It might surprise you to know that there are many different shapes of duct. The most basic is rectangular or square, used for the main trunk lines in a house. But how do we connect rectangular ductwork together? The answer is a simple two-part system that has been holding duct together for generations: the s-cleat and the drive. Let’s dive in.
Isometric view of an assembled slip-and-drive rectangular duct joint with s-cleats on top and a drive cleat on the side
What Is an S-Cleat?
The word of the day is s-cleat. A cleat is something that is used to connect things together. Teaming cleat with an S means that the cleat is in the shape of an S. Let’s talk about how to use s-cleats.
The s-cleat goes on the top and the bottom of the rectangular or square duct. It is a strip of sheet metal folded into an S shape, which gives it two pockets. One pocket slides onto the edge of the first piece of duct. The other pocket is left open, waiting to receive the edge of the second piece.
Here is the detail that separates a proper install from a sloppy one: the cleat goes on so that the bottom of the S rides on the duct, not hanging off of it. That way, when the second piece of duct is set into the top pocket, its weight is carried by the metal of the first duct — not by the cleat. If the weight hangs on the cleat, it stretches the cleat open and loosens the whole connection.
Diagram of an s-cleat joining two sections of rectangular duct, with duct 1 in the bottom pocket and duct 2 in the top pocket
Using the Drive to Lock It Together
The drive is the key to keeping the duct locked together. A drive that is not installed properly can lead to air leaks, unintended movement of duct during air movement, sagging, and loose connections. Bends in the duct reduce air flow — keeping duct straight and aligned is best for peak performance.
The drive is another folded strip of metal, but it works differently than the s-cleat. The side walls of each duct section have a small flange bent outward at the end. When the two sections come together, those flanges sit face to face. The drive slides down over both of them at once and clamps them together.
Then comes the part that ties the whole joint into one solid unit: the drive is cut a little long on purpose. That extra length gets bent over the top and the bottom of the duct — about an inch of overlap. When the tab folds over the corner, it locks down the end of the s-cleat too. That is how the s-cleat and the drive work together: the cleats hold the top and bottom, the drives hold the sides, and the bent tabs tie all four corners together.
Diagram showing how a drive cleat clamps the flanges of two duct sections and folds over the corner to lock the s-cleat
Step by Step: Installing a Slip-and-Drive Joint
Here is the full operation, in the order you would do it on the job.
Step 1 — Slide the s-cleats onto duct one.** One cleat on the top edge, one on the bottom edge. Remember the orientation: the cleat rides on the duct so the added duct sits on top of the first so the cleats are not stretched.
Step 2 — Set duct two into the cleats.** The edge of duct two slides into the open top pocket of each cleat. Done right, the weight of duct two is resting on duct one’s metal. The cleat is just keeping things aligned and sealed — it is not a load-bearing part, and it can loosen up if you make it one, especially on bigger ducts.
Step 3 — Drive the drives.** Slide a drive down or up each side wall, over the mated flanges. Cut the extra off the drive so an inch is left at top and bottom. Then bend the extra inch of tab over the top and the bottom. You are doing this on all four corners — two drives, four tabs. When the tabs are bent, the joint is mechanically locked in every direction.
Step 4 — Seal all the seams.** On commercial work, every seam gets sealer. Most homes do not use sealer on the duct, but we are keeping it professional here: seal it. Air that leaks out of a joint is wasted dollars. Big buildings spends big dollars to make sure air stays in systems. Also remember, leaking air whistles from small holes, you will hear a train all night if seams are left unsealed (tape could be an alternative but not perfect, a mastic is the best product).
Four-step diagram for installing a slip-and-drive rectangular duct joint — cleats, setting the second duct, driving the sides, and sealing the seams
Why This System Works So Well
There is a reason tinners have used slip-and-drive joints for generations. No welding, no special fasteners, no power tools required for the joint itself — just formed metal holding formed metal. The joint goes together fast, it comes apart if a section ever needs to be replaced, and when it is assembled with the cleats oriented right and the tabs bent over clean, it is straight, tight, and quiet.
The tools for the job are the same basic kit I cover in my ductwork tools list — snips to cut cleats to length, a hammer to set the drives and bend the tabs, and hand seamers if a flange needs dressing, even the beater screw drive to open that cleat a little if it is pinched from cutting to size. If you are just getting started, my five essential hand tools for HVAC beginners covers what to buy first.
Conclusion
Connecting rectangular ductwork comes down to two pieces of folded metal doing two different jobs. The s-cleat joins the top and bottom, with the weight of the duct riding on metal instead of on the cleat. The drive clamps the sides and locks the corners with its bent-over tabs. Seal the seams and the joint will outlast the building’s paint.
Next time you are in an unfinished basement, look up at the trunk line and find the joints. Now you know exactly what is holding them together.
*I also built a free invoice and estimate tool for tradespeople — no signup, no subscription: quickdrawinvoice.com*
