Removing old sheet metal air duct from an attic

[Stephen Tashiro]

I need to remove an old sheet metal air duct about 20 ft long that's over the ceiling of a hallway. I can get to it by working in the attic, but I'll have to cut it into pieces to get it out from under the ceiling joists. An alternative would be to tear out the ceiling of the hallway, including the 2x4's under the duct and take the duct out in one piece. I suppose that would make sense if the duct is replaced by a new sheet metal duct. However, from a DIY perspective, it would be simpler to replace the old duct by several flexible ducts.

What's the best tool and technique to use if I cut up the old duct? (A large section of the bottom of the duct rusted away from being under the swamp cooler. Cutting up the rest of it will require the same effort as cutting new sheet metal.)

[Lee Schierer]

I need to remove an old sheet metal air duct about 20 ft long that's over the ceiling of a hallway. I can get to it by working in the attic, but I'll have to cut it into pieces to get it out from under the ceiling joists. An alternative would be to tear out the ceiling of the hallway, including the 2x4's under the duct and take the duct out in one piece. I suppose that would make sense if the duct is replaced by a new sheet metal duct. However, from a DIY perspective, it would be simpler to replace the old duct by several flexible ducts.

What's the best tool and technique to use if I cut up the old duct? (A large section of the bottom of the duct rusted away from being under the swamp cooler. Cutting up the rest of it will require the same effort as cutting new sheet metal.)

I would use and oscillating multitool with a metal cutting blade. Be sure to wear ear protection and a dust mask. A rotary tool with a cut off wheel will throw a lot of sparks in an area where they could be dangerous.

[Tom M King]

Multitool (since you probably don't have a battery powed, handheld bandsaw) to start a cut, then tin snips with offset handles (I recommend Milwaukee), and power shears. Some are lined with fiberglass insulation. Can you just replace the part that needs replacing, where it's rusted?

[Matt Day]

I don’t know, not sure I’d want to hear the racket of a multitool cutting sheet metal in an attic for 20’ of cutting. I’d consider something like a metal shear like this one from HF:
https://m.harborfreight.com/14-gauge-4-amp-heavy-duty-metal-shears-62213.html

[Mike Cutler]

Pneumatic shears.
The Tin Knockers at work have some electric shears which are pretty cool.
There is probably a battery powered version available now.

[Brad Chenoweth]

Isn't the duct assembled in sections? Most large ducts are assembled in 5' sections, with drive clips.

[Roger Feeley]

My tool of choice would be a pneumatic nibbler. it runs on your air compressor. Just use a step drill to drill a starter hole and then the thing will go through the metal like butter. Like the multi-tool, it will be noisy but, in my view, a LOT safer.

$28

https://www.harborfreight.com/16-gauge-air-nibbler-46061.html

[Bruce Wrenn]

I would SKIP the nibbers as they leave a bunch of half moons of metal. I take each section apart at the drives, and then break it down using the electric shears from HF. At each joint, you will have two drives, and two "Pittsburg" locks. Knock the drive off, and the duct slips out of the Pittsburgs.

[John Terefenko]

If it does not have splines which you can take apart, I would knock a hole with a screwdriver and use snips. If there is toxic dust in there you do not want to be spreading that around with electric rotor snips. Also be careful tipping it to get out the hole. I would cover the hole with a large plastic bag and duct tape it. So you do not track through the house. Wear a dust mask.

[Stephen Tashiro]

Isn't the duct assembled in sections? Most large ducts are assembled in 5' sections, with drive clips.

I see only 1 joint in the duct. It appears to consist of two 10-ft sections that are joined. It's 1956 vintage construction.

[Stephen Tashiro]

I donÂ’t know, not sure IÂ’d want to hear the racket of a multitool cutting sheet metal in an attic for 20Â’ of cutting. IÂ’d consider something like a metal shear like this one from HF:
https://m.harborfreight.com/14-gauge-4-amp-heavy-duty-metal-shears-62213.html


I happen to have that particular tool , as well as a similar tool from DeWalt. They work well for cutting flat sheet metal. I don't know a good way to cut around the corner of a square duct with them. The duct is wrapped with a thin (about 1 inch) sheet of fiberglass insulation that must be removed or cut through.

[Bob Glenn]

Let sleeping dogs lay.

[Pat Barry]

If its a gable roof you might he able to take it out through one of the gable end vents. Maybe you cut it to fit or flatten it and bend it so it will fit.

