In this video, This Old House general contractor Tom Silva shows how to properly install a roof-mounted bath-fan vent.
1. From inside the attic, drive a nail through the roof directly above the bathroom vent fan. Center the nail between two rafters.
2. From the roof, find the nail and use the utility knife to cut roof shingles from around the nail.
3. Slide a flat pry bar underneath the shingles around the nail to break the weather seal.
4. Use 4¼-inch-diameter hole saw to drill through the roof, centered on the nail hole.
5. Crimp the end of a sheet metal sleeve. Slide the sleeve into the 4-inch-diameter insulated duct. Secure sleeve to duct with foil duct tape.
6. Remove the connector fitting from the bath vent fan and attach it to the opposite end of the insulated duct. Secure the fitting with foil duct tape.
7. Use a nylon zip tie to securely hold the insulation to each end of the duct.
8. Take the end of the duct with the connector fitting and attach it to bath vent fan.
9. Pass the opposite end of the duct through the hole in roof.
10. Next, use aviation snips to cut slits into the sheet metal sleeve. Bend over the tabs, then nail the tabs to the roof.
11. Run a thick bead of tri-polymer sealant along the underside of vent hood. Apply sealant to the sides and top edge, but not along bottom edge of the hood.
12. Slide the vent hood under the roof shingles, centered on the duct, and press down flat.
13. Lift shingles along the side of the vent hood and nail through the hood's flange with roofing nails.
14. Apply sealant under the shingles to each side of the vent hood, then press them down flat.
15. Lift the shingles above the vent hood and nail through the flange along the top of hood.
16. Run bead of sealant under the shingles along top of vent hood, then press them down.