10 is greatly increased and a large portion of that surface area extends into the interior of the building where convective currents of air will flow thereover and increase heat transfer rates accordingly preferably the roof panels 11 are secured directly to the purlins 12 by screws 13 and preferably no insulation is used to break the thermal path between the panels 11 and the purlins direct contact is preferable because it provides a rigid structure which will not shake and move objectionably when thermal expansion and contraction high winds machinery in the building or the like apply forces to the building since these forces move the building the screws 13 holding the roof panels 11 tend to work within the screw holes in the roof panels thereby enlarging the holes these enlarged holes provide paths through which heat can escape from the building by convection and by which rain water can enter the building referring now to fig2 a portion of one of the purlins 12 is shown the purlin 12 has a z cross section in which a web 14 which provides bending strength for the purlin has oppositely directed flanges 15 and 16 attached at the ends thereof a great deal of heat can be transferred.