Grease Duct Enclosure & Fire-Rated Shafts

Overview

When a grease duct passes through floors, fire-rated walls, or areas where the standard 18-inch clearance to combustibles cannot be maintained, NFPA 96 and the International Mechanical Code require the duct to be enclosed in a fire-rated shaft or protected with a listed fire-rated wrap system.

Enclosure requirements are among the most misunderstood aspects of commercial kitchen exhaust design. Getting them wrong leads to failed inspections, costly rework, and potential fire code violations. This guide covers when enclosures are required, the available options, and key design considerations.

The requirements in this guide are drawn from NFPA 96, the IMC, and common AHJ interpretations. Always verify specific requirements with your local authority having jurisdiction.

When an Enclosure Is Required

A fire-rated enclosure or listed protection system is required in these common situations:

• Multi-story penetrations: Grease ducts passing through fire-rated floor/ceiling assemblies must be enclosed in a fire-rated shaft from the point of penetration to the roof termination
• Fire-rated wall penetrations: Where a grease duct penetrates a fire-rated wall or partition, the penetration must maintain the wall’s fire rating
• Clearance reduction: Where the standard 18-inch clearance to combustible construction cannot be provided, a listed enclosure, wrap, or factory-built zero-clearance duct system is required
• Concealed spaces: Grease ducts routed through concealed spaces, above ceilings, or within wall cavities typically require enclosure per the locally adopted code

Fire Rating Requirements

The required fire-resistance rating for grease duct enclosures depends on the number of stories the duct serves:

Some jurisdictions require higher ratings or have additional enclosure requirements. Always verify with the AHJ.

Enclosure Types

There are three primary approaches to enclosing a grease duct:

Shaft Enclosure (Gypsum or Masonry)

A dedicated fire-rated shaft constructed from gypsum board, masonry, or other rated assembly materials. This is the traditional approach and the most commonly accepted by AHJs.

• Shaft walls must achieve the required fire-resistance rating (1-hour or 2-hour)
• Must maintain clearance between the duct exterior and the shaft interior per code
• Shaft must extend continuously from the floor penetration to the roof termination
• Fire-rated access doors required for duct inspection and cleaning access

Field-Applied Fire Wrap

Listed fire-rated wrap systems applied directly to the grease duct exterior. These products are tested to provide a specific fire-resistance rating when installed per the manufacturer’s instructions.

• Reduced clearance: Many listed wraps permit reduced clearance (as low as 0–3 inches) to combustible construction
• Must be installed per the specific product listing and manufacturer instructions
• Wrap must be continuous and properly sealed at all joints, supports, and penetrations
• Listed access doors compatible with the wrap system must be used

Factory-Built Listed Assembly

Some UL 1978–listed grease duct systems include integral fire protection that eliminates the need for a separate enclosure or wrap. These systems are tested as complete assemblies.

• Zero-clearance ratings available from some manufacturers
• Modular installation with no field welding or separate wrap
• Entire system (duct, insulation, cladding) covered under one listing
• Highest material cost but lowest total installed cost in tight-clearance conditions

Single Duct Rule

NFPA 96 requires that only one grease duct be installed in a shaft enclosure. No other ductwork, piping, wiring, or building services may share the grease duct shaft.

• Each grease duct must have its own dedicated shaft
• No supply air ducts, exhaust ducts from other systems, plumbing, or electrical may share the shaft
• Multiple grease ducts from separate hood systems cannot share a single shaft
• The shaft interior must remain accessible for inspection and cleaning

Venting & Drainage

Grease Drainage

Grease duct slopes must direct grease toward the hood or an approved grease reservoir. Where a shaft enclosure is used, provisions must be made for grease that may accumulate at the base of the shaft:

• Duct slope of at least 1/4 in per foot toward the hood or reservoir
• Grease-tight construction at shaft base to contain any leakage
• Cleanout access at the lowest point of the duct run within the shaft
• No floor drains or other building drainage connections within the shaft

Shaft Ventilation

Some jurisdictions require the shaft enclosure to be ventilated to prevent heat buildup in the annular space between the duct and the shaft walls. Check local requirements:

• Ventilation openings at the top and bottom of the shaft (where required)
• Openings must not compromise the fire rating of the shaft
• Fire dampers are typically not permitted in grease duct shafts

Clearance Reduction Options

The standard 18-inch clearance to combustibles can be reduced using listed protection methods. The allowable reduction depends on the specific product and its listing:

Reduced clearance values are listing-specific. Always verify the exact clearance permitted by the product’s UL listing and confirm AHJ acceptance.

Inspection & Cleaning Access

Grease duct enclosures must provide adequate access for inspection, cleaning, and maintenance of the duct system:

Fire-Rated Access Doors

• Rating: Access doors in fire-rated shaft walls must have a fire-protection rating equal to or greater than the wall rating
• Size: Large enough to permit cleaning equipment access and visual inspection of the duct
• Location: At each duct access panel location and at changes in direction
• Labeling: Access doors must be permanently labeled for identification during inspections

Working Space

Adequate working space must be maintained around access points. Cleaning contractors need room to operate rotating brush equipment, cameras, and chemical application tools. Plan for a minimum 30-inch clear working space in front of each access point.

Frequently Asked Questions

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