a method for heat sealing a portion of an upper heat sealable thermoplastic membrane to a portion of a lower heat sealable thermoplastic membrane which comprises placing a detachable stabilizer strip on the outer surface of the upper membrane over the over - lap seam portions to be sealed . the stabilizer strip must fully adhere or have mechanical contact to retain the dimensional stability of the upper membrane while maintaining the relative positions of the membranes . a hot air medium is directed between the upper and lower membranes thereby effecting a heat seal therebetween . once the overlapped and sealed portions of the membranes are permitted to cool , the stabilizer strip may be removed .