Rectangular duct sits on a strap. Round duct gets wrapped by one.
That one difference changes everything downstream — how long you cut the strap, where the hang point has to land, how many screws go in, and what you do when the run comes out crooked.
This is the hands on part. For how far apart the hangers go, that is its own question with its own answer.
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How long to cut a strap for round duct
The strap has to go all the way around the pipe and then come back far enough to screw into the vertical strap it hangs from.
So the length is the circumference — pi times the diameter — plus a tail. An 8 inch pipe is about 25 inches around. A 12 inch is about 38. Don’t for get to add the tail.
Hang it long. Get a rough length, wrap it, and tune the tail after the final measurements are made. Cutting to a number and hoping is how you end up one strap too short.
Strap comes on a roll — 500 feet is what we ordered — and you cut it with snips. Unistrut and all-thread come in 10 foot lengths or longer.
The hang point has to be top center
This is the part that decides whether the run stays where you put it.
The center of the pipe is the straight line of downward force. Hang from the top center and the weight pulls straight down through the strap. Hang from anywhere else and the strap is pulling the pipe sideways as well as up — and the run will move to wherever that pull puts it.
You will not notice it on one strap. You will notice it on ten.
Wrapping it and pinching it
Straps go up first, hung long. Then lift the pipe to the strap — with a buddy and a ladder if you have one, and here is how it goes if you do not.
While the pipe is held, bring the strap from the top center, around the pipe, and back to the vertical piece it came down on. It does not matter which way it laps over itself. What matters is that it starts and ends at the same place on top and stays vertical.
Then pinch the two together with a pair of swivel C-clamp locking pliers (paid link) before you drive anything.
Why locking pliers and not just your hands: once they are clamped you can let go. That gives you both hands for the screw gun, and the pliers themselves become a handle to hold against while you drive. Clamp, screw, unclamp.
If you do not have a pair, lineman’s pliers will do it. I have done it that way. You just have to hold the pinch and drive the screw at the same time, and you will feel the difference on the tenth hanger.
Locking pliers, snips, a screw gun. Here is everything else that goes in the bag.
How many screws in the strap
Two. Put them through the lap, after the wrap is pinched.
Round gets fewer than rectangular because the strap is fastening to itself rather than to the duct. A rectangular hanger gets three — one in the tab that bends under the bottom, and one in each side wall.
What screw
Hex head self-tapping screws. A self-tapper is bit-tipped — it cuts its own hole and you do not pre-drill.
- 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, not sheet metal (Make sure to account for gyp board layers in your screw length if it is applicable.)
Those are a general idea and they held on the jobs I worked. Your specification or the SMACNA standards may call for something else, and whichever is more stringent is the one you follow. Check before you buy a case of the wrong screw.
Two ways to join round pipe: a crimped end or a collar
These get talked about as if they are the same thing. They do the same job, but they are not the same part.
- A crimped end is part of the pipe. The end is squeezed down so the next piece of duct slides on over it. Nothing extra to buy — the joint is built into the duct.
- A collar is an internal coupling. A short sleeve that goes inside both pieces to join two ends with no crimp between them.
Same job, different parts. Only one of them is something you order.
Which way does a crimped end go
Most people learn this as “the crimp goes away from the furnace,” and then it bites them the first time they run a return.
The real rule is that the crimp points the way the air is going. Everything follows from that.
- Supply side — air moves away from the equipment, so the crimped end goes on the edge furthest from the unit.
- Return side — air moves toward the equipment, so it flips. The crimp faces the unit.
Either way, the air passes over a trailing edge instead of running into a leading one. Get it backwards and you have built a little scoop into every joint in the run.
If you have to crimp a piece in the field, crimp whichever end the air is heading toward.
Running this backwards in a dryer vent is very dangerous, be sure if you are crimping to have the crimps follow the air flow.
How many screws in a round duct connection
Four, at the quarters. Twelve, three, six and nine o’clock. That goes for every round connection, not just a collar.
A collar takes eight — four into each pipe it joins. It is one fitting but it is two connections.
