What a Grease Duct System Has to Do
A grease duct system has three jobs: move exhaust air from the hood collar to the fan at a velocity that keeps grease moving, hold any grease that condenses on its walls without leaking, and contain a grease fire long enough that it does not spread into the building. Every rule about material, welding, slope and enclosure traces back to those three jobs.
Grease duct is not ordinary HVAC duct. It is only used on Type I hood systems, the hoods over fryers, griddles, ranges, charbroilers and other appliances that produce grease or smoke. Type II hoods over dishwashers and steamers use regular exhaust duct. If you are not sure which hood type applies, our Type I vs Type II comparison explains the split.
The duct runs from the hood to the exhaust fan, usually straight up through the roof, sometimes horizontally to a side wall. It is designed with NFPA 96 and the International Mechanical Code in mind, and the authority having jurisdiction (AHJ) decides what is accepted on a given project.
Field-Welded Black Steel vs Listed Factory-Built Grease Duct
There are two ways to build a grease duct system: fabricate it from heavy sheet steel and weld every joint in the field, or install a factory-built grease duct that is listed to UL 1978 and assembled exactly as its listing describes.
Field-welded duct
The traditional grease duct is made from carbon steel (often called black iron) or stainless steel, cut and formed into round or rectangular sections and joined with continuous welds. The commonly referenced minimums are 16 gauge carbon steel or 18 gauge stainless steel. Welded duct can be built to almost any shape, so it fits tight ceiling spaces and odd offsets. The tradeoff is labor: every joint is welded on site, often overhead, by a qualified welder.
Factory-built listed duct
Factory-built grease duct is usually round, double-wall stainless with insulation between the walls. Sections join with listed clamps and sealant rather than field welds. Because the assembly is tested as a system, many products are listed for reduced clearance to combustibles and some are listed as an alternative to a rated shaft enclosure. Installation is faster, but the duct has to follow the listing exactly: approved fittings only, specified supports, the manufacturer's cleanout and access components.
| Factor | Field-welded steel | Listed factory-built (UL 1978) |
|---|---|---|
| Joints | Continuous liquid-tight field welds | Listed clamped or sealed joints |
| Fire protection | Rated shaft or listed wrap, plus clearances | Built into the listing, check what it covers |
| Shapes | Any size or transition | Catalog fittings, usually round |
| Field labor | High, welding on site | Lower, assembly on site |
| Material cost | Lower | Higher |
| Best fit | Short runs, custom routes, budget jobs | Long runs, tight clearances, occupied buildings where welding is hard |
Gauges, Welds and Liquid-Tight Construction
Grease duct must be liquid-tight because liquid grease that escapes through a seam soaks into the surrounding construction and becomes fuel. For field-built duct this means continuous welds on all joints and seams, inspected before the duct is closed in.
- Material. Commonly 16 gauge carbon steel or 18 gauge stainless steel as a minimum. Galvanized duct is not used for grease exhaust.
- Longitudinal seams. Welded the full length, not snap-lock or button-punched.
- Transverse joints. Welded all the way around, using joint styles the code recognizes (for example, telescoping or bell-type joints with the inner section oriented so grease drains back toward the hood).
- Hood collar connection. Welded or made with a listed connection, liquid-tight to the hood.
- Penetrations. No screws, rivets or fasteners that pierce the duct wall where grease can leak out.
Welding grease duct is specialized work. Weld spatter and burn-through are common on thin stainless, and a single pinhole can fail an inspection. We confirm the welding approach and weld inspection plan as part of the submittal package.
Slope, Drainage and Duct Velocity
Horizontal grease duct must slope so grease drains back to the hood or to an approved grease reservoir instead of pooling in low spots. The commonly referenced minimum slope is 1/4 inch per foot, with a steeper slope required on long horizontal runs; confirm the values in the edition adopted locally.
Low points are the enemy. A sag between hangers collects grease that eventually ignites or leaks. Supports should be spaced and sized so the duct holds its slope when it carries the weight of accumulated grease and, in field-welded systems, the weight of the steel itself.
Air velocity matters for the same reason. A commonly referenced minimum design velocity is 500 fpm, and many designers aim for roughly 1,500 to 1,800 fpm so grease stays entrained instead of settling. Higher velocity raises static pressure and noise, so duct sizing is a balance worked out alongside the exhaust fan selection. Oversizing a duct "for the future" can drop velocity below what keeps it clean.
Cleanouts and Access Openings in the Grease Duct
Every section of a grease duct system has to be reachable for inspection and cleaning, which is why the code calls for access openings at changes of direction and at intervals along horizontal runs. A duct that cannot be cleaned will eventually load with grease, no matter how well it was built.
- Access doors are grease-tight, with gaskets rated for the conditions and fasteners that do not penetrate the duct.
- Openings are placed on the side or top of horizontal duct, not the bottom where grease collects.
- Openings must be large enough for a cleaning crew to reach the full interior, and spacing follows the adopted code.
