The spacing question gets all the attention. How far apart do the hangers go is what everybody asks and what everybody writes about.
Almost nobody answers the other half: what is the strap itself. What gauge, how long you cut it, how you get a bend in it standing on a ladder, and how many screws go where.
That is this one.
What gauge strap to hang duct
We followed the SMACNA standards or the job specification, whichever was more stringent. That is the honest answer and it is also the only defensible one — if the spec is tighter than the standard, the spec wins, and if the standard is tighter, it wins.
In practice, most of what we hung was 24 gauge strap. That covers the great majority of duct on a normal job.
This is worth saying plainly because the tables get quoted as though somebody is looking them up on every run. The tables are how you check yourself, and how you defend a decision later. Day to day, a shop runs one strap for most of the work and steps up when the duct tells it to.
Snips, seamers and a screw gun will get you through everything on this page. If you are still building the bag, here is the ductwork tools list.
The ladder: strap, then rod, then cable
Three rungs, and you move up them as the duct gets bigger and heavier.
- Strap. 24 gauge, most of the time, most of the job.
- 3/8 inch all-thread and Unistrut. When the duct is big enough that a strap is not the right support anymore.
- Steel cable and clamping accessories. Rare. I have used it, but not often and on low budget projects.
Past that rung there is a fourth one where an engineer designs the support and you install to a drawing. That end of the ladder is covered here. Most work never gets near it, but it is worth knowing the ladder keeps going.
What the strap comes as
A roll. We always ordered 500 feet at a time. You cut what you need off the roll with a pair of snips — there is no pre-cut length and there is no measuring at a supply house counter.
Unistrut and all-thread come in 10 foot lengths. I am sure both come in other sizes; that is what showed up on our jobs.
How a strap holds rectangular duct
The strap comes down the side of the duct and bends under the bottom. That bent piece is the tab, and it wants to terminate about an inch under the duct — enough to take a screw, not so much that it hangs out into the room.
The screw goes in the tab. That is what keeps the duct from walking off the strap.
Round is a different animal entirely — the strap wraps the whole way around the pipe and screws to itself. Different length, different screw count, different everything. That is its own post.
How many screws go in a duct hanger
- Rectangular: three per strap. One in the tab underneath, and one in each side wall.
- Round: two. Through the lap where the strap meets itself.
SMACNA has a standard for this and your specification may too. Whichever is more stringent is the one you follow. The numbers above are what we ran.
What screw
Hex head self-tapping screws. A self-tapper is bit-tipped — it cuts its own hole and there is no pre-drilling.
- Residential, into duct: 1/4 inch hex head, 1/2 inch long
- Commercial, into duct: 5/16 inch hex head, typically 3/4 inch long
- Into a joist: 2 inch long. You are into wood now, and a half inch screw is holding nothing.
These are a general idea and they held on the jobs I worked. Check your specification or the SMACNA standards for anything that deviates before you buy a case of the wrong screw.
How to get a clean bend in strap without a brake
Here is the real problem nobody addresses. The bend has to be tight, and you do not have a brake.
You are not measuring straps, driving back to the shop, running them through the brake and driving out again. The bend happens where you are standing, with what is on your belt.
Bend it cold with a pair of hand seamers and you get a wide bullnose — a soft, rounded curve instead of a corner. It holds, but it looks like nobody cared and the duct sits a little differently than it should.
Bite it first
Mark your line. Then take your snips and bite about a thirty-second of an inch into each edge of the strap, right on the mark. Both sides. You are not cutting through — you are just breaking the edge. Be sure not to cut through or you are getting new strap.
Now bend on that line with your seamers.
The bend goes exactly where you put the bite, and it comes out crisp. The two little cuts give the metal a place to fold instead of letting it roll. Same tools, same thirty seconds, and the difference is a corner instead of a curve.
Word of the day: bullnose. The fat, rounded edge you get when metal bends without being told exactly where.
Fold the top end over itself
This is the one I would put on a card and hand to a first-year.
