TDC Duct Flange Connections: What TDC Means, How to Assemble

TDC - Transverse Duct Connection - has become the dominant flange system for rectangular sheet metal ductwork in commercial and increasingly in residential applications. Along with TDF (Transverse Duct Flange), the same kind of joint off a different maker's rollformer, it replaced bolted angle-iron companion flanges as the go-to method for high-integrity rectangular duct joints. Understanding how TDC works, when to specify it, and how to assemble it correctly separates contractors who get air-tight systems from those who fail leakage tests.

What TDC Actually Is

TDC is a roll-formed flange that is integrated directly into the end of a duct section. A specialized roll-forming machine (a "TDC machine") runs along the end of the sheet metal panel and forms a specific profile: a flat bearing surface perpendicular to the duct wall, with a return leg that locks a clip or bar in place. When two TDC-equipped duct ends are brought face to face, the flanges mate flat against each other and are held together with corner clips and continuous bar clips along each leg.

The critical characteristic: the flange is part of the duct panel itself, not a separate angle iron bolted on. This eliminates fastener holes through the duct wall - a major source of air leakage in older flanged systems. The entire joint is compressed between the two mating flanges and sealed with the gasket material.

TDC vs TDF: What Is the Difference

TDC and TDF are two trade names for the same type of joint. Both are flanges roll-formed from the duct wall itself, both close with corner pieces, a gasket and cleats, and SMACNA lists them side by side as transverse joints T-25a (TDC) and T-25b (TDF). The difference is the machine that forms them: the two profiles are close but not identical, so each takes its own corner pieces. Order corners to match the flange a shop runs, and do not mix the two profiles in one joint.

What both replaced is the companion flange: separate angle iron riveted or welded to each duct end and bolted together. The integral flange improved on that in three ways: it eliminated the leak path at the flange-to-duct connection; the roll-formed profile is more consistent than shop-attached angle iron; and the cleats need no bolts or screws through the duct wall. Angle-iron flanges are still found on older commercial systems and are still specified for some heavy industrial duct.

Corner Clips and Bar Clips

A TDC joint uses two types of hardware to clamp the flanges together:

Corner clips. L-shaped or C-shaped stamped metal clips that engage the flange profile at each corner of the duct. On a 24" × 18" duct, four corner clips are used - one at each corner. The corner clip prevents the joint from opening at the highest-stress points (where the flanges tend to separate first under internal pressure). SMACNA specifies minimum corner clip thickness and engagement depth by duct pressure class.

Bar clips (drive cleats). Continuous clips that run along each straight leg of the flange between the corners. For larger ducts, these may be hammered on with a mallet or driven with a pneumatic cleat driver. The bar clips are typically spaced at 6-inch intervals maximum per SMACNA requirements, though many contractors use continuous bar clips for higher-pressure work.

Gasket Selection and Application

The gasket between TDC flanges is what actually seals the joint. Without a proper gasket, the flange faces alone will not achieve an airtight seal. Three gasket types are in common use:

• Closed-cell neoprene foam. The standard choice. Typically 1/8" thick, self-adhesive, applied to one flange face before assembly. Compressed to approximately 50% of its thickness when the clips are installed. Rated for temperatures from -40°F to 250°F. Use for all standard HVAC work.
• EPDM gasket. Better ozone, UV, and weather resistance than neoprene. Specified for outdoor ductwork and systems handling industrial exhaust. Slightly more expensive than neoprene.
• Silicone gasket. Required for high-temperature applications above 250°F - industrial ovens, exhaust systems, and some commercial cooking exhaust. Significantly more expensive. Not needed for standard HVAC.

Apply gasket tape continuously around the full perimeter of the flange, including through the corners. A gap in the gasket - even 1/2 inch - can produce a measurable leak rate. For duct systems targeting SMACNA Seal Class A (the lowest allowable leakage), gasket continuity is non-negotiable.

TDC Pressure Ratings

SMACNA rates TDC connections by static pressure class:

Higher pressure classes require heavier gauge metal for the duct itself, more frequent bar clip spacing, and in some cases, additional reinforcement bars across the flange face. The duct gauge, flange gauge, and clip spacing must all be matched to the pressure class - upgrading one element without the others does not achieve the rated performance.

Assembly Sequence for TDC Joints

1. Clean both flange faces of oil, dust, and any metal burrs from the roll-forming process.
2. Apply gasket tape to one flange face only - the tape on both faces can bunch up and create uneven compression.
3. Bring the two flanges together, aligning the duct sections before the gasket contacts.
4. Install corner clips on all four corners simultaneously, using a mallet to seat them fully into the flange profile.
5. Drive bar clips along all four legs at the required spacing, working from the center toward the corners on each leg.
6. Verify that the corner clips are fully engaged - the clip should not be able to rock or shift. Any play in a corner clip indicates it was not fully seated.
7. Inspect the gasket compression around the full perimeter. The gasket should be visible but compressed to approximately 1/16" to 1/8" visible thickness.

When to Use TDC vs Slip and Drive

Slip and drive connections are faster to assemble in the field and do not require a roll-forming machine. They are appropriate for residential systems operating below 0.5" static pressure. TDC becomes the right choice when:

• The system is above 1" static pressure class
• Duct leakage testing is specified - TDC systems test significantly better
• The duct is accessible for clip installation but will be concealed afterward
• The system uses any degree of positive or negative pressure beyond standard residential ranges

PMX Ductwork fabricates custom straight duct sections, elbows, tees, and all other fittings with TDC ends in any gauge from 26 to 20. Specify TDC at both ends, one end, or mixed with slip/drive as needed for each section of your system.

Frequently Asked Questions

What does TDC mean in ductwork?

TDC stands for Transverse Duct Connection. It is a flange that is roll-formed directly into the end of a rectangular duct section, so the flange is part of the duct panel and not a separate piece bolted on. Two TDC ends mate face to face with a gasket between them and are held together with corner clips and bar clips.

How do you assemble a TDC duct joint?

Assemble a TDC joint in this order: clean both flange faces, apply gasket tape to one face only, align the duct sections and bring the flanges together, seat corner clips on all four corners with a mallet, then drive bar clips along each leg working from the center toward the corners. Finish by confirming no corner clip can rock and the gasket is evenly compressed around the full perimeter.

What gasket is used on TDC flanges?

Closed-cell neoprene foam is the standard TDC gasket. It is typically 1/8 inch thick, self-adhesive, and compresses to about half its thickness when the clips go on. Use EPDM gasket on outdoor ductwork and industrial exhaust, and silicone gasket for service above 250 degrees F. Run the gasket continuously around the perimeter, including through the corners, with no gaps.

When should you use TDC instead of slip and drive?

Use TDC when the system is above the 1 inch static pressure class, when duct leakage testing is specified, or when the system runs at any positive or negative pressure beyond standard residential ranges. Slip and drive connections are faster to assemble in the field and are appropriate for residential systems operating below 0.5 inch static pressure.