First off its code!
NFPA 96/ 184.108.40.206(8) reads in part – A hinged up-blast fan supplied with flexible weatherproof electrical cable and service hold-open retainer to permit inspection and cleaning that is listed for commercial cooking equipment with the following conditions
(a) Where the fan attaches to the duct-work, the duct-work shall be a minimum of 0.46m (18in.) away from any roof surface.
(b) The fan shall discharge a minimum of 1.02m (40in.) away from any roof surface.
Secondly if it doesn’t have hinges & hold-open retainers the fan will need to be placed on the roof. The sharp unfinished metal corners of the fan’s base can easily cause roof damage. The fan’s base will also become deformed, bent disfigured as well has the bowl from laying it on the roof.
Grease running on the roof from damaged kitchen exhaust fans is not a rare problem amongst restaurants. If there’s no containment or it’s installed improperly then grease will be on the roof and create some very costly damage. The investment in grease containment can not be overstated. Are you willing to protect your investment from thousands if not ten’s of thousands worth of damage to your roof and keep you in compliance with fire codes.