Duct termination height for a takeaway extraction system

Extraction duct termination height is the detail that decides most hot food takeaway applications. Filtration matters, but a well filtered discharge at first floor level next to a bedroom window will generate complaints, while a modest system terminating a metre above the ridge with a vertical discharge usually will not. Environmental health officers know this, which is why the condition they write is almost always about where the duct ends rather than what is inside it.

If your drawings show the flue as a dashed line up the rear elevation and stop there, expect a request for further information before the application is even validated.

Why the discharge point matters more than the filtration

Odour control reduces the concentration leaving the system. Dispersion reduces the concentration reaching somebody's nose. The second is usually doing more work than the first.

A discharge released above the roofline into moving air dilutes quickly. The same discharge released at low level, in a courtyard, or under a canopy sits in a pocket of still air and finds its way into open windows and along the ground. No realistic amount of carbon will fix a badly placed termination, and officers have seen enough of them to be sceptical of any application that offers filtration as the answer to a poor position.

The logic runs the other way too. Where the termination is well placed, the level of abatement needed falls, and the capital cost of the system with it.

The usual expectation relative to roof level

The accepted starting point in England, which comes from the long standing national guidance on odour and noise from commercial kitchen exhaust systems, is that the duct should terminate at roof level, discharging vertically, at least a metre above the ridge of the building it serves and clear of the roofs of adjoining buildings.

Two qualifications follow. Terminating above the eaves rather than the ridge is sometimes accepted where the ridge is remote from the kitchen and the surrounding buildings are lower, but it needs justifying rather than assuming. And a cowl or a weather cap that deflects the discharge downwards defeats the point of the height, so the termination detail matters as much as the height itself.

Discharge velocity is part of the same design. A velocity in the region of 10 to 15 metres per second is commonly specified to carry the plume clear of the building, and a fan sized for pressure drop through a new carbon filter will deliver a very different velocity once that filter is loaded. Ask for the velocity at the design condition and at the filter change point.

Dispersion and nearby openings

The separation from openings is the point most often missed on terraced high streets, where the flue runs up a rear elevation past the windows of flats above and beside the unit.

Check the elevations of every building within reach, not just your own. A termination a metre above your two storey ridge is below the second floor windows of a three storey neighbour, and that is a refusal waiting to happen. Where the surrounding buildings are taller, the termination has to rise with them or the use has to change.

Air intakes deserve separate attention. A mechanical ventilation intake for flats above, or a fresh air intake for a neighbouring premises, will draw the plume in directly. The position of those intakes belongs on the drawing.

Where the odour risk is material, the assessment approach and the factors officers weigh are set out in the note on odour assessment and the FIDOL factors, and the selection of the abatement itself is covered in the note on extract termination height and odour control selection.

Routing through a building you do not own

This is the practical obstacle that stops more schemes than policy does. A first floor flat in separate ownership sits between your kitchen and the roof, and the duct has to pass it.

Three separate permissions are in play and none of them is planning. You need the right under your lease or a deed of easement to run the duct through or against another party's property. You need the freeholder's consent for a flue fixed to the building. And the installation has to satisfy building regulations, in particular fire separation where a duct passes through compartment walls or floors, because a grease laden kitchen extract duct cannot generally be protected with a fire damper and instead needs a fire rated enclosure for its full route.

Do that legal work before submitting. A permission for a flue you have no right to install is worth nothing, and landlords are slow.

An external route is often the answer on a mid terrace. It has its own consequences: a large stainless duct on a rear elevation in a conservation area is a design issue, and on a listed building it needs listed building consent, which is a separate application with a different test.

Fan noise and attenuator selection

Extraction systems fail on noise as often as on odour, and the two solutions pull against each other. Adding filtration raises the pressure drop, which means a harder working fan and more noise.

What usually appears in a workable specification: the fan selected for the actual system resistance rather than the free blowing duty, an attenuator sized for the dominant frequencies rather than a generic length, anti-vibration mounts and flexible connections at the fan, a rigid well supported duct to avoid drumming, and where the fan sits within the building an acoustically treated plant space.

The assessment itself follows the standard method for industrial and commercial sound, with a rating level compared against the measured background, and the character corrections matter because fan noise is frequently tonal. Where the application needs it, a Noise Impact Assessment should set out the plant data, the receptor positions and the mitigation, and it is a document councils expect from a suitably qualified author.

Access for cleaning and maintenance

Grease accumulation is a fire risk and a performance problem, and both are planning relevant because a system that is not cleaned stops controlling odour.

Design in access hatches at the intervals the industry cleaning guidance expects, at bends and at fan positions, and make them reachable. A hatch 8 metres up a rear elevation with no safe access will not be opened. Where the duct runs internally through another party's property, the right of access for cleaning has to be in the legal documentation as well as on the drawing.

Officers increasingly condition a maintenance regime: filter changes at stated intervals, cleaning to a recognised standard with records kept on the premises and made available on request. Offer that in the application rather than waiting for it to be imposed, because the version you write will fit how the kitchen actually runs.

Evidencing the design in an application

A Ventilation and Extraction Statement should carry scaled elevation and section drawings showing the full duct route and the termination relative to your ridge and to the roofs, windows, balconies and air intakes of every nearby building. Alongside it: the kitchen type and cooking processes, the extract volume, the abatement proposed with the reasoning, fan and attenuator data, discharge velocity, the maintenance regime, and confirmation that the necessary property consents are in place or being obtained.

Getting the termination right at the outset changes the tone of the whole application. Enter your postcode on PlanScope to see which reports a change of use to a hot food takeaway is likely to need.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.