/ Field knowledge · IMC 801.18.4
Relining is not zero-clearance permission.
A liner that fixes the flue-gas condition does not automatically fix clearance around it. Existing chimneys and vents must keep airspace to combustibles per the International Building Code (IBC) and the manufacturer. Relining is not zero-clearance permission — the listing sets the reduced-clearance condition. Fireblocking is a separate IBC requirement at every assembly the chimney passes through.
Educational 2024 International Mechanical Code (IMC) 801.18.4 and 801.18.4.1 from Daily Code Talk. Confirm the adopted edition. Distinct from passageway condition (801.18.2) and liner compatibility (801.15–801.16). This is not a clearance layout.
- Verify airspace at wood framing, roof and floor penetrations, wall assemblies, shafts and chases, attics, concealed spaces, and adjacent finishes or storage — the full path, not only the appliance connection.
- An existing masonry chimney without the required airspace may remain in use if it is lined or relined with a UL 1777 chimney lining system specifically listed for reduced-clearance applications. Remaining clearance cannot be less than the liner listing and manufacturer allow.
- 801.18.4.1: noncombustible fireblocking per the IBC at every assembly penetration to limit concealed airflow and fire spread. Combustible fireblocking, or fireblocking assumed, is the miss.
Two questions that get blended: does the passageway support reuse, and does the surrounding construction meet clearance and fireblocking? A compliant liner answers the first. It does not automatically answer the second.
Relined for size. Clearance to framing never measured against that listing
Fireblocking at the attic or floor not shown. The chimney can pass the flue-gas review and still carry an unresolved fire-exposure condition. Escalate unverified clearances, a UL 1777 reduced-clearance liner without documented listing support, and concealed framing that cannot be confirmed from existing drawings.
Source: Daily Code Talk — IMC 801.18.4 · passageway: 801.18.2 · lining: 801.15–801.16 · IBC: 801.3.
What to send
Chimney route in section (attic / floor / roof if you have it), any liner product named, and photos of framing around the chimney.
If it was relined and clearance was never traced
Send the liner cut sheet and the section. Masterbuild will say whether 801.18.4 still has no listing for the reduced clearance — or 801.18.4.1 still has no fireblocking at the attic.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. UL 1777 reduced-clearance is 2024 IMC 801.18.4 education from Daily Code Talk — not a liner specification. This page is not legal advice.