TDC Flange Connections: The Sheet Metal Standard Explained

TDC — Transverse Duct Connection — has become the dominant flange system for rectangular sheet metal ductwork in commercial and increasingly in residential applications. It replaced the older TDF (Transverse Duct Flange) system and the even older Pittsburgh lock with button punch 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 Changed

TDF (Transverse Duct Flange) was the earlier iteration. TDF flanges were separate angle-iron pieces welded or riveted to the duct end. The joint integrity depended on the quality of the weld or rivet and on a separate gasket compressed between the angle pieces. TDF joints are still found on older commercial systems and are still specified in some heavy industrial applications.

TDC improved on TDF in three ways: the integral flange eliminated potential leak paths at the flange-to-duct connection; the roll-formed profile was more consistent than field-applied angle iron; and the clip system required no bolts or screws through the duct wall. TDC joints consistently test at lower leakage rates than TDF joints of similar size.

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.