Residential Building Inspector B1Public Safety and Special ConstructionHard
An inspector observes a solid masonry chimney that serves a wood-burning fireplace. What is the minimum required thickness of the chimney wall, if not lined with fireclay flue lining?
- A8 inches
- B6 inches
- C12 inches
- D4 inches
Show answer & explanationAnswer & explanation
Correct answer: A. 8 inches
IRC R1003.3 specifies that where a flue lining is not required, the chimney walls shall be constructed of solid masonry units not less than 8 inches (203 mm) thick. This is a critical safety measure when a dedicated flue liner is absent.
Why the other options are wrong
- B. This thickness is insufficient for an unlined masonry chimney wall.
- C. While safer, this thickness is not the minimum required for unlined masonry chimney walls.
- D. This is the thickness for firebox walls, not typically unlined chimney walls.
Unlined Masonry Chimney Thickness
For masonry chimneys that do not utilize a fireclay flue lining, the surrounding masonry walls must be substantially thicker to provide adequate containment and thermal protection.
- Applies to solid masonry chimneys.
- Only when a flue lining is NOT required/present.
- Minimum 8 inches solid masonry thickness.
Memory trick: No liner? Eight inches thick, for a chimney that's solid and quick.