What is the minimum clearance for a single-wall metal flue pipe from combustibles?

• A. 12 inches
• B. 3 inches
• C. 6 inches
• D. 9 inches

Answer: (C)

Hot flue gases in a single-wall metal pipe can transfer enough heat to ignite nearby combustibles. To reduce that risk, a minimum distance is required between the pipe and any combustible materials. For single-wall metal flue piping, the standard minimum clearance is six inches. This provides a reliable heat barrier so wood framing, insulation, or other combustibles aren’t exposed to excessive heat. A smaller gap, like three inches, would not meet the safety standard for this type of pipe, while larger gaps (nine or twelve inches) aren’t required by the baseline rule unless a specific installation or local rule calls for it. If you use a listed double-wall pipe or a shielded installation, clearances can sometimes be reduced per manufacturer listings, but the bare minimum for single-wall pipe is six inches.