, the smoke will end up hitting the brick and have to build up and be forced from brick to brick to brick before reaching the flue and going up the chimney. This is a significantly slower process than if the smoke can roll right up the sides and out the chimney. You can find much more information about this in the creosote build-up page, but the longer it takes for your chimney to draft, the cooler the smoke gets, and the cooler the smoke gets, the more creosote it leaves behind, leading to higher cleaning costs and even the potential for a chimney fire.
While issues in the smoke chamber can be some of the most expensive repairs you will face due to the difficulty of getting inside the smoke chamber after the chimney has been completed, this area is certainly one of the most important parts of your chimney.