Abstract: An insulated roof support system including a plurality of transversely spaced longitudinally extending purlins having bottom flanges with insulation layers occupying the spaces between the purlins, a lattice work of transverse and longitudinal support straps extending beneath the insulation layers so that the transverse support straps extend beneath the bottom flanges of the purlins, a plurality of elongated channel-shaped support bars containing elongated insulation boards secured in flush engagement co-extensively against the bottom surface of each purlin bottom flange and the transverse straps crossing the purlins, to completely thermally insulate the roof structure including the purlins from the space below the roof structure.