How it works

Heated gases leaving the firebox expand rapidly. The smoke chamber’s sloped walls, ideally angled at no more than 45 degrees from vertical, guide that expansion into a smooth column that accelerates as it enters the flue. When the walls are corbeled brick with stepped ledges, turbulence forms eddies that trap creosote and slow exhaust velocity. This is why modern code requires the interior surfaces to be parged smooth with refractory mortar or coated with a UL-listed product such as HeatShield Smoktite.

A properly built smoke chamber has symmetric walls, a level smoke shelf at its base, and the flue centered above it. Asymmetry, oversized chambers, or rough corbeling are among the most common causes of chronic fireplace smoking complaints, and a CSIA Level 2 inspection">Level 2 inspection with video scan is typically what reveals these defects.

DFW context

In the Dallas-Fort Worth metroplex, most pre-1995 masonry fireplaces were built with unparged corbeled smoke chambers because regional code enforcement was inconsistent. Combined with the Blackland Prairie’s expansive clay soils, which shift foundations seasonally and crack chimney masonry, these chambers often develop gaps that leak combustion gas into attic spaces. A parge coat applied today must withstand DFW’s freeze-thaw swings, where overnight temperatures drop into the teens and afternoon highs can climb above 60 degrees in the same January day.

Related terms

• [Smoke shelf](/glossary/smoke-shelf/)
• [Throat damper](/glossary/throat-damper/)
• [Flue tile](/glossary/flue-tile/)
• [Refractory mortar](/glossary/refractory-mortar/)
• [CSIA Level 2 inspection](/glossary/csia-level-2-inspection/)

Sources

• NFPA 211 (2024 edition), Section 10.5 – Smoke Chamber Construction
• CSIA Certified Chimney Sweep Reference Manual, Chapter 7
• IRC 2021, Section R1001.8