Guide

Clearance to combustibles around hoods and ducts: what managers should know

Updated 2026-10-14

Grease ducts run hot in a fire. Code keeps wood, paper stock, and other combustibles a measured distance from hoods, filters, fans, and ducts — or requires a listed enclosure / wrap that permits a shorter distance. Managers do not redesign clearances, but they do catch storage shoved against a duct, wrap torn at an access door, and “temporary” plywood left after a remodel. This page is the operator view of those rules.

Short answer Secondary NFPA 96 §4.2.1: where enclosures are not required, hoods, grease-removal devices, exhaust fans, and ducts need at least 18 in (457 mm) to combustible material, 3 in (76 mm) to limited-combustible material, and 0 in to noncombustible material — unless the component is listed for a lesser clearance. Clearance can be reduced with code-described sheet-metal / insulation assemblies or a listed field-applied grease duct enclosure evaluated to ASTM E2336 (clearance then follows the listing, including zero-clearance products). Shaft or wrap enclosures are a separate construction path; access doors must stay usable. See fire wrap — what a cleaner can and can’t touch. Your AHJ’s adopted edition wins.

Default clearances (no enclosure required)

Adjacent material	Minimum clearance (secondary §4.2.1)	Manager check
Combustible (wood framing, cardboard, paper goods, many plastics)	18 in (457 mm)	No storage, pallets, or temp walls against bare duct / hood sides
Limited-combustible	3 in (76 mm)	Confirm the material is actually limited-combustible — do not guess
Noncombustible	0 in	Still keep access panels clear of clutter
Listed hood / duct / device with reduced clearance	Per listing	Keep the listing sheet; installers follow the label, not a verbal “it’s fine”

How clearance gets reduced (secondary §4.2.3)

Method	What secondary text allows	Manager note
28-gauge sheet metal on 1 in noncombustible spacers	Down to 9 in to combustibles	Field-built; not the same as a listed wrap system
22-gauge sheet metal on 1 in mineral wool / ceramic fiber with mesh, on 1 in spacers	Down to 3 in to combustibles	Same — construction detail, not kitchen-staff work
Listed field-applied grease duct enclosure (ASTM E2336)	Clearance per the listing (often zero)	Continuous coverage; damaged wrap is a repair job, not a cleaner improvisation
Listed factory-built grease duct enclosure	Per listing	Same continuity and firestop rules at penetrations

Partial wrap applied only to “shave” clearance at one tight spot is a common red flag in mechanical-code commentary — enclosure systems are meant to run continuous, with listed firestops where the duct penetrates rated construction. Confirm with your AHJ / design professional before accepting a patch job.

What managers should flag on a walk-through

Cardboard, linen carts, beer cases, or wood shelving within 18 in of an unenclosed grease duct or hood side
Torn, sagging, or incomplete duct wrap; missing access-door covers or labels on wrapped ducts
Cleanout openings blocked by insulation, paint, or new walls after a remodel
Wood or plastic “temporary” soffits left against the duct after construction
Storage on top of a horizontal duct chase in the basement or mezzanine
After a kitchen remodel: clearances, wrap, and access doors re-checked before cooking restarts — see kitchen remodels and hood relocations

A grease fire that finds a breach into a combustible ceiling is how small cookline events become structure losses. See fire incident case studies.

Who calls whom

Finding	Who	Not
Storage too close to duct / hood	Kitchen / facilities — move it now	Waiting for the next hood clean
Damaged wrap, missing enclosure, questionable clearance reduction	Mechanical / fire-protection contractor + AHJ as required	Hood cleaner “patching” with leftover blanket
Access door that no longer opens or is buried	Sheet metal / GC; cleaner documents inaccessible areas	Skipping that duct section indefinitely
New hood or relocated duct	Design professional + plan review	Manager measuring with a tape and calling it code

Who it’s for / not for

For: Facilities managers walking roofs and basements; operators reclaiming storage space after a failed inspection; landlords reviewing remodel punch lists
Not for: Selecting or installing a duct-wrap system; calculating fire-resistance ratings; substituting Class K extinguishers for clearance compliance

Sources

HoodFilters: NFPA 96 clearance summary (secondary §4.2.1 default clearances)
UpCodes — Grease Duct Enclosures (IMC shaft / field-applied ASTM E2336 / factory-built enclosure pattern; continuous coverage; firestops)
UL Solutions Evaluation Report ER14229-01 (PDF) (example listed zero-clearance wrap; access-door protection — secondary illustration of listing-driven clearance)
Hood Clean Index — Grease duct fire wrap and enclosure (operator / cleaner split on wrap and access doors)

Next step

Walk the duct path from canopy to roof tonight with a tape measure and a flashlight. Photograph anything combustible within 18 in of unenclosed duct, and any torn wrap at access doors — then assign move-storage vs call-contractor before the next busy weekend.