IMC 801.18.4 addresses one of the most important existing-chimney reuse checks: is the chimney or vent properly separated from combustible construction? A liner that resolves flue-gas condition does not automatically resolve the clearance condition around it.
Plain-English Highlights
Chimneys and vents must maintain airspace clearance to combustible materials in accordance with the International Building Code and the chimney or vent manufacturer's installation instructions. For plan review, verify clearances at wood framing, roof and floor penetrations, wall assemblies, shafts and chases, attics and concealed spaces, and adjacent finishes or storage areas. An existing masonry chimney without the required airspace clearance may remain in use if it is lined or relined with a UL 1777 chimney lining system specifically listed for reduced-clearance applications.
The remaining clearance cannot be less than what is permitted by the liner listing and manufacturer's instructions - the liner does not automatically eliminate clearance requirements, its listing establishes the permitted reduced-clearance condition. Section 801.18.4.1 separately requires noncombustible fireblocking in accordance with the IBC to limit concealed airflow and fire spread where the chimney or vent passes through or alongside building assemblies.
Permit Proof Chain
Existing chimney or vent route identified -> surrounding construction and combustible materials documented along the full path -> required clearance confirmed against the IBC and manufacturer instructions -> any reduced-clearance condition matched to a specific UL 1777 liner listing -> fireblocking locations identified at every assembly penetration -> concealed-condition verification and inspection hold point assigned.
Where To Show It
M-001: chimney clearance and liner notes. M-101: chimney / vent routing and surrounding construction. M-501: clearance, liner, penetration, and fireblocking details. A/LS sheets: walls, floors, roofs, shafts, and rated assemblies. S sheets: framing and chimney support coordination.
**Do** - Verify required airspace clearance along the full chimney or vent path, not only at the point of appliance connection. - Identify any UL 1777 reduced-clearance lining system by listing and manufacturer instructions. - Show the specific clearance permitted by that liner listing, not a generic reduced dimension. - Coordinate noncombustible fireblocking with architectural framing and penetration details.
**Do Not** - Assume an existing masonry chimney has compliant clearance without verifying the surrounding construction. - Treat relining as automatic permission for zero clearance; the listing sets the permitted condition, not the code section alone. - Ignore concealed framing, attic, or roof-penetration conditions that are difficult to inspect after construction. - Use combustible materials for required fireblocking or leave fireblocking undetailed.
Masterbuild QA Lens
Split the reuse review into two separate questions that are easy to blend by mistake: does the passageway condition support reuse, and does the surrounding construction meet clearance and fireblocking requirements? A compliant liner answers the first question; it does not automatically answer the second.
Drawing / Submittal Check
Trace the chimney or vent through every framing, floor, roof, shaft, and rated assembly it passes through or alongside. Confirm the clearance dimension shown at each location matches either the prescriptive requirement or a specific liner listing, and confirm fireblocking is detailed at each penetration rather than assumed.
Common Review Risk
An existing masonry chimney is relined to resolve a passageway or sizing issue, and the drawings stop there. The clearance to nearby framing was never measured against the liner's actual listing, and fireblocking at the attic or floor penetration is not shown. The chimney can pass the flue-gas review and still carry an unresolved fire-exposure condition.
When To Escalate
Escalate reduced or unverified clearances anywhere along an existing chimney or vent path, any UL 1777 reduced-clearance liner without documented listing support, concealed framing or attic conditions that cannot be confirmed from existing drawings, and missing or combustible fireblocking at penetrations through rated or fire-resistance-rated assemblies.
Field Tip
On existing chimney projects, do not stop at the liner. Trace the entire chimney path and verify clearances, penetrations, and fireblocking wherever combustible construction is present - a passing liner listing is not a substitute for that trace.