Installing a Range Hood (or Why I Decided to Cut a Hole in my Roof)

My range hood finally died. I had been waiting for it to happen since we moved in. The hood was from the '70's and despite a white coat of paint to cover the awesome brown color, I still didn't like it and it didn't work well.

Before removal, I flipped the circuit breaker and confirmed the hood was not live by testing the light. Upon removal, I discovered that it was not vented properly. The first indication was the insulation that not only fell out of the opening, but was also stuck in the vent opening. I guess that explains why it never worked well.

Installed roof cap.

Installed range hood, now properly vented too.

Per the building code, a range hood must be vented to the exterior (unless it recirculates). A hood vented to the outside will perform better than a recirculating hood.
The cabinet had a vertical void built in for a duct, but no duct was installed inside. The ducting for the range hood must be metal. Do not use flexible duct. Refer to the hood's instruction manual for duct size recommendations.

Ducting, 3.25"x10". This hood can also accept 8" round.

Since I now had a new hood, but no ducting, I purchased 3.25"x10" ducting. This will attach to the opening in the cabinet with nails and run up through the roof. The range hood vent damper, which comes with the hood, will feed into the duct. Three 24" long sections of 3.25"x10" ducting will first be fed into the vertical opening. You may need more, depending on the location of the hood and size of the attic. The joints of the ducts will be taped to ensure they are air tight. Use foil tape as duct tape is not sufficient.

The hood damper installed. This hood provided adjustments for the damper location.

Two nails were installed at each end to secure the bottom of the duct.

Once the duct is fed through the vertical opening, I will use it as guide to mark an opening to cut in the roof. I had to remove the oven to gain enough clearance to feed the duct into the opening. Even then, due to the length of my duct, I had to feed part into the opening, and then attach the last piece of duct and tape the joints.

Use foil tape for the duct joints.

The roof hole will be .5" wider and longer to accommodate the duct. The duct will be trimmed 3/4" taller than the roof, per the roof cap instructions. Once the roof hole is cut, the bottom of the duct can be tacked in place. Then the excess ducting above the roof will be cut with a pneumatic saw.

Feed the ducting through the opening.

Use the ducting as a guide to mark the roof opening before lowering it back down.
Then use a drill to make a pilot hole and a jig saw to cut the opening.

To feed the duct through to the roof, one person was in the attic
and another in the kitchen

The duct in place before sealant at the roof opening.

The roof cap fits under the shingles. I purchased a metal roof cap for durability. It also includes a bird screen and damper. The ducting was trimmed above the roof  using a marker and piece of plywood as a guide. I used a pneumatic saw, but a hack saw would work too. Be careful while performing any work on the roof. Follow the roof cap installation instructions. For this cap, the duct cannot be taller than 3/4" above the roof. If the duct is trimmed taller, the cap will not sit flat on the roof.

The top of the roof cap.

The bottom of the roof cap. The 3.25"x10" ducting can insert into the cap
parallel or perpendicular to the vent opening

The duct before trimming.

Once the top edge of the roof cap is placed under the shingles, the shingles are trimmed around the roof cap. I sealed around the opening in the roof for the duct. I put the roof cap in place after putting a bead of sealant around the bottom. The perimeter of the roof cap and nail heads also received a bead of sealant, and the shingles were caulked around the top and sides of the roof cap.

The roof cap in place. Roof cement was applied at the duct opening in the roof,
on the bottom of the cap, at the perimeter over each nail hole, and
where the shingles meet the cap.

Now that the ducting and roof cap are in place, the hood can be installed. Filler strips were installed underneath the cabinet for a stable connection. The strips are 1/2" deep and 2" wide. Screws 1" long were used to avoid penetrating the cabinet. The hood damper had to be shifted to fit the existing duct location. I had to drill new holes for the screws and I taped the void in the top of the hood with foil tape.

Filler strips made of 1/2" plywood, screwed to the cabinet.

The wiring on the new hood is on the opposite side from the original, so the wires had to be extended. Wire nuts were used to properly connect the wire. Any time you splice or extend the house wiring, you should also install a junction box that is attached to the structure to provide a suitable mechanical connection for the wires. A wire strain relief was installed in the hood. This was not done on the old one. I had to remove the old hood to access the wire splices which were above the hood. The new wire connections to the hood are made under the hood in the access panel.

This is the wire strain relief that will clamp the electrical wiring.

The hood is held in place by hand and the keyhole openings are marked on the wood filler strips with a pen. The screws that will hold the hood are pre-drilled and installed into the filler strips. As the hood was slid into place, the wiring was fed through the strain relief clamp at the same time. The strain relief was clamped down and the hood screws were tightened.Then the wiring connections from the house  to the hood were made.

Once the electrical connections were made, the circuit breaker was turned back on and everything tested. The light and fan work. I also confirmed the damper on the roof cap operated as it should.