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SMACNA standards, seam types, and some tips on the basics of manufacturing and installing HVAC rectangular duct

Let’s start at the top – a rectangular duct section 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 running 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

Snap lock longitudinal duct seam

Pittsburgh Seam

Pittsburgh longitudinal duct seam


The transverse seam is running perpendicular to the airstream, connecting joints of duct or fittings 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.

Slip and drive duct acessories

*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



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.

TDF vs TDC Profile
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 (or screwing) at 6″ intervals along the flange.

Exploded view of a TDF/TDC joint connection


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.

  1. All ductwork will have stiffening beads that provides rigidity to the surface and keeps it from flapping and making noise.
  2. In accordance with SMACNA, internal conduit stiffeners will be present on most medium/large ductwork. Our tags will indicate a pattern in the following format X – X – X. The first and last number will indicate the quantity at either end. The middle number tells how many stiffeners in the middle of the duct.


The necessary evil – insulation is effective, necessary, but can be a headache for whoever is installing it. When insulating your duct you have three main options: internal liner, external duct wrap, or double-wall duct.

  1. Liner –  comes in variety of densities, compositions, thicknesses, and dimensions – anything the engineer can design. Liner is the most cost effective option for insulating ductwork, as it is all done in the manufacturing process and does not have to be done onsite. Liner, however, does not work on applications that require the smooth metal surface be exposed to the airstream (exhaust, high pressure, etc).
  2. External Duct Wrap – typically a fiber glass base with a blanket-type pattern with a foil back. External duct wrap can be done before hanging the ductwork, and will usually be more effective in this manner. Wrapping ductwork, however, often times happens after the duct has already been installed and can be a very cumbersome process to get the insulation wrapped, stapled, and taped.
  3. Double-wall Duct – An inner duct (perforated or solid) will be wrapped with liner (fiber glass, mineral wool, etc) and fastened to the duct. An outer portion of duct will then be slid over this piece, and locked into place with nosing on either end. This option is the most expensive and difficult to manufacture and install.


Start by beating the longitudinal seams together. If worksite conditions allow, then put together as many sections as possible to speed up the install. When installing or sectioning duct together, always 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 of duct you are connecting to the main run 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. In similar fashion as the S & Drive duct, start by beating together the longitudinal seams. Then install the TDF/TDC corners in the duct (typically using a hammer/screw driver).  While the TDF/TDC joint is still standing, apply the gasket to the flange.  We also glue the corners to ensure there is an airtight connection. Install clips or shoot screws in 6”-8” intervals. Pro Tip: We recommend buying a drift pin to help align the corners, or at the very least a screwdriver that can fit the corners. Also, we put the bolts in the direction of the airflow to keep things neat and uniform.

A fairly new alternative to the typical corner, bolt and nut is the lag screw with a corner that accepts the threads. We found the system to be slightly faster, but ultimately much more expensive due to the high material cost of the custom corner and lag bolts.



How to Hang HVAC Ducts

Metal Ductwork

Duct Sealing