Commercial kitchen extract ductwork carries grease-laden air and may face a fire originating inside the duct. Its route, compartment crossings, construction, supports, access doors and cleaning arrangements therefore need to be considered together.
Why kitchen extract is different
Cooking processes can produce grease, heat, smoke and products of combustion. Deposits that accumulate within the extract route increase the importance of internal-fire performance, grease-tight construction, cleaning access and an effective maintenance regime.
- The extract should be separate and independent from ventilation serving other parts of the building.
- Extracted air should not be recirculated.
- The route should be kept as direct and accessible as the project permits.
- The fire strategy should address fire exposure both outside and inside the duct where relevant.
- Cleaning and inspection must remain practicable after the surrounding building services are installed.
When is fire-resisting ductwork relevant?
If the extract can discharge directly to atmosphere while remaining within the kitchen fire compartment, the fire-resisting route may differ from a system that passes through another compartment, shaft or protected escape route. Where compartmentation is crossed, the fire performance of the separating construction must be maintained.
Approved Document B for England identifies fire-resisting enclosures and fire-resisting ductwork as possible methods. The selected construction must be supported by appropriate test, classification and field-of-application evidence for the proposed route.
Why a conventional fire damper is not the answer
Approved Document B says thermally activated fire dampers and automatically activated fire-and-smoke dampers should not be used as the compartmentation method for extract ductwork serving kitchens. Grease accumulation within the duct can adversely affect damper operation. BESA DW145:2024 likewise warns against fitting fire dampers in kitchen extract systems and other pollutant-carrying systems where deposits may obstruct their action.
This restriction concerns the kitchen extract route. Dampers on a kitchen supply-air system are a separate design question and must be assessed from the fire strategy, compartmentation and applicable product evidence.
The standard and evidence route
For new EN-based specifications, BS EN 1366-1 is the relevant fire-resistance test route for ventilation ducts. The resulting classification and system evidence must cover the proposed function, duration, exposure direction, orientation, dimensions, pressure, construction, supports, penetrations and access openings.
Legacy specifications may refer to BS 476-24 Type A and Type B exposure. These descriptions should not be treated as an automatic EN classification or as a complete project specification. Existing reports need to be checked on their own terms and against the regulatory route applicable to the project.
Construction and grease control
- Confirm the required duct material, gauge, jointing and grease-tightness.
- Avoid flexible ductwork in the kitchen extract route.
- Avoid internal linings or features that trap grease or prevent cleaning unless specifically supported.
- Coordinate drainage, falls and discharge details where the design requires them.
- Check any protective board, wrap or insulation as part of the complete tested construction.
Access doors and cleaning
The full internal surface should be capable of inspection and cleaning. Access locations should be coordinated with bends, branches, risers, changes of direction and equipment, while remaining physically usable after ceilings and other services are complete.
In fire-resisting ductwork, an access door is part of the fire-resisting system. Its construction, size, location, orientation, frame, fasteners, sealing and protection must remain within the applicable system evidence. A separately rated door does not automatically preserve the duct classification after an opening is cut.
Kitchen grease inspection and cleaning requirements should be checked against TR19 Grease and DW/172. TR19 Air is not the kitchen grease-extract cleaning specification. Access panels must be sufficient in number, size and quality, unobstructed in use and suitable for the duct construction. See our Access Doors and Maintainability guide.
Grease-risk management and cleaning evidence
A cleaning interval should follow the responsible person’s fire-risk assessment and the observed rate of grease accumulation, rather than an assumed universal timetable. Inspection and cleaning records should identify the system, accessible and inaccessible areas, representative pre- and post-clean observations or measurements, defects, photographs and recommended corrective action.
Adding or changing an access panel in fire-resisting ductwork must not compromise the tested construction. The relevant system manufacturer or competent fire-protection specialist should confirm the permitted detail and any evidence or re-certification needed.
Odour, discharge and planning
Fire safety is only one part of a commercial kitchen extract design. The cooking type and intensity, nearby receptors, discharge location, stack dispersion, particulate removal, odour-control equipment, fan noise and maintenance access may all affect planning and environmental-health acceptance.
Particulate grease and smoke should be addressed before gaseous odour treatment. The appropriate combination of filtration, treatment and discharge arrangement is project-specific and should be checked against current planning conditions, local-authority requirements and specialist design advice. Odour-control equipment also needs safe access and a documented maintenance regime.
Supports, penetrations and interfaces
Fire performance depends on the complete installation. Support channels, rods, fixings, spacing, proximity to joints, vertical restraint, wall or floor construction, penetration sealing and clearance from combustible materials must be checked against the evidence and project design.
Solid-fuel cooking and fire suppression
Solid-fuel appliances and high-risk cooking processes can introduce additional hazards. The project may require specialist consideration of appliance isolation, fire suppression, extract-fan operation, make-up air, gas or energy interlocks and the fire-alarm cause-and-effect strategy. These matters sit alongside, rather than replace, the ductwork fire-resistance assessment.
Kitchen extract project checklist
- Kitchen and appliances: identify the cooking processes, fuels and higher-risk appliances.
- Route: confirm whether the duct discharges directly or crosses another compartment, shaft or protected route.
- Required performance: establish the fire-resistance duration and applicable integrity and insulation criteria.
- Exposure: confirm the required internal and external fire exposure evidence.
- Duct details: record dimensions, orientation, pressure, material, joints and grease-tight construction.
- System evidence: obtain the test, classification and field-of-application documentation.
- Supports and penetrations: compare the proposed arrangement with the evidenced installation.
- Access and cleaning: provide an accessible door schedule, cleaning-risk assessment and evidence-recording strategy.
- Odour and discharge: record the cooking intensity, nearby receptors, proposed filtration or treatment, discharge position and maintenance access.
- Controls: coordinate fire suppression, fan operation, alarms and energy interlocks where applicable.
Source basis: DW/172 Second Edition 2018, TR19 Grease (supplied July 2019 edition), EMAQ+ Control of Odour and Noise from Commercial Kitchen Exhaust Systems (2018), BESA DW145 Second Edition (November 2024), BS EN 1366-1:2014+A1:2020 and Approved Document B Volume 2 for England. The May 2024 DW/172 Appendix D addendum has not yet been reviewed for this page. This is an original practical overview and does not reproduce or replace the publications, system evidence, fire strategy, planning requirements or competent professional advice.