Commercial Kitchen Exhaust Fan with grease all over the roof
This picture is from a facility that was abandoned a while back. You see grease here on the roof either because the Kitchen hood did not have any grease filters, or the personnel just removed the filters and operated the hood without them. People don't like grease filters because they sometimes catch fire. But they catch fire because you don't periodically wash them and one day when the heat is really high the grease collected on the filters starts burning.
Sometimes grease receptacles are provided at the fan, on the roof. But I am not a big 'fan' of that scheme. Out of sight, out of mind grease receptacle is seldom attended to.
A good kitchen exhaust system consists of grease hood with filters, grease receptacle, and fire suppression system; all this with an upblast fan on the roof. But you still need to periodically empty the grease receptacle, wash the filters, and check the fan on the roof.