slope direction of the bottom of grooves 48 or of the lower membrane 26 as the case may be in the case of panels 22 having small dimensions e g one square meter or less the upper face of the panels may be flat and grooveless the insulating panels are maintained in a common plane by lap joints 58 which are conventionally formed by a panel edge notch defined by the upper and lower vertical faces 60 62 connected by the horizontal face 64 as shown in fig6 to 9 the notch in two adjacent sides of the panel is inverted relative to the notch in the two other sides of the panel however the panels 22 may have flat edges and be endwisely connected with spacers if required which spacers may be integrally molded to the panels the insulating panels may have a constant thickness as shown in 22a and 22b 22b 39 in fig3 and 4 respectively or of variable thickness as shown at 22c in fig5 wherein the draining direction in grooves 48 is indicated by arrow 66 when panels 22c having a sloped upper face are employed those of these panels which are sufficiently spaced from the upper drain must have a larger thickness than that required to satisfy the established norms for roofing insulation properties too much insulating material would then be required to circumvent this drawback one needs only to modify the panels 22c as shown in fig1 and 17 the panels 22 39 c are provided with cavities 146 at their lower face the depth of these cavities is in direct relation to the thickness of the panel section in register therewith the lower grooves 54 39 open into these cavities which.