Low Ceiling, Big Hood: Designing a Kitchen Exhaust System When Headroom Is Tight

Pictured above: the underside of a servery counter at one of our projects (Heritage, Yishun) — heat lamps, shelving and cabling all competing for the same shallow space. It is not a hood cavity, but it is the same problem in miniature: fit-out services stacking up in a void that was never as deep as the drawing suggested.

Every kitchen exhaust hood on a supplier's spec sheet is drawn against a wall that has nothing else on it. Real ceilings do not work that way. By the time you get the keys to a unit — mall, shophouse, coffeeshop, or an industrial space converted to F&B — the void above the false ceiling is usually already carrying someone else's aircon ducting, sprinkler pipework, electrical trunking and the landlord's own services. What's left after all of that is your actual headroom, and it is very often less than the equipment catalogue assumed.

A kitchen exhaust system often has to be designed into that leftover space rather than into an empty shell. Here is what actually eats the height, what happens when nobody checks it before ordering a hood, and what to ask before you sign a lease or commit to equipment.

The clear height on the drawing is rarely the clear height on site

An as-built drawing tells you what was there when the building was handed over. It does not tell you what every tenant since has added to the ceiling void — a new aircon run for the last tenant's shop, extra sprinkler branches, cabling for signage, someone else's ducting routed through on its way to a shared riser. Every one of those takes height, and none of them show up until you are standing under the ceiling tile with a tape measure.

This is why we measure on site rather than working from a drawing alone. The number that matters is the real clear height from the highest point your duct can occupy down to where the hood needs to sit above head height for the people working the line — not a figure taken off a floor plan from years ago.

What headroom actually has to fit

A kitchen exhaust duct is not just a pipe. In the space above your hood, headroom has to accommodate:

• The duct itself, sized to carry the airflow the hood needs — see how we size a hood to the cooking line.
• The pitch (fall) along the duct run that lets condensed grease drain back towards the hood instead of pooling in a low point — a duct forced flat by low headroom loses this, and a duct that cannot drain fills with grease faster than it should.
• Access panels at intervals along the run, for our cleaning crews to physically reach inside the duct. A route with no room for panels is a route nobody can clean properly, whatever the cleaning contract says.
• Fire-rated enclosure, where the duct crosses into a different fire compartment — see fire-rated ductwork — which adds real thickness around the duct, not just around the hood.
• Everyone else's services that were there first: aircon ducting, sprinkler branches, electrical trunking, other tenants' exhaust.

Stack all of that against a shallow void and the honest answer is sometimes that the straight-line route on the kitchen designer's drawing does not physically exist.

What happens when nobody checks before the hood is ordered

A common sequence: a hood is ordered to a catalogue spec — correctly sized for the cooking line — with no site check on what is above it. Two things tend to happen next.

The first is that the duct route gets forced into more bends and offsets than the original design allowed for, working around beams, other services and any fire-rated sections, to reach the discharge point within the available height. Every extra bend adds resistance the fan has to overcome — we go through why in how ductwork routing affects exhaust performance. A hood that was correctly sized for the cooking line can still end up underperforming, because the route it was forced into afterwards was never accounted for in the fan selection.

The second is that the hood itself gets pushed down to make the route work, eating into the actual working headroom for the staff standing under it. That is not a paperwork problem — it is someone ducking under a hot canopy on every shift, for as long as the kitchen operates.

Both are avoidable. Neither is fixed cheaply once the ductwork is fabricated and the ceiling is closed up.

"Just use a smaller duct" does not solve it

When headroom is tight, the obvious-sounding fix is to shrink the duct so it fits under whatever is in the way. It rarely works out as intended. A duct undersized for the airflow the hood needs increases velocity through it, which increases the resistance the fan has to overcome — the same physics that makes extra bends costly. Push it far enough and the fan cannot move enough air to keep the hood capturing properly, no matter how strong the motor rating looks on paper. The duct has to be sized to the airflow first; the route is then designed to fit that duct into the space available, not the other way round.

Coordinating with other trades above the ceiling

On a fit-out with a tight void, the exhaust duct is rarely the only thing trying to get through the same gap. Aircon, sprinkler and electrical contractors are usually working the same ceiling on the same timeline, and whoever finalises their route first effectively claims the space. We coordinate directly with the other trades on site rather than designing in isolation and finding out at installation that someone else's ducting has already taken the path we needed. Where that coordination happens early — ideally before any of the services are fixed in place — everyone's routing gets easier, including ours.

What we do on site when headroom is tight

When we survey a unit with limited headroom, the sequence is the same regardless of how the space looks on paper:

1. Measure the real clear height, at several points along the likely route, not just under the hood position.
2. Find out what is already in the void — from the landlord's M&E drawings where they exist, and by opening a ceiling tile and looking where they do not.
3. Route around what is already there, choosing a path with fewer forced bends over a shorter one with more, because bends cost more in performance than length does.
4. Fabricate to the actual constraint, not a standard size — we build our own duct sections and transitions in-house rather than adapting off-the-shelf pieces to an awkward void.
5. Confirm the fan against the route we actually built, not the route on the original drawing, sometimes backed by air balancing at commissioning to prove it is moving air the way it was designed to.

A tight void does not have to mean a system that underperforms. It means the design has to respond to the real space instead of the drawing.

The shophouse version of this problem

Conservation shophouses take this to its extreme — no false ceiling void at all in some units, just the underside of the original roof structure, and conservation guidelines limiting what can change on the outside. We wrote about that specific case in kitchen exhaust in a conservation shophouse. A tight mall unit and a shophouse are different buildings asking the same underlying question: how much real headroom is actually available, and what route does the duct have to take to work within it.

Before you sign the lease or order the hood

1. Measure the clear height yourself, floor to the underside of the structural slab or the lowest service crossing it — not to the ceiling tile.
2. Ask the landlord for as-built M&E drawings of what already runs through that ceiling void, and treat them as a starting point, not the full picture.
3. Confirm the discharge point and the likely route before you commit to a hood size or position.
4. Check whether the route crosses into another fire compartment, which adds enclosure thickness to plan for.
5. Get the headroom checked against the actual hood and duct, not a generic spec sheet, before you order anything.

The short version

• The clear height on an old drawing is rarely the real clear height today — other tenants' services accumulate in the void over time.
• Headroom has to fit the duct, its drainage pitch, access panels, any fire-rated enclosure and everyone else's services already there — not just the hood.
• A hood ordered without a site headroom check often forces extra duct bends later, which quietly costs performance, or forces the hood down into the staff's working space.
• Shrinking the duct to fit is not a free fix — an undersized duct adds resistance of its own.
• We measure on site, coordinate with the other trades in the ceiling, route around what is already there, and fabricate to the actual constraint rather than a standard size.
• Confirm headroom before you sign the lease or order the equipment, not after.

Working with a unit where the ceiling void looks too shallow for what you need? WhatsApp BC Air on 8989 2833 with your location and the clear height you are seeing, and we will tell you what is realistic before you commit to a hood. Full design and build scope is on the design and build page, and CFD modelling and air balancing on the CFD and air balancing page. Scope and pricing follow a site survey.

Related reading: how we size a kitchen exhaust hood · how ductwork routing affects exhaust performance · kitchen exhaust in a conservation shophouse