An exhaust fan sits atop a prefabricated and insulated curb by the mechanical contractor, requiring flashing and counter flashing by the general contractor. The sheet metal duct connects to the curb with screws, maintaining a twelve inch minimum height above the finished roof. The roof penetration is sealed with flexible waterproof mastic around the opening. A backdraft damper installs inside the duct drop, with an attenuating curb provided where noted on the schedule.
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