Sheet of roofcovering material and method to apply this sheet in or as a roofcovering

Claim 1

A sheet member, used as a roof-covering material comprising: a first base layer having portions made of a porous material and having a plurality of apertures throughout disposed between said portions of porous material, said base layer having a lower and an upper surface, said lower surface to be applied on a roof; a second layer of moisture-tight coating disposed over said upper surface of said porous portion of said base layer; and a third layer of bituminous material disposed on the upper surface of said second layer, whereby said sheet member is adapted for attachment to the roof by heating said third layer of bituminous material while disposing it over said second layer to cause a flow of the molten bitumen from said third layer through said apertures in said base layer to firmly attach said sheet to the roof surface at the locations of said apertures, said flow of said bitumen being prevented from penetrating said porous material portion of said base layer by said second layer whereby the porous base layer allows escape of the vapor trapped under the sheet member to the surrounding and prevents formation of blisters in said sheet member.