[Tom M King]

Cutting the sheet metal wouldn't be bad, but that compressed fiberglass insulation glued to it is the worst of the worst fiberglass to have to deal with.

I'd be looking for one of these types of lining systems: https://www.ductarmor.com/ Just put a patch over anywhere that's rusted all the way through.

[Andrew Fleck]

You can easily cut that sheet metal up with some compound tin snips.

[Charlie Velasquez]

If its a gable roof you might he able to take it out through one of the gable end vents. Maybe you cut it to fit or flatten it and bend it so it will fit.

Never took something like that out of an attic; did have to put in a 2x12x20’ beam.

Something similar to Pat’s suggestion, but no gable vent.
$1.79 for a vinyl siding removal tool.
Pulled a couple of rows of siding, about 5 minutes.
Cut an opening in the sheathing big enough for the beam.
Slid it in,
Screwed a 1/4” backer over the hole
Glued and screwed the plug back in
Reinstalled the siding, (another $1.79 tool),

Seems rectangular ductwork can be pushed almost flat. Even if not, pulling a couple more rows of siding is simple.
.... just thinking....

[Tony Pisano]

I have a dewalt electric shear for sheet metal. I use it to cut up old washers and dryers, microwave oven housings, aluminum and steel. I think they are rated for 18 ga steel. Picked mine up at a pawn shop. With some coaxing, you can cut around a 90 degree bend. Cut away a narrow strip of insulation where you want to make your cut, then go to town. It will leave a long curly strip, not a bunch of small pcs. There is one moving jaw that pivots between 2 stationary jaws. Once you start you can cut all the way across a washing machine in about 15 or 20 seconds.

[Stephen Tashiro]

The old duct has been removed. The useful tool turned out to be a saber saw with a metal cutting blade. Both a Bosch corded saber saw and a Ryobi cordless saw worked well. Often the saw had to be run in bursts instead of continuously to keep vibrations of the duct from kicking the saw out of the cut. When cutting across the bottom of the duct I tipped the base of the saw away from the duct at an angle to keep the end of the blade from hitting the ceiling below the duct. ( Perhaps a better solution would be to buy a very short blade or to shorten a normal length blade by cutting the end off. )

I used a spray bottle with water and vinegar in it to mist the fiberglass insulation before I gently pulled it off the duct. The worst incidents of dust happened when a freed section of duct was pulled down from the attic through the opening I cut in the hallway ceiling. Debris from inside the duct spills out. It's best to block the end of the duct before pulling it down or pull it down so it faces sideways.

My method was to first cut out a rectangular section all the way across the top of the duct, wide enough so I could use the saber saw to make a cut across the bottom of the duct. I used a cordless drill to make holes for starting blind cuts. After making the cut across the bottom, I made cuts down the sides of the duct and used (manual) metal snips to finish the very end of the side cuts where they met the bottom cut. Connections of small ducts to the main duct were pryed apart with a small crowbar.

My observations on other tools:

Tools that cut too slow to be useful: Bosch and Porter Cable oscillating mutli-tools. Harbor Freight electric and air powered body saws, manual hack saw.

Tools that cut well in ideal conditions, but didn't work well on rusty ducts that are free to vibrate: Harbor Freight air nibbler, Harbor Freight and DeWalt electric metal cutting shears. ( I don't understand why the jaws of electric sheet metal cutting shears aren't designed to open wider. Perhaps the type of shear used to cut corrugated metal roofing would work better. Judging from pictures, the jaws on that type of shear seem to have a different design, but I've never used one.)

A Dremel Saw Max cuts the duct well but shoots a shower of sparks, so I didn't use it in the attic. (Also, the abrasive disks for it don't last long.)

[roger wiegand]

Sawzall.

But then that's how I solve most problems.

Actually first I'd try taking it apart and bending it flat. But pretty quickly I'd reach for the sawzall to cut it up. Noisy but it will only take a couple minutes-- use ear and eye protection. Snips are fine if you have patience and strong hands. I have neither anymore.

[Tom M King]

I know you're glad to have that job behind you!

[Rollie Meyers]

I have a obsolete version of this tool, https://www.kett-tool.com/products/KD-400 works great for sheet metal. Mine came from a scrap yard, needed new blades and a power cord and getting rid of the Twistlock® plug.

[Bill Dufour]

Why vinegar with the water? Good idea to wet the dust done before touching it and shaking it with power tools.
Bill D