Most designers require that duct construction follow the SMACNA HVAC Duct Construction Standards. That manual is an excellent structural guide for building duct and follows a simple premise – what is the minimum metal thickness and reinforcing to limit wall deflection to a specific amount? Round spiral duct has no wall deflection in positive pressure, but both rectangular and flat oval ducts have flat surfaces that will deflect. Flat oval will have less than a comparative rectangular because you can subtract out the curved sides. You also have those 4-ply spiral lock-seams every 4 ½” to help break up the flat surface. The reason we bring up “2 in. WG or more” is that static pressure determines how much deflection a duct surface will have. The SMACNA manual has gauge deflection charts for seven different pressure classifications for rectangular duct – ½”, 1”, 2”, 3”, 4”, 6”, and 10” WG. Only one for flat oval spiral duct – 0” to 10” WG.