The present invention concerns an insulating roof support assembly for a roof structure comprising a plurality roof elongated rafters spaced apart in a predetermined distance with insulation boards therebetween, wherein elongated mineral wool fiber insulation elements are provided on the top of each of the elongated roof rafters and elongated wooden elements on top of said insulation elements with at least one impermeable membrane between at least two neighbouring insulation elements sandwiched between the wooden elements and the insulation elements.