A support assembly for a screen enclosure includes a collar device having a sleeve section with a central passageway for receiving an upright post of the enclosure and permitting the sleeve section to be adjustably positioned and fastened to the post. At least one channel is attached exteriorly to the sleeve section for receiving and being secured to a respective structural beam such that the beam is supported by and extends transversely to the upright post. The structural beam includes wide front and back face sections interconnected by narrow top and bottom side sections. At least one of the face sections has a spline groove formed longitudinally therethrough for receiving a screen panel and complementary spline component to secure the screen panel to the structural beam.