Three is the minimum, and there is a real reason you end up at three: in a joist bay you often cannot reach the top of the pipe. When that happens put the three in thirds around what you can actually get to, rather than leaving a quarter pattern with one corner missing.
Why the quarters, and how I learned it
I ran them at ten, two, four and eight o’clock for years. Four screws, evenly spaced, and it holds fine. Nobody ever said a word about it and no inspector is out there with a magnifying glass on this connection.
I had to adjust so many of my own and then I had to straighten an attachment an old-timer did, that is when it clicked.
At the quarters, you can back out one opposing pair and the other pair still does its job. Pull the left and right and the top and bottom are still carrying the pipe. At ten-two-four-eight there is no clean pair — nothing is holding straight up while you loosen the sides, so the pipe drops on you as soon as you back anything out.
Nobody requires this. It is not in a spec and it will never get flagged. It is for whoever has to fix it later, and some of the time that is you.
How to straighten a round duct run that is not straight
You will hang a run, step back, and see it wandering. It happens.
You do not have to pull it down.
Back out the left and right screws at the connection — the top and bottom stay in and hold the pipe up while you work. With those two out, the pipe can rotate and shift. Reposition the run straight, then drive the two back in.
This is the whole payoff of the quarter pattern. Top and bottom carry it, left and right hold the line, and you can release one pair without losing the other. Screws put in wherever they landed do not give you that, and neither does ten-two-four-eight.
Where the straps land on the run
Spiral pipe ships in ten foot lengths, so the strap layout follows the pipe rather than a tape measure.
- The first piece off a take-off gets strapped twice — once near each end
- Then space off the farthest strap, not off the take-off
- Coming off a high efficiency take-off, a spin-in or a saddle, that first span often lands nearer eight feet than ten
Bigger pipe changes this. Heavier duct wants tighter spacing, and there is a size past which none of these numbers apply — I have been on projects with duct big enough that an engineer designed the supports. These are the field rules I worked to for years, not a substitute for the tables. The spacing question in full is here.
Spiral versus snap lock
Two different products and they do not hang the same.
Spiral is what you see in commercial work. Ten foot minimum lengths, no seam running the length of it, and sturdier construction. It holds its own line.
Snap lock is the house product. Five foot sections, and it needs supporting more often for exactly that reason — more joints in the same run.
I have never seen snap lock round bigger than 6 inch in a five foot length, and 6 inch is the common one. Below that, 5 inch exists but is not common, and 4 and 3 inch are mostly exhaust fans and dryer vents rather than supply.
Common questions
How long do you cut a strap to hang round duct?
Circumference plus a tail. Pi multiply by the diameter gets you around the pipe, then add enough to lap back onto the vertical strap and take two screws. Cut it long and trim the tail once the run is set.
How many screws go in a round duct strap?
Two, through the lap where the strap meets itself. Round needs fewer than rectangular because the strap fastens to itself rather than to the duct — a rectangular hanger takes three.
How many screws go in a round duct collar?
Eight — four into each of the two pipes it joins. Any single round connection gets four, at the quarters, and a collar is two connections. Three is the minimum where you cannot reach the top of the pipe, and those three go in thirds around what you can get to.
Which way does the crimped end of round duct go?
The crimp points the way the air is going. On a supply that means away from the equipment. On a return it flips — air is moving toward the unit, so the crimp faces the unit. Either way the air passes over the edge instead of into it.
What is the difference between a collar and a crimped end?
A crimped end is part of the pipe itself — the end is squeezed down so the next piece slides on over it. A collar is an internal coupling, a separate sleeve that goes inside both pieces to join two ends that have no crimp. They do the same job and get used interchangeably, but only one of them is a part you order.
Can you straighten round duct after it is hung?
Yes, without taking it down. Back the left and right collar screws out, leave the top and bottom in to carry the pipe, shift the run straight and drive the two back in. You may need a new hole if the shift is small enough, just be sure to seal the old holes with duct mastic.
Does round duct hang differently than rectangular?
Yes. Rectangular sits on a strap that bends under the bottom and takes a screw in the tab. Round gets wrapped all the way around and the strap screws to itself. Different strap length, different screw count, different hang point.
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