AEO Answer · MEP

What Drives Safe Grease Duct Routing in the Commercial Kitchen?

By Jeremy Mills, CEO & Founder, Apex Grid Engineering — USAF Veteran. · Updated 2026-09-15

I'm Jeremy Mills, CEO & Founder of Apex Grid Engineering and a U.S. Air Force veteran. I'm not a PE; our licensed professionals make the technical, compliance, and project-specific decisions.

Illustrative diagram of building MEP systems: ductwork with diffusers, an electrical panel with conduit, and piping runs.

The concise answer

Grease duct routing carries commercial cooking exhaust in welded, liquid-tight steel duct sloped for drainage, with cleanout access at every turn and fire-rated enclosure or separations from combustibles. Safe routing minimizes length and direction changes, keeps the entire run accessible for cleaning, and terminates above the roof per code. Grease duct is unforgiving of casual design. Every extra elbow is a cleaning problem, every inaccessible section is an inspection failure, and every combustible too close to the duct is a fire risk. The routing has to be designed like the fire-protection component it is — because in a grease fire, that's exactly what it becomes.

The design constraints that shape the route

The route starts at the hood and takes the shortest practical path to the roof — up is almost always better than sideways, because horizontal runs need slope, collect more grease, and are harder to clean. Every change of direction gets a cleanout, the duct is sloped so condensed grease drains back to the hood or an approved collection point, and the entire run is either in a rated shaft or built from listed factory-built grease duct where it passes through the building. Coordination with the other trades is critical because grease duct is the least flexible system in the ceiling: it can't be rerouted casually, its enclosure takes real space, and its clearances are code-mandated. I get the grease duct route locked early in coordination and protect it — everything else routes around it, not through it.

Grease duct rules I enforce

These rules come straight from the code and from fire investigations. None of them are optional.

Welded liquid-tight construction: no screws, rivets, or gaps inside the duct — smooth and drainable
Shortest route with fewest turns: up to the roof beats sideways through the building every time
Cleanout at every change of direction: the entire interior must be reachable for cleaning crews
Rated enclosure or listed duct: protect the run through the building per code, with verified clearances
Proper roof termination: upblast fan with code clearances from intakes, parapets, and property lines

Which related engineering resources can help?

When should I hire a structural engineer?What does a PE stamp mean?What is MEP engineering?Kitchen exhaust grease duct design Kitchen hood suppression design Grease interceptor design Request a project review Engineering service for this question Metro engineering guides Verified service areas

What else do project teams ask?

Why must grease ducts be welded instead of screwed together?

Because any fastener, seam gap, or rough joint inside the duct catches grease and creates a fuel deposit — and gives a fire something to hold onto. Welded liquid-tight construction presents a smooth interior that drains properly and can be cleaned effectively. It's a code requirement, not a preference, and inspectors check it.

How often do grease ducts need cleaning access?

Cleanouts are required at every change of direction and at intervals along straight runs so the entire interior can be reached for cleaning. The cleaning frequency itself depends on the cooking volume — high-volume solid-fuel cooking needs the most frequent service — but the design requirement is that every inch of the duct is accessible to the cleaning crew.

Can grease duct run through other occupied spaces?

It can, but only with proper protection: fire-rated enclosure (typically a rated shaft) or listed factory-built grease duct, plus the required clearances from combustibles. Routing through concealed spaces without access is asking for trouble. I always push for the shortest, most direct route to the roof with the fewest direction changes.

What terminates a grease duct at the roof?

An upblast exhaust fan (or approved equivalent) discharging above the roof with required clearances from parapets, air intakes, and property lines. The discharge can't contaminate outdoor air intakes or create a nuisance, and the fan itself must be rated for grease duty with cleanout access. Hinge kits on the fan for cleaning access are standard good practice.

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