40 to allow any water which penetrates through the system to drain across the surface of the membrane fig1 a 15b and 15c are side and perspective views of an alternative form of foam backing 36 the upper surface of which is shaped to co operate with the undersurface of a roofing element channels 60 and cut outs 62 are provided in the foam to allow the flow of air into the air flow passages created by the spacer members such a network of channels and cut outs enhances the bidirectional air flow provided by the spacer members of the roofing elements in order to maximise the benefits of the insulation by keeping the foam backing on adjacent elements in abutment an overhang 66 and an undercut 68 are provided on at least one pair of opposing side edges towards the base of the foam backing each co operating in use with an undercut and overhang respectively on an adjacent element of course the undercut and overhang are not restricted to the shape shown in the drawings for example they could simply be generally rectangular in cross section the dashed line of fig1 b shows an example of a type of contoured.