Fire, insurance & risk
Grease pools at the roof fan and discharge, hidden in the void. When it lights, fire spreads sideways above the ceiling and can take the roof - attacking the building from above.
The short answer
Grease pools at the roof fan and at low points near the discharge, in a concealed void above the ceiling that is the last place anyone cleans. When it ignites, fire spreads laterally through the void and can break out at the roof itself - attacking the building from above, past the wall and riser barriers the fire strategy relied on.
Why the void collects grease
Every internal surface of an extract system gathers grease, because no filter is one hundred per cent effective and vaporised fat carries right through to the fan and beyond. The end of the run - the vertical rise, the roof fan and the discharge to atmosphere - is where condensate and grease settle at low points and around the fan, and it is usually the hardest section to reach, hidden in a roof void, plant room or up an external wall.
That combination is the problem: the dirtiest part of the system is the part a canopy-only clean never touches. Grease even accumulates on the roof around the fan, damaging the membrane and adding an external fuel bed. A clean that stops at the canopy leaves the most concentrated fuel load exactly where it is least visible.
What ignition does up here
A duct fire that reaches this end has three ways to spread. Grease that has leaked from duct seams into the void can ignite the surrounding roof structure directly. A fire reaching the roof fan or discharge can set the roof itself alight and be fed by the grease pooled around the termination. And because the void is a large, interconnected concealed space above the ceiling, flame travels sideways through it - bypassing the building's compartmentation from above rather than through the wall and riser penetrations most people picture.
That lateral, hidden spread is what makes roof-void fires so dangerous: they are well advanced before anyone sees them, and they can cross the footprint of the building over the heads of the people below.
Controlling the discharge end
The control is to clean the whole run to atmosphere - fan and discharge included - not to stop at the visible canopy, and to make sure there is enough access to the void and fan to inspect and clean them properly. At the termination, grease containment or collection at the roof - closed, non-combustible and drained from low points - keeps discharge grease off the roof surface. Where the duct leaves the kitchen compartment it should be fire-rated, so a fire in the run is held even as it travels; the section inside the kitchen compartment need not be, but everything beyond it does the load-bearing work of protecting the rest of the building.
In short, the roof void is where a neglected extract system does its worst damage - so it is precisely the part that must be reached, cleaned and kept accessible.
Questions
Because grease pools at the roof fan and low points near the discharge, in a concealed void that is the hardest part to reach - so it is often the dirtiest and least cleaned, yet holds the most concentrated fuel.
Three ways: grease leaked from duct seams ignites the roof structure, a fire at the roof fan or discharge sets the roof alight, and flame travels sideways through the interconnected void above the ceiling - bypassing wall and riser barriers from above.
No. A canopy-only clean leaves the vertical rise, roof fan and discharge untouched, which is exactly where grease concentrates. TR19 requires the whole run to atmosphere to be cleaned.
Even maintained systems discharge some vaporised grease, which settles around the fan and on the roof membrane. It damages the roof and adds an external fuel bed that can feed a duct flare-up out onto the roof.
Clean the complete run to atmosphere, keep enough access to the void and fan, contain grease at the rooftop termination, and ensure the duct is fire-rated where it leaves the kitchen compartment.
Phoenix Duct Clean · by the numbers
A canopy-only clean leaves the most dangerous grease where you can't see it. We clean to atmosphere, fan and discharge included.