Venting Two Bathroom Fans With One Roof Vent

Updated April 17, 2017

Bathroom fans vent warm, moist air from the bathroom to the outdoors to prevent mould, mildew and the destruction of drywall. Sometimes in the course of an installation, it becomes necessary to vent two fans (either from a large bathroom or two separate bathrooms) from the same roof vent. Presuming you have already installed the fans, you are ready to work on the ventilation system.

Determine the location of the roof vent and cut a hole for its installation. You can install a vent either in the sofitt or on the roof itself. Secure the vent using the manufacturer supplied mounting hardware. Install the rain cap, caulk around the installation, and install metal flashing around the edge of the vent to prevent leakage.

Locate each bathroom vent in the attic and visualise the best run to the roof vent for the duct work. Find as straight and short a shot that you can, as the longer the run is, the less efficient your fans will perform.

Install the duct work from each fan running toward the roof vent. Attach a "T" or "Y" section of duct work to the roof vent and then tie the fan runs into this.

Turn the fans on and measure the air velocity in each duct to make sure they are drawing equivalent amounts. You may need to install a baffle system in one or both ducts to ensure proper air pressure.


Have a friend or spouse help you move and install duct work. Ducts are not heavy, but are bulky and can be expensive, so drop them as little as possible.


Wear safety glasses and gloves while cutting holes in or trimming the duct work. The metal can be extremely sharp.

Things You'll Need

  • Galvanised duct work
  • Hand tools
  • Jigsaw
  • Air velocity meter
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