How to install a bathroom fan in a soffit

Updated February 21, 2017

Building code restrictions require that bathrooms have either a window or vent fan. Vent fans remove excess moisture from bathrooms, which are high-humidity areas because of hot showers and sinks. Without a vent, moisture absorbs into the wall and ceiling materials, leading to deterioration or mould and mildew growth, which causes breathing problems for residents. You'll need one CFM of airflow per square foot of bathroom space. Installing the vent to run to a house's soffit--the overhanging portion of the roof--conceals the vent, eliminating the need to install a vent to the side of the house.

Unscrew the blower motor from the fan housing and unplug the electrical connection. A new bathroom vent comes with the blower motor intact, but you must remove it to provide access for installing the fan housing.

Feed a cable connector through the knockout plug hole on the side of the fan housing box and secure the nut tightly with a pair of pliers.

Position the fan so it points toward the desired exhaust location on the soffit. Locating the fan near the tub or shower makes it more effective at removing steam from the bathroom.

Screw the fan housing to the ceiling joists through the screw holes on the fan housing.

Position the metal brackets of the fan housing onto the ceiling joists so that the fan housing is level, then secure the brackets to the joists with wood screws.

Secure 4-inch flex duct over the fan exhaust port with foil duct tape. Foil duct tape is very sticky, so make sure you have it properly positioned before securing to the duct.

Stretch the flexible duct over to the desired vent location in the soffit, adding a plastic duct hanger strap every 2 feet to hold the duct in place. The straps nail directly into nearby joists.

Secure the end of the flex duct to a 4-inch metal 90-degree elbow fitting with foil duct tape and position it so it points down into the soffit.

Push the electrical cable through the connector and tighten the screws.

Tie the fan wiring in with the electrical cable. Read the label instructions carefully for specific details, or hire a professional electrician for this step.

Plug in the blower motor, put the blower motor inside the fan housing and screw it into place.


Cutting the soffit is not necessary because the moisture and air escapes through the tiny holes in the soffit.

Things You'll Need

  • Screwdriver
  • Electrical cable connector
  • Slip groove pliers
  • Wood screws
  • Power drill driver
  • Torpedo level
  • 4-inch flex duct
  • Foil duct tape
  • Plastic duct hanger strap
  • 4-inch metal 90-degree elbow fitting
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About the Author

A former cake decorator and competitive horticulturist, Amelia Allonsy is most at home in the kitchen or with her hands in the dirt. She received her Bachelor's degree from West Virginia University. Her work has been published in the San Francisco Chronicle and on other websites.