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All the basics you need to know about manufacturing and installing HVAC rectangular ductwork.

Let’s start at the top – a rectangular duct is a combination of the following items:

  • Longitudinal Seam (Pittsburgh | Snap Lock | Welded)
  • Transverse Seam (Slip & Drive | TDF/TDC | Ductmate | Raw | Welded | Various Bends)
  • Strength (Stiffening Beads | Stiffening Rods/Conduit)
  • Insulation (Double Wall | Internal Liner | External Duct Wrap)
  • Basic Install (Assembly | Hanger | In-air installation)

The longitudinal seam on ductwork is the joint that is parallel with the air stream, typically either 56” or 59” long on full joints of duct. Both the pittsburgh seam and the snap-lock seam are male and female systems, with a difference in the locking system. Pittsburgh seams are used on heavier gauge, larger, and higher static pressure ductwork as the end of the female seam folds over the male portion – locking it in place. Snap-lock is typically used in smaller, lighter gauge applications as the male has a button depressed into the metal that catches inside the female seam when hammered together.

Snap-Lock Seam

Pittsburgh Seam


The transverse seam is where two joints of duct or fittings are connected together. Most jobs use a combination of these transverse seams; slip/drive, TDF/TDC, ductmate, raw, and various outside flanges.

Slip and Drive – common style on smaller, lighter gauge duct. When ordering duct, typically the Slip typically runs the width of the duct and the drive is the height of the ductwork. [SEE TABLE BELOW FOR SMACNA SIZING].

Slips or commonly referred to as “S” allows the metal to overlap within the S. As the width of the duct gets larger, the “S” goes from flat to standing, giving it more rigidity. The slips should always be ½” smaller than the width of the ductwork.

The drive flange is a 180° outside flange formed onto two opposing sides of the duct. The external drive connector is then hammered on from the top or bottom, pulling the two flanges tightly together into the “C” shaped metal connector. Drives are 3” longer than the height to allow a 1 ½” bend over the top and bottom.

*Basic information, based on 1” static pressure.*

Duct Dimension Gauge Length Connection Type Duct Reinforcement
0-16” 26 5’ Flat S & Drive None
17-28” 26 5’ Standing S & Drive None
29-42” 24 5’ TDF None
43-84” 22 5’ TDF Yes
85-96” 20 5’ TDF Yes
97”+ 18 5’ TDF Yes


TDF/TDC Profiles

Although these are used somewhat interchangeably, they have one major difference. The TDF (T-25b) uses a profile that hems the metal to the inside, leaving a more friendly edge to grab. The TDC (T-25a) on the other hand, hems the metal towards the outside, leaving a raw edge exposed. Both profiles require additional items to complete the connection (corners, bolts/nut assembly, clips, gasket, and glue). We use the TDF profile on all of our duct as there is no convincing argument for using TDC, and it exposes workers hands to even more handling injuries.

This style of connection for rectangular ductwork is formed directly onto the ductwork. This provides a solid metal connection that reduces leakage. A TDF connection is completed by installing the corners, applying gasket to the flange, mating it to another flange, placing nuts and bolts in the corners, and installing cleats at 6″ intervals along the flange.


There are a few things working together to provide integrity to the rectangular ductwork. (1) stiffening beads running parallel with the transverse seam, (2) internal conduit stiffeners, (3) angle on the outside of the duct (4) transverse seams themselves. We adhere to SMACNA standards, which come standard on all our ductwork.


The necessary evil – insulation is effective, necessary, but can be a headache for whoever is installing it. There are three main options: internal liner, external duct wrap, or double-wall duct. The double-wall duct is the most expensive, extensive as there are two ducts sleeved inside each other, sandwiching insulation.


Assuming all the duct has been correctly ordered, start by beating the longitudinal seams together. If space and conditions allow, have someone start installing hangers (hanger strap or trapeze hangers), while you are sectioning joints together. Alternate longitudinal seams to keep the duct from twisting. Seal as much of the ductwork as possible before install.

When installing the S & Drive duct, start by putting the two slips on the stationary duct. Then swing the joint you are connecting to the side at an extreme angle so only the corners of the top and bottom will enter the slips. Start your drive on the side that is touching. Swing the joint (bumping it slightly to ensure it doesn’t catch) around and installing the other drive. Hang. Seal.

Installing TDF duct is similar, but using gasket, bolts and nuts. I would absolutely recommend buying a drift pin to help align the corners, or at the very least a screwdriver that can fit the corners. We also glue the corners to ensure there is an airtight connection. Install clips or shoot screws in 6”-8” intervals.