Abstract: A duct assembly system for joining duct sections each having a plurality of flat intersecting walls includes angle member connectors respectively associated with the duct walls, each connector being generally L-shaped with one leg portion extending perpendicular to the duct wall and having an elongated channel formed therein and the other leg portion comprising two biased-together flanges adapted for frictionally receiving the adjacent duct wall edge therebetween, each flange having elongated ridges which interlock with elongated ribs on the associated duct wall. Each duct wall edge is embedded in a mastic sealant in a trough in the associated connector, the trough having diverging sides for guiding the duct wall edge thereinto. Connectors on intersecting duct walls are joined by corner members formed of sheet metal and flanged for rigidity and tight fit in the connector channels. Each corner member has flanges for gripping the channel edges of the joined connectors.