FIELD: manufacture of building materials. SUBSTANCE: at least one component A containing at least one alkylhydroxypolysiloxane, at least one hydrocolloid B containing at least one galactomannane, and at least one polysaccharide C containing at least one gum are added to plaster powder to form powdered composition containing at least one hydrocolloid B in amount 0.05-0.5%, solid based on at least one supported component A in amount 0.25% of the weight of plaster, and at least one polysaccharide C in amount 0.01-0.5%. Waterproofed plaster is used to manufacture products in the form of sheets and boards. EFFECT: optimized waterproofed plaster preparation. 27 cl, 3 tbl, 3 ex
