Three duct types
The extract fan on the roof is the last part of the system and the first to be forgotten. A fouled fan drags down the airflow the whole duct depends on - yet cleaning often stops at the ductwork and leaves it.
The short answer
Every extract system ends at a fan, and on most commercial buildings that fan sits on the roof. It is the last component in the run, the one out of sight and out of reach, and it is routinely left out when the ductwork gets cleaned. That is a mistake, because the fan is what drives the whole system - a fouled roof fan drags down the airflow every other part depends on, so cleaning the duct and skipping the fan only does half the job.
Why the roof fan gets ignored
The roof fan is ignored for simple, practical reasons. It is at height, so reaching it means roof access and working-at-height precautions that the ductwork inside the building does not. It is at the far end of the run, the last thing on the list. And it is genuinely out of sight - nobody in the building sees it, so nobody thinks about it. The result is that a clean often runs along the accessible ductwork and stops where the duct meets the fan, leaving the dirtiest, most important component untouched.
Yet the fan collects exactly what the system carries. In a grease extract the impeller and housing load up with grease; in a general or laundry extract they gather dust and lint. Whatever the run carries ends up caked on the fan that is trying to move it.
Why a fouled fan matters
The fan sets the airflow for the whole system, so its condition matters more than any single length of duct. As grease, dust or lint builds on the impeller blades and inside the housing, the fan moves less air, works harder and runs hotter. That drop in airflow is felt everywhere upstream: the canopy or grilles pull less, the ductwork clears more slowly, and in a grease system a fouled fan is itself a concentrated fire load at roof level. A build-up on the blades can also unbalance the fan, adding vibration and wear that shortens its life.
So a clean that leaves the fan dirty never really restores the system. The ductwork might be spotless, but the component that determines how well the whole thing breathes is still choked.
Cleaning the fan as part of the system
Cleaning the roof fan means treating it as part of the run, not an optional extra. With the right access and working-at-height provision, the fan is isolated, and the impeller, housing and blades are cleared of the grease, dust or lint they have collected, so it moves air freely again and runs balanced. Done alongside the ductwork, it means the whole system - from the intake through the duct to the fan that drives it - is actually restored, not just the parts that were easy to reach. If a clean has quoted only the ductwork, the fan is the question worth asking about.
Questions
Because it is at height and needs roof access, it is the last thing at the far end of the run, and it is out of sight. Cleaning often runs along the accessible ductwork and stops where the duct meets the fan.
The fan sets the airflow for the whole system. As grease, dust or lint builds on the impeller and housing it moves less air, so everything upstream pulls less - and in a grease system it is a fire load at roof level.
Whatever the run carries. A grease extract cakes the impeller and housing with grease; a general or laundry extract gathers dust and lint. It is often the dirtiest single component.
Yes. Build-up on the blades can unbalance the fan, adding vibration and wear that makes it work harder, run hotter and fail sooner.
As part of the run - with proper roof access and working-at-height provision, the fan is isolated and the impeller, housing and blades are cleared so it moves air freely and runs balanced again.
Phoenix Duct Clean · by the numbers
The roof fan is part of the system and needs cleaning too. We reach it, clear the impeller and housing, and restore the airflow. Call to include it in the job.