- Where the duct runs inside a rated enclosure, each access door needs a matching rated access panel in the enclosure.
- Ceiling tiles, sprinkler pipes and cable trays must not block the doors.
We mark every cleanout on the shop drawings so the general contractor and ceiling installer leave access open. How often the duct actually gets cleaned depends on cooking volume; our kitchen exhaust cleaning and inspection guide covers the frequency side.
Fire-Rated Enclosures, Listed Wraps and Clearances
Where a grease duct passes through floors, ceilings or walls inside a building, it generally has to be enclosed in a fire-resistance-rated shaft or protected with an approved alternative from the point it leaves the room with the hood to the point it exits the building.
Rated shaft enclosures
A traditional shaft is built from rated gypsum or masonry assemblies with a rating that matches the building, often 1-hour or 2-hour depending on the number of stories and the code path. The shaft serves only the grease duct, with nothing else run inside it.
Listed field-applied wraps
Listed grease duct wrap systems are installed directly on field-welded duct as an alternative to a shaft. They save floor area and are popular in tenant fit-outs. The wrap manufacturer's listing controls the number of layers, overlap, fastening and penetration firestopping, and the AHJ must accept the system.
Clearance to combustibles
Unprotected grease duct commonly needs 18 inches of clearance to combustible construction. Listed wraps and listed factory-built duct can reduce that clearance to what their listing allows. Wood framing, combustible roofing and stored materials in the ceiling space all count.
| Protection method | What governs it |
|---|---|
| Rated shaft | Building code rating, shaft construction details |
| Listed wrap on welded duct | Wrap listing and installation instructions |
| Listed factory-built duct | UL 1978 listing and manufacturer instructions |
| Unprotected in open space | Code clearances to combustibles |
Leak Testing a Grease Duct Before Concealment
Before a grease duct is enclosed or covered, the code commonly requires a leakage test witnessed by the building official, and the most common method is a light test. A bright lamp is pulled through the full length of the duct while the inspector checks the outside of every joint and seam for light leaking through.
A few practical points keep the test from becoming a schedule problem:
- Run the test before drywall, shaft liner or wrap goes on. Fixing a weld through a finished shaft is expensive.
- Darken the area around the duct so small leaks show.
- Have the welder on site with equipment, so any pinhole can be repaired and retested the same visit.
- Ask the AHJ in advance which test method it accepts. Some jurisdictions allow or require other methods.
Listed factory-built duct may have its own test procedure in the manufacturer's instructions. Once the duct passes, enclosure work can proceed.
Routing the Grease Duct to the Roof or Wall Termination
The grease duct ends at the exhaust fan, and the fan location sets requirements that feed back into the duct route. Most kitchens use an upblast fan on a roof curb, with the discharge commonly at least 40 inches above the roof surface and a hinge kit so the fan can tilt open for duct cleaning. Wall terminations are possible but have their own distance limits from openings and property lines.
Good duct routes are short, use few elbows and keep horizontal runs to a minimum. When the kitchen sits on a lower floor of a multi-story building, the route and its enclosure become the largest coordination item on the job. We usually lay out the route with the architect and structural engineer early, so shafts line up floor to floor and the roof penetration lands where the fan curb can go. If you are planning a kitchen and want a grease duct route reviewed, send your drawings for a quote.
Grease Duct Questions
- What gauge steel is required for a grease duct?
- The commonly referenced minimum for field-built grease duct is 16 gauge carbon steel or 18 gauge stainless steel, with all joints and seams continuously welded and liquid-tight. Factory-built duct listed to UL 1978 uses whatever construction its listing specifies. Confirm the requirement against the code edition adopted in your jurisdiction.
- Can grease duct joints be sealed instead of welded?
- Not on field-fabricated duct. Field-built grease duct relies on continuous liquid-tight welds. Sealed or clamped joints are only acceptable on factory-built grease duct systems that are listed for that joint method, and then only when installed exactly as the listing describes.
- How far does a grease duct need to be from combustible materials?
- Unprotected grease duct commonly needs 18 inches of clearance to combustible construction. Listed wraps and listed factory-built duct can reduce that distance to what their listing allows. The AHJ has the final word, and the clearance applies to everything combustible, including framing and stored materials.
- What is a grease duct light test?
- It is a leakage test done before the duct is concealed. A bright lamp is passed through the full length of the duct while an inspector checks the outside of every welded joint and seam for escaping light. Any visible light means a leak that must be repaired and retested.
- Does a grease duct need to slope?
- Yes. Horizontal grease duct must slope back toward the hood or toward an approved grease reservoir so grease drains instead of pooling. A commonly referenced minimum is 1/4 inch per foot, with steeper slope on long horizontal runs. Hangers must hold that slope without sagging.
- Is factory-built grease duct worth the higher price?
- It often is on long vertical runs, in occupied buildings where hot work is difficult, or where the listing lets you avoid building a rated shaft. On short, simple runs straight through a roof, field-welded steel is usually the lower-cost option. Comparing the full installed cost, including enclosure work, gives the real answer.