A duct route that looks straightforward on a drawing can become the deciding factor in a kitchen fit-out or workshop extraction project. The choice between spiral versus rectangular ducting affects available headroom, installation time, cleaning access, system resistance and the number of fabricated components required.
Neither format is automatically right for every job. Spiral duct is often the practical choice for long, open runs, while rectangular ductwork earns its place where ceiling voids are tight, connections are non-standard or a canopy requires a purpose-made transition. The right answer comes from the route, the extract duty and the standard the system must meet - not from choosing one profile by habit.
Spiral versus rectangular ducting: the main difference
Spiral ducting is round, galvanised steel duct made from a continuously formed spiral seam. It is normally supplied in standard diameters and joined with matching fittings, such as bends, tee pieces, reducers, spigots and caps. Its circular form distributes pressure evenly around the duct wall, giving it good rigidity for its material thickness.
Rectangular ducting is fabricated from flat sheet metal into square or oblong sections. It can be produced to particular widths and depths, making it useful where a circular duct would take too much vertical space. It also allows a system to match the shape of canopy outlet spigots, structural openings and confined ceiling zones.
For commercial kitchen extraction, both can be used within the same system. A common arrangement is a rectangular plenum or canopy connection, followed by a fabricated transition into spiral duct for the main run. This is often a sensible way to use each format where it performs best.
Where spiral ducting works well
Spiral ducting is a strong, efficient option for straight extraction runs through plant areas, service corridors, workshops and exposed ceilings. The round profile uses less sheet material than a rectangular duct of comparable air capacity, which can make it a cost-effective choice where standard diameters suit the design.
Straight runs and standard fittings
A round duct is particularly convenient when the route contains repeatable bends, branches and reductions. Standard spiral fittings are readily matched to standard pipe sizes, helping installers assemble a consistent system without waiting for every section to be fabricated specially.
The circular shape also tends to offer lower resistance than an equivalent rectangular section when comparing suitable duct sizes. That does not mean a round duct will always improve a poorly designed extract system. Sharp bends, unnecessary changes of direction, undersized sections and unsuitable fan selection can still restrict airflow. The full route must be considered alongside the fan duty.
Exposed commercial installations
In an open-ceiling restaurant, production area or workshop, spiral duct can provide a tidy industrial finish. Its regular profile is generally easier to align visually than a long run of rectangular panels and flange joints. Where appearance matters, the installation should still be planned carefully: neat supports, correctly positioned access doors and properly sealed joints make the difference between a professional result and a rushed one.
Strength and handling
Round ducting has good inherent stiffness. This can simplify handling and support over longer straight runs, particularly where the duct is suspended below a soffit rather than enclosed within a ceiling void. Correct support spacing and fixing methods remain essential. Ductwork should not be treated as self-supporting simply because the section is round.
When rectangular ducting is the better fit
Rectangular ducting comes into its own where physical space dictates the design. A shallow, wide section can pass through a limited ceiling void where the diameter of a spiral pipe would be impractical. This is a frequent issue in commercial kitchens, where lighting, pipework, electrical containment and building structure all compete for the same space.
Low headroom and awkward routes
If a route needs to stay close to the underside of a ceiling, a rectangular section can be sized to preserve headroom. It can also be fabricated around structural constraints more effectively than a series of circular components. That flexibility is valuable, but it needs competent design. Making a duct excessively wide and shallow may increase friction and create a less efficient route.
Rectangular ductwork is also often the natural interface at the top of an extraction canopy. A canopy may have one or more rectangular outlet positions, and a fabricated plenum can collect air evenly before transitioning to the main duct route. For a made-to-measure canopy installation, the canopy, plenum, duct size and fan should be treated as one system rather than bought as unrelated parts.
Bespoke dimensions and transitions
Standard spiral sizes are useful, but they cannot solve every site constraint. Rectangular duct can be fabricated to suit a particular opening, offset, rise or equipment connection. This is especially relevant where an installation needs a non-standard transition between a canopy, an electrostatic precipitator unit, an inline component or an existing duct route.
Custom fabrication should be specified clearly. Confirm the required dimensions, material grade, thickness, connection detail and intended use before manufacture. A duct section that fits the drawing but cannot be physically manoeuvred into the building is an expensive problem to discover on installation day.
Airflow, noise and pressure loss
Duct profile influences airflow, but it is only one part of system performance. Air velocity, total route length, fitting geometry, changes in section, filtration equipment and discharge arrangement all contribute to pressure loss. The fan must be selected for the resistance of the completed system, not simply for the size of the canopy or the diameter of the first duct section.
Spiral duct has a smooth, consistent internal profile when correctly installed. Its standard bends and fittings can support a predictable route, particularly on straightforward runs. Rectangular duct can perform equally well when it is correctly sized and fabricated, but poor proportions, abrupt transitions and poorly formed bends can add resistance and turbulence.
Noise follows the same principle. High air speed, restrictive fittings and an overworked fan are common causes of noise, whatever duct shape is used. Where noise control is required, include suitable silencers and avoid treating them as an afterthought. Their size and position need space within the route.
Cleaning access and compliance
For commercial kitchen extraction, grease-laden air changes the discussion. Ductwork needs to be accessible for inspection and cleaning, with access panels positioned where grease is likely to accumulate and where cleaning contractors can genuinely reach them. A duct route that cannot be cleaned properly is not a successful installation, even if it fits neatly above the ceiling.
DW172 provides guidance for kitchen ventilation systems, while DW144 covers the manufacture and installation of sheet metal ductwork. These standards matter when selecting materials, jointing, access arrangements and construction quality. They should be considered from the first layout, not added after the ductwork has been ordered.
Round duct is not automatically easier to clean, nor is rectangular duct automatically harder. The deciding factors are the length of straight runs, the number and type of bends, the location of access doors and whether the system has been designed for safe maintenance. In grease extract applications, avoid unnecessary pockets, dead ends and complicated changes of direction.
Cost is about the installed system
It is tempting to compare ducting by the price of a single length of pipe or a single rectangular section. That rarely gives a useful project cost. Spiral duct may be economical on long standard runs because standard components reduce fabrication time. Rectangular duct may cost more per section where it requires bespoke manufacture, flanges and complex transitions, but it can prevent costly building alterations by fitting within a restricted space.
Installation labour also matters. Standard spiral components can be quick to assemble on an accessible route. Conversely, a large circular duct may be difficult to lift through a crowded ceiling void, while a rectangular section can be fabricated in manageable lengths to suit site access. The practical route from delivery point to final position should be considered before placing an order.
Material selection is another cost and durability decision. Standard ducting is commonly galvanised steel, while stainless steel may be appropriate where the environment, cleaning regime or specification calls for it. For associated canopy and stainless fabrication, confirm whether standard 430-grade material is suitable or whether 304-grade stainless is required for the application.
Specify the route before choosing the profile
Start with the equipment being extracted, the required canopy or pickup position, the fan location and the discharge point. Then map the route, including structural beams, ceiling depth, access for installation, access for cleaning and space for silencers or treatment equipment. Once those constraints are visible, the duct profile usually becomes much easier to choose.
For many projects, the most practical answer is not spiral or rectangular throughout. It is a properly sized combination: rectangular where a canopy connection or tight void demands it, spiral where a standard circular run keeps the route efficient and straightforward. CanopyMan manufactures made-to-measure extraction canopies and stainless fabrications in Birmingham, and can supply the ducting components needed to build out a considered system.
A well-planned duct route pays back long after installation. Give the same attention to access, transitions and discharge as you give to the canopy and fan, and the system will be easier to fit, easier to maintain and better placed to perform as intended.