An 18" high-heat metal chimney shall have a minimum sheet steel thickness of ____________.

• A. 10 gauge
• B. 12 gauge
• C. 14 gauge
• D. 16 gauge

Answer: (B)

When a metal chimney is exposed to high heat and must stand up to repeated heating and cooling, the shell needs enough thickness to resist deformation, denting, and possible leaks over time. Using a thick enough wall helps maintain a solid flue path and prevents heat from warping the metal or distorting clearance to adjacent materials. In this range, 12-gauge steel provides that balance of strength and practicality: it’s thick enough to resist bending under thermal stress and handling while not being so heavy or costly as to be wasted for an 18-inch high chimney. Thinner options would be more prone to deformation and leaks, while thicker options are often unnecessary for this size. So, the minimum thickness chosen for this application is 12 gauge.