What’s the difference between a snap lock and a Pittsburgh seam?
Snap lock is the standard seam on most commercial rectangular duct. The
machine forms a female edge with a turned down lip and a male edge with
punched notches. Push them together and the notches catch. Pittsburgh is
for the pieces that cannot go through the snap lock machine — most often
the elbow.
A seam is not a joint
This is worth getting straight before anything else, because first-year guys mix
these up constantly.
A seam runs along the length of a piece of duct. It is how the flat metal
becomes hollow.
A joint is where two finished sections meet end to end. That is s-cleats
and drives, and that is a different article.
Snap lock and Pittsburgh are both seam.
How a snap lock works
The machine puts a female edge on one side and a male edge on the other.
The female edge has a piece turned down inside it. The male edge has
notches punched into it. When you drive the male edge into the female
edge, those notches catch on the turned down piece and lock.
That is the whole mechanism. No screws, no sealant (unless the spec calls for it), nothing to tighten. It is why snap lock is the standard on most commercial duct — the machine does the work and the seam holds itself shut.
You will know when it is closed when the male lip is flush with the female lip. After you set one end in the snap lock, hold the other end a few inches above the female snap lock seam. If you do not hold it up the male insert will wrinkle and it is hard to straighten to fit properly.
Why the elbow gets a Pittsburgh
A snap lock needs to run through the machine. Some pieces cannot.
The rectangular elbow is the one you will meet most. The inside corner
does not give the machine what it needs, so the seam gets made a
different way.
The top and bottom pieces get turned down into a male insert edge. Those
go into a female bent angle on the vertical length. Then the extra lip on
the female side gets hammered over the top and bottom faces, and that is
what locks it.
Same idea — male into female — but you are closing it with a hammer
instead of a machine button punch. The lip does not need a pounding, it needs a gentle rolling tap to lock it over the male end.
Is the Pittsburgh seam going away?
You could absolutely use a Pittsburgh machine for most of the seams. In fact, the Pittsburgh machine has been around for nearly 90 years. It revolutionized how duct was made and the machine and name are still around.
The snap lock seam is faster to assemble and is the most common in my experience. The Pittsburgh seam is still necessary and will be around until an invention is made to take care of the inside 90 degree angle of a turning rectangle duct route. This is why the elbow gets a Pittsburgh.
When the lock is loose
Snap lock usually holds fine on its own. Pittsburgh usually does too,
once that lip is hammered over properly.
But you will get pieces where it does not. Bad forming, a lip that did
not take, a corner that opened up when hanging.
That is where a few screws belong. Not everywhere — the seam is not
designed to be screwed shut and a row of screws down a seam tells anybody
looking that something was wrong with the piece.
Usually when the lock is loose, the defect is in manufacturing the duct in the shop. The piece of metal didn’t move along the guide properly or the forming machine needs some tuning or grease (or the new guy was getting his feet wet). A few screws will hold it together but be sure to seal the duct, the seam is not tight.
The biggest draw back to poorly manufactured duct is time, what takes 1 minute to assemble a snap assembly turns into 20 minutes of keeping a flimsy seam together and it really does look terrible. If the connection is not decent, try a new piece of duct.
Common Questions
Which seam is standard on commercial rectangular duct?
Snap lock. It comes off the machine already male on one edge and female
on the other, and it locks itself when the two are connected by inserting the male side into the female side.
Why can’t a rectangular elbow use a snap lock?
The inside corner cannot go through the machine, a manual 1/4″ male turn down needs to be bent and inserted into the Pittsburgh female receiver. Roll tap with your hammer over to lock it in.
What is a button punch?
Notches punched into the male edge when machining is button punch snap lock.
Do you need screws on a snap lock seam?
Maybe, lean toward less often than every piece of duct. a seam only needs help if it is not setting properly. Bad forming, a lip that did not take, a corner that opened up when hanging. That is where a few screws belong. Not everywhere.
Is a Pittsburgh stronger than a snap lock?
This is an opinion. Using a Pittsburgh seam for the entire length of a 4 to 5 foot piece of duct would take so much time to turn over the lip. It would be a stronger seam but the snap lock is much quicker and easier to install.
Just starting out? Here are the hand tools worth buying first.