A slurry coat of formulated concrete is applied to a new or existing top coat of lightweight insulating concrete to repair cracks and holes and attach a base ply roof membrane, thereby eliminating the need for a base sheet and mechanical fasteners when installing a roof membrane system on the lightweight concrete deck of a roof structure. The formulated concrete slurry comprises a mixture of Portland cement (type I), cellulose, poly vinyl alcohol, and water. A base ply (modified or single ply) roof membrane having a sanded or fleece bottom side is set into a 1/4 inch thick slurry coat of the formulated concrete to effectively attach the base ply roof membrane to the lightweight concrete deck.