A strap fails at the screw head. The strap is weak and doesn’t stay tight to the anchor point, the loose bend anchored the support stretches to the screw head or it catches only the edge of a joist and the whole weight of the run tears it out. Either way you find the duct out of plumb and having to readjust.
So cut the strap long, fold the top end back over itself, and put the screw through the doubled part. Be sure to hit is flat with your hammer. The strap becomes its own washer. Two thicknesses of metal spread the load under the screw head instead of one, and the head has something to bear against.
The bottom end does not need it. That end is screwing into duct, not carrying the whole run.
It costs an inch of strap and one extra bend. Nobody inspects for it and nobody will ever know you did it, which is roughly the definition of doing it right.
Unistrut, and the one thing it does that strap cannot
When it moves up to Unistrut, the geometry changes. The strut runs full length underneath the duct and is held up by all-thread, washers, nuts and an anchor into the structure.
And here is the part that matters: all-thread is adjustable after it is installed. Spin the nuts and the height changes. A strap is a fixed length the moment you screw it off — if it is wrong you cut a new one.
That is not a small thing on a long run that has to hold a line, or on a job where somebody else’s work is going in above you and something has to give.
The anchor is different too. Unistrut goes up on a lag bolt or clevis hanger into a receiver thread, not a self-tapper — so the fold-it-over trick does not apply. Different fastener, different failure.
Getting the straps perpendicular
Straps should look straight up and down. I used to hear that as a cosmetic instruction. It is not, and it is not purely structural either — it is both, and the two are not separable.
The physics half
Duct hangs at the lowest point of force. Every strap on the run is agreeing on where that is.
One strap at an angle is pulling sideways as well as up. It drags the duct to a different resting point than the straps on either side of it wanted, and that hanger’s effective height ends up different from all the rest. There is probably a tolerance on it somewhere. But if it looks wrong, it probably is.
The other half
Physics is real and so is the eye.
One piece of a system that does not look like the rest of it gets noticed. It reads as a lack of care from the people who installed it, and that is what comes back as feedback — not the strap, the impression.
It comes down to doing it right to show how much you care about the end product, and giving the person who ends up with it the best experience of it. That is worth an extra thirty seconds per hanger.
What I have seen fail
Straps pulling out of joists. Almost always if the screw caught only an edge of the joist instead of landing in solid wood. It holds while you are looking at it. Then the weight of the system works on it and out it comes.
Two fixes and you should do both: land the screw properly in the joist, and fold that top end over so the strap spreads the load instead of concentrating it to the screw head.
Common questions
What gauge strap do you use to hang duct?
24 gauge covers most duct on a normal job. The real answer is SMACNA or the job specification, whichever is more stringent — but in practice a shop runs one strap for the majority of the work and steps up to rod and Unistrut when the duct gets big enough to need it. If needed, an engineer may need to design the supports.
How many screws go in a duct hanger strap?
Three on rectangular duct — one in the tab that bends underneath, one in each side wall. Two on round, through the lap where the strap screws to itself.
How do you bend hanger strap cleanly on the job?
Bite about a thirty-second of an inch into both edges of the strap with your snips, right on your mark, then bend that line with hand seamers. Without the bite the metal rolls and you get a wide bullnose. With it, the bend lands exactly where you marked and comes out crisp.
What size self-tapping screw for duct hangers?
Into duct, 1/4 inch hex head at 1/2 inch long on residential and 5/16 inch hex head at about 3/4 inch on commercial. Into a joist, 2 inches long. Check your specification or SMACNA for anything that deviates. And be sure to account for the thickness of the gypsum board, if applicable.
When do you use Unistrut instead of hanger strap?
When the duct is big and heavy enough that strap is no longer the right tool. Unistrut runs full length under the duct on all-thread, and unlike a strap it stays adjustable after installation — you spin the nuts to change the height instead of cutting a new piece.
Why do duct hanger straps need to be straight up and down?
A strap at an angle pulls the duct sideways as well as up, so it drags the run to a different resting point and sits at a different effective height than the hangers either side of it. It also looks wrong, and a system that does not look consistent reads as a lack of care to whoever ends up with it.