Signs & diagnostics
A dark, greasy stain spreading across the roof around the extract fan is not weathering. It is fats, oils and grease leaking from the discharge - which means grease has travelled the full length of the run to reach the very end.
The short answer
When a greasy, dark stain spreads across the roof around the extract fan, it is fats, oils and grease leaking from the fan discharge and settling on the roof surface. Grease travels up through the hood, along the ductwork and out through the fan, and when it reaches the discharge in enough quantity to stain the roof, it has made it all the way to the very end of the system. So roof staining is one of the clearest signals that the whole run is carrying a grease load - because the grease has physically travelled its full length to get there.
Why the discharge stains the roof
The extract fan is the last point in the system, discharging the air - and whatever grease it still carries - to the outside. On a system that is under control, grease is largely captured and contained upstream, and rooftop terminations are meant to include provision to drain grease from any traps or low points into a proper collection container rather than let it wash across the roof. When staining appears anyway, it means grease is reaching the discharge faster than any containment can handle, and spilling out onto the roof around the fan.
The stain spreads because grease-laden air leaves the fan and condenses on the cooler roof surface nearby, building a residue that widens over time. It is unsightly, but more importantly it is diagnostic: for grease to be discharging in visible quantity, the filters, ductwork and fan upstream must all have loaded up enough to let it pass through the entire system. The stain on the roof is the outward evidence of an internal condition that runs the full length of the extract route.
What the stain tells you - and costs you
As a diagnostic, roof staining tells you the same thing a struggling fan does, from the other side: grease has reached the end of the run, so the whole system is loaded. It is a late-stage, whole-system signal rather than an early or local one - by the time grease is staining the roof, the ductwork behind it has been carrying a substantial load for some time. It belongs alongside the fan struggling and the canopy dripping as evidence of an advanced grease problem.
There is a building cost too. Grease on a roof can degrade the roof membrane over time, creates a slip hazard for anyone working up there, and can wash into drainage where it causes further problems. So the stain is worth acting on for two reasons at once - it flags a system that needs cleaning end to end, and it protects the roof from the grease that is currently accumulating on it.
What to do about it
Roof staining is a prompt to clean the whole extract route, because the grease has reached the end of it. Cleaning the fan discharge, the ductwork and the components feeding it removes the load that is reaching the roof, so the staining stops getting worse. Where rooftop grease containment is missing or overwhelmed, dealing with the underlying grease is what actually solves it - a clean system does not discharge grease onto the roof in the first place.
Questions
A dark, greasy residue spreading around the extract fan discharge is fats, oils and grease from the system - not weathering or general dirt. If it feels greasy, spreads from the fan outward and builds over time, it is grease reaching the end of the run and leaking onto the roof.
That grease has travelled the full length of the run to reach the discharge, so the whole system - filters, ductwork and fan - is carrying a substantial load. It is a late-stage, whole-system signal: by the time grease stains the roof, the concealed duct has been loaded for some time.
Yes. Grease can degrade the roof membrane over time, creates a slip hazard for anyone working on the roof, and can wash into drainage. So the staining is both a system signal and a building maintenance issue worth dealing with promptly.
Rooftop terminations are meant to include provision to drain grease from traps or low points into a proper collection container rather than let it spread across the roof. When staining appears, that containment is either missing or overwhelmed - which points back to too much grease reaching the discharge in the first place.
Cleaning the fan and the run feeding it removes the grease load that is reaching the roof, so the staining stops getting worse. A system that is clean end to end does not discharge grease onto the roof, so dealing with the underlying grease is the real fix - not just wiping the deck.
Phoenix Duct Clean · by the numbers
Roof staining means grease has reached the end of the run. We clean the fan and the whole route feeding it, so the discharge stops leaking and the system is clear end to end.