The best kitchen extract route is usually the shortest practicable grease-tight route to a suitable outdoor discharge, but it must also preserve fire safety, structural integrity, cleaning access and planning acceptability.
Installation boundary: This article explains design and coordination principles. Drainage, ventilation, fire, electrical and fabrication work must be designed and carried out by competent people using current project documents, manufacturer instructions and the requirements that apply in the relevant UK nation.
The short answer
Begin at the canopy discharge spigot and work to an agreed terminal using the fewest necessary changes of direction. Do not finalise the canopy, fan or steelwork until the route has been surveyed through ceilings, risers and roof zones. Every bend adds resistance and a grease-collection area; every compartment crossing creates a fire-strategy decision; every hidden section needs a way to inspect and clean it. A route that is short on plan can be unbuildable in section.
Route planning is a three-dimensional survey
A reflected ceiling plan does not show beam depths, transfer structures, sprinkler mains, cable trays, historic joists or roof falls. Survey the route in plan and elevation, record the available opening at each pinch point and reserve space for insulation, fire protection, supports and access doors—not only the bare duct dimensions. Where the route passes a landlord or neighbour's area, establish legal access and future cleaning rights before design approval.
The duct cross-section can change, but transitions need enough length to avoid abrupt turbulence and must not create an internal shelf that collects grease. Turning vanes or internal features need careful consideration because anything within a grease extract must remain cleanable. The fan selection must be based on the final resistance of the complete system, including filters, attenuators and odour-control equipment, rather than a straight-duct estimate.
The terminal position drives the whole design
A convenient external wall grille may be unacceptable if it discharges near windows, air intakes or a courtyard. Planning and environmental-health submissions often require roof plans and elevations showing the complete route and vertical discharge. Establish the terminal first, then test whether the internal route can reach it without compromising fire compartments or maintenance.
The route should be reviewed alongside planning permission for kitchen extraction and the extraction quotation checklist. If the quotation excludes builder's work, structural openings, roof weathering or fire protection, the cheapest duct price is not the project cost.
What common failures reveal
| Site symptom | Likely installation issue | Practical consequence |
|---|---|---|
| Duct clashes with a beam on site | Route designed from a 2D plan without intrusive survey | Late offsets, reduced duct area and programme delay |
| Fan cannot achieve design flow | Extra bends, filters or undersized transitions added after selection | Poor capture and gas-safety implications |
| Grease collects at a transition | Abrupt shape change or internal ledge | Fire load, leakage and cleaning difficulty |
| External discharge causes complaints | Terminal chosen for convenience rather than dispersion | Odour nuisance and enforcement risk |
| No contractor can clean the riser | Route has no physical access or legal access agreement | System cannot be maintained as designed |
Installation and coordination checklist
- Fix an acceptable terminal location before freezing the internal route.
- Survey structure, services, ceiling voids, risers and roof build-up in three dimensions.
- Reserve space for insulation, fire protection, supports and cleaning doors.
- Minimise bends and use smooth, cleanable transitions without grease shelves.
- Recalculate fan duty from the final route and all treatment components.
- Identify every fire compartment, escape route and landlord boundary crossed.
- Show builders' openings, weathering and external appearance on coordinated drawings.
- Confirm a safe physical and legal route for inspection and cleaning.
Commissioning and handover
The as-built drawing should record duct dimensions, route levels, bends, transitions, access doors, fire-resisting sections, supports, fan and treatment plant. Measure and record the commissioned airflow and pressure readings against the design duty. Photograph concealed sections before ceilings close, and provide the cleaning contractor with the actual route rather than the tender drawing.
Questions to ask before approving the work
- Has the whole route been physically surveyed?
- Where are the worst resistance and grease-collection points?
- Which fire compartments and third-party areas are crossed?
- Can every internal surface be reached for cleaning?
- Is the fan selected from the final as-built resistance?
Capture the installation detail before it becomes a site problem
Capture the service positions, levels, route constraints and access risks before equipment or fabrication is ordered.
Technical sources and further reading
- HSE CAIS10: Ventilation of kitchens in catering establishments
- Approved Document B, Volume 2: Buildings other than dwellings
- Spelthorne Borough Council: Planning guidance for kitchen extraction systems
Last technical review: 14 August 2026. Always check the latest manufacturer instructions, statutory guidance, standards and project approvals before installation.