Rectangular Duct Joints

Rectangular duct joints represent the critical connection points in sheet metal ductwork systems where individual duct sections are mechanically fastened to create continuous air distribution pathways. Joint selection directly impacts system air leakage rates, structural integrity, installation labor costs, and long-term reliability.

Joint Classification and Function

Rectangular ductwork requires two primary joint categories: transverse joints connecting sequential duct sections along the airflow path, and longitudinal seams forming the duct body from flat sheet metal. Each joint type addresses specific structural and sealing requirements based on duct pressure classification, size, and construction class.

Transverse Joint Systems

Standing S-slip joints provide economical connections for low-pressure systems, utilizing interlocking edges that create mechanical engagement without additional reinforcement. The standing S configuration forms a continuous hook that maintains alignment during thermal expansion while permitting limited angular movement.

Pocket lock joints offer enhanced strength for medium-pressure applications, creating a four-thickness metal lock that resists separation under positive or negative pressure. The pocket depth typically ranges from 0.375 to 0.5 inches, providing mechanical advantage that distributes stress across the connection interface.

Button punch snap lock joints combine rapid field assembly with adequate strength for commercial installations. Punched buttons create positive mechanical engagement that prevents joint separation, while the snap-lock edge facilitates tool-free connection during installation.

Drive slip joints serve as the fundamental connection for rectangular ducts in low to medium-pressure systems. The slip joint depth varies with duct size and pressure classification, typically ranging from 1 to 2.5 inches. Proper engagement requires uniform seating around the duct perimeter to prevent air leakage and structural weakness.

Flange Connection Systems

Companion angle flanges represent the most common reinforced connection method, utilizing continuous formed angles mechanically fastened to duct ends. Angles are typically formed from 20 or 22 gauge galvanized steel, with corner assemblies bolted at 3 to 6-inch spacing depending on pressure requirements.

TDC (Transverse Duct Connection) angle flanges provide factory-installed corner pieces with integral bolt holes, reducing field labor while ensuring consistent gasket compression. The four-bolt corner design creates predictable sealing characteristics when paired with appropriate gasket materials.

Flat bar flanges offer maximum strength for high-pressure systems and industrial applications, utilizing structural steel bar stock mechanically attached to duct ends. Bar thickness ranges from 0.125 to 0.25 inches, with width selected based on duct size and operating pressure.

C-channel flanges combine light weight with adequate stiffness for medium-pressure systems, utilizing roll-formed profiles that provide resistance to deflection while minimizing material mass. The channel depth typically ranges from 0.75 to 1.5 inches.

Rolled edge flanges create hemispherical reinforcement along duct edges, providing increased stiffness without added components. This method suits low-pressure systems where labor costs favor integral reinforcement over applied flanges.

Pressure and Seal Classification

Joint selection correlates directly with SMACNA pressure classifications and seal class requirements. Seal Class A installations typically require flanged connections with continuous gaskets, while Seal Class C permits slip joints with mastic sealant application.

Positive pressure systems experience outward stress on joints, requiring mechanical fastening that resists separation. Negative pressure creates inward stress that may collapse inadequately reinforced connections, particularly at aspect ratios exceeding 3:1.

Installation Considerations

Joint alignment affects both air leakage and system static pressure loss. Misaligned joints create turbulence that increases pressure drop while reducing effective flow area. Proper installation requires square cuts, consistent engagement depth, and uniform fastener spacing.

Thermal expansion accommodations must address expected temperature differentials, particularly in systems handling outdoor air or serving process equipment. Expansion joints or flexible connections should be positioned to prevent stress concentration at rigid joint interfaces.

Material Compatibility and Corrosion

Dissimilar metal contact at joint interfaces accelerates galvanic corrosion in humid environments. Stainless steel fasteners provide superior corrosion resistance compared to carbon steel alternatives, particularly in coastal or industrial atmospheres.

Joint sealants must remain pliable throughout the design service life while resisting ultraviolet degradation, ozone exposure, and thermal cycling. Water-based mastic sealants offer low VOC emissions, while solvent-based formulations provide superior adhesion to oily or dusty surfaces.