Non-asbestos flexible sheet material suitable for use in gaskets for the cylinder head of an internal combustion engine is made by dewatering on a water-permeable conveyor a layer of aqueous slurry and compressing and drying the dewatered layer, the aqueous slurry employed being one that contains the following ingredients in the following proportions by dry weight:______________________________________ ball clay 25-44% calcium silicate of high specific 15-40% surface area cellulose fibres 3-15% organic polymer binder 5-15% fibrillated poly (aromatic amide) 1-15% fibres ______________________________________