Listed vs. Field-Fabricated Grease Duct

Overview

Every commercial kitchen exhaust project requires a grease duct system. NFPA 96 recognizes two approaches: field-fabricated duct built on-site from welded sheet metal per prescriptive code requirements, and listed (factory-built) duct tested to UL 1978 and installed per the manufacturer’s instructions.

The choice affects clearance requirements, installation method, labor type, and total project cost. There is no universal “better” option — the right choice depends on building conditions, budget, and local code requirements.

This guide provides a neutral comparison to help specifiers, contractors, and plan reviewers evaluate both approaches for a given project.

Side-by-Side Comparison

Cost estimates are rough national averages and vary significantly by region, duct size, and project complexity. Get local bids for accurate pricing.

Field-Fabricated Grease Duct

Field-fabricated grease duct is built on-site from sheet metal per the prescriptive requirements in NFPA 96 and the International Mechanical Code. It is the traditional approach and remains the most common method for new construction with adequate chase space.

Material & Construction Requirements

• Carbon steel: Minimum 16 gauge (0.055 in) with continuous liquid-tight external welds on all joints and seams
• Stainless steel: Minimum 18 gauge (0.044 in), same welding requirements
• Prohibited materials: Aluminum, galvanized steel, and FRP are not permitted
• Slope: Minimum 1/4 in per foot toward the hood or an approved grease reservoir
• Clearance: 18 inches to combustible construction on all sides

Advantages

• Lower material cost per linear foot
• Custom shapes and sizes for any chase geometry
• Widely understood by plan reviewers and inspectors
• No single-source dependency on a manufacturer

Limitations

• Requires certified welder on-site for the full run
• 18-inch clearance requirement consumes significant chase space
• Longer installation time vs. modular listed systems
• Weld quality is inspector-dependent

Listed / Factory-Built Grease Duct

Listed grease duct systems are factory-built assemblies tested to UL 1978 (Standard for Grease Ducts). The listing defines allowable clearances, installation methods, and any restrictions. Many listed systems offer reduced or zero clearance to combustibles.

UL 1978 Testing

UL 1978 subjects the duct assembly to a grease fire exposure for a specified duration. The test evaluates the duct’s ability to contain the fire and limit heat transfer to surrounding construction. Products that pass receive a listing that specifies the minimum clearance to combustibles — which can be as low as zero inches for some assemblies.

Installation

• Modular assembly: Snap-together or bolted sections with factory-applied gaskets — no field welding required
• Clearance per listing: Reduced clearance (3″, 6″) or zero clearance depending on the specific product listing
• Must follow listing: All installation details (supports, penetrations, terminations) must match the manufacturer’s listed instructions exactly
• Access panels: Listed access doors matched to the system

Advantages

• Reduced or zero clearance to combustibles saves valuable chase space
• Faster installation — no welding, modular snap/bolt assembly
• Consistent quality — factory-controlled manufacturing
• Fire-tested performance documented by UL 1978 listing

Limitations

• Higher material cost per linear foot
• Limited to shapes and sizes offered by the manufacturer
• Single-source — replacement parts must come from the listing holder
• Some AHJs are less familiar with listed duct approvals

When to Choose Each

The decision depends on the specific project conditions. Here are common scenarios:

Field-Fabricated Is Usually Better When:

• New construction with a dedicated open chase and plenty of clearance space
• Simple, short duct runs with few penetrations
• Budget is the primary constraint and labor rates for welders are reasonable
• Local AHJ strongly prefers prescriptive code compliance over listed assemblies

Listed Duct Is Usually Better When:

• Retrofitting a duct into an existing building with tight clearances
• Multi-story penetrations where 18-inch clearance would require an oversized shaft
• Zero-clearance installation is needed (e.g., duct running against a rated wall)
• No certified welder is available or welding cost is prohibitive
• Speed of installation is critical — modular assembly is significantly faster

Code Requirements

Both field-fabricated and listed grease duct systems must comply with NFPA 96 and the locally adopted mechanical code (typically IMC). Key code sections:

NFPA 96 Sections 7.1–7.3

• 7.1 — General: Ducts must be designed for effective removal of grease-laden vapors
• 7.2 — Field-fabricated: Prescriptive requirements for material, gauge, welding, slope, and clearances
• 7.3 — Listed systems: Factory-built systems must be installed per their listing and manufacturer instructions

IMC Section 506

The International Mechanical Code Section 506 covers grease duct construction, clearances, and enclosure requirements. It parallels NFPA 96 and adds local amendments adopted by many jurisdictions. When NFPA 96 and the IMC conflict, the more restrictive requirement typically governs.

Frequently Asked Questions

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