Welded grease duct requirements are set primarily by NFPA 96 and the International Mechanical Code, and they cover far more than the welds themselves. A compliant system has to meet specific standards for seam construction, clearance from combustible materials, proper slope and support, accessibility for cleaning, and how the duct terminates outside the building. Understanding these categories helps explain why Commercial grease duct fabrication services involves considerably more planning than simply connecting a hood to a fan with metal ducting.
Each of these requirement categories exists to address a specific risk grease duct presents, whether that is fire exposure, grease accumulation, or long-term structural reliability. Understanding them separately makes it easier to see why grease duct is treated as one of the more heavily regulated components of a commercial kitchen exhaust system.
Seam and Joint Construction Requirements
The foundation of grease duct construction is continuous, liquid-tight welding along every seam and joint, rather than mechanical fasteners, tape, or sealant. This requirement exists because grease-laden vapor condenses inside the duct as it travels, and any gap in a seam becomes a point where grease can accumulate and eventually leak, often inside concealed building spaces where the leak would go unnoticed.
This requirement does not have exceptions based on convenience or routing complexity. Every seam along the duct’s path, however the duct is routed through the building, needs to meet the same continuous welding standard.
Clearance Requirements From Combustible Materials
Grease duct carries hot exhaust, which means it needs to be installed with adequate clearance from combustible materials like wood framing or other flammable building components. The exact clearance required depends on several factors, including whether the duct is enclosed in a fire-rated shaft, what type of insulation or wrap system is used, and how the duct is routed relative to nearby structural elements.
Because these variables differ from building to building, clearance requirements are not something that can be estimated generically. They need to be calculated based on the duct’s actual construction and the specific materials surrounding it in a given installation.
Slope and Support Requirements
Horizontal sections of grease duct are typically required to be sloped toward either the hood or an approved drain point, allowing any grease that does condense inside the duct to flow back rather than pooling in low spots. Proper slope is a design requirement, not something that can be adjusted after installation, which means it has to be planned into the duct’s routing from the start.
Support requirements work alongside slope requirements to keep the duct properly positioned over its service life. A duct that is inadequately supported can sag over time, which not only disrupts the intended slope but also places additional stress on welded seams that were not designed to bear that kind of structural load.
Access and Cleanout Requirements
Grease duct is required to remain accessible for regular cleaning and inspection throughout its service life, which means access panels and cleanout points need to be built into the duct’s design and routing rather than added after installation. How frequently a system needs cleaning, and where access points make the most sense, depends on the type of cooking equipment feeding the hood and how much grease-laden vapor the system handles.
A duct fabricated without adequate access planning can meet code at the moment of installation and still create serious maintenance problems later, since technicians may struggle to properly clean and inspect sections of duct that were never designed to be reached.
Termination Requirements
Where and how grease duct terminates, typically at the roof or an exterior wall, is also governed by specific requirements meant to keep exhaust properly directed away from the building, air intakes, and adjacent structures. Termination point location, height above the roofline, and distance from other building openings all factor into whether a termination point meets code.
These requirements exist to prevent exhaust from re-entering the building through windows, doors, or air intakes, and to reduce fire exposure risk at the point where hot exhaust interacts with the roof or exterior wall assembly.
Material and Construction Standards
Grease duct is required to be constructed from specific gauge steel or stainless steel, with the exact requirement depending on factors like duct size and local code interpretation. This is different from general HVAC ductwork, which can often use lighter-gauge materials since it is not exposed to the same heat and grease conditions.
Because material requirements can shift based on project specifics and local amendments, the right specification for a given duct run should come from a fabricator evaluating the actual project conditions rather than defaulting to a generic industry standard.
Licensing Requirements for Fabrication and Installation
Because grease duct construction is tied directly to fire and mechanical code, fabricating and installing it typically falls under the same licensing requirements that govern the rest of the exhaust system. In several states, purchasing a commercial hood and exhaust system, including the ductwork connecting it, legally requires working with a company holding proper HVACR licensing.
This requirement reflects how seriously grease duct construction is regulated. A general sheet metal contractor without specific commercial kitchen exhaust experience may not be familiar with the full scope of seam, clearance, slope, access, and termination requirements that apply to this type of work specifically.
How Local Jurisdictions Add to These Requirements
NFPA 96 and the IMC establish the baseline for grease duct construction, but local jurisdictions often apply their own amendments on top of that baseline, which can affect clearance distances, termination requirements, or inspection frequency expectations. A duct system that meets code in one jurisdiction is not automatically compliant in a different one, which is why local code review needs to be part of the design process rather than an assumption based on the general NFPA 96 or IMC standard alone.
Why These Requirements Function as One System, Not a Checklist
Seam construction, clearance, slope, access, termination, and material requirements are not independent items that can be addressed one at a time in isolation. A decision made about routing affects clearance requirements. A decision about access points affects how the duct needs to be supported. Treating these requirements as a single, integrated design problem, rather than a checklist to satisfy item by item, tends to produce a more reliable outcome than addressing each one separately.
Coordinating Duct Work With the Rest of a Project
Grease duct fabrication often overlaps with structural planning, roofing, and general construction during a build-out. It is worth being clear that coordinating those other trades and vendors across a project ultimately falls to the owner or the general contractor managing the build. A fabricator’s role is to design, fabricate, and install a code compliant duct system for the specific building involved, not to manage the surrounding construction schedule.
Getting the Requirements Right From the Start
Welded grease duct requirements cover seam construction, clearance, slope, access, termination, and material specifications, all working together to address the specific fire and maintenance risks grease duct presents. Getting each of these right depends on evaluating the specific building and routing conditions involved, not applying a general standard across different projects.
If you are planning a new hood installation or evaluating existing ductwork, a professional site assessment is the most reliable way to confirm what your specific project requires under NFPA 96, the IMC, and any local code amendments that apply.
Frequently Asked Questions
Why do grease duct seams need continuous welding instead of sealant or fasteners?
Grease-laden vapor condenses inside the duct as it travels, and any gap in a seam can allow grease to leak, often inside concealed spaces where the leak would go unnoticed. Continuous welding eliminates those gaps entirely, which sealant or fasteners cannot reliably guarantee over time.
What determines the required clearance between grease duct and combustible materials?
Required clearance depends on factors like whether the duct is enclosed in a fire-rated shaft, what insulation or wrap system is used, and how the duct is routed relative to nearby structural elements. These variables mean clearance has to be calculated per project rather than assumed.
Why does grease duct need to be sloped?
Sloping horizontal sections toward the hood or an approved drain point allows condensed grease to flow back instead of pooling in low spots. This has to be planned into the duct’s routing from the start, since slope cannot be easily adjusted after installation.
How does access planning affect long-term maintenance?
Access panels and cleanout points need to be built into the duct’s design so technicians can properly clean and inspect the system throughout its service life. A duct fabricated without this planning can meet code initially while still creating maintenance difficulties later.
Does grease duct termination affect anything beyond where the exhaust exits the building?
Yes. Termination requirements address the point’s location, height, and distance from other building openings to prevent exhaust from re-entering the building through windows, doors, or air intakes, and to reduce fire exposure risk at the exterior wall or roof assembly.
Why does grease duct fabrication require specific licensing rather than general sheet metal experience?
Grease duct construction involves fire and mechanical code requirements covering seams, clearances, slope, access, and termination. In several states, this work legally requires a company holding proper HVACR licensing, which general sheet metal experience alone does not necessarily cover.
