AS 3958.1 stipulates minimum thicknesses of 15 mm for bonded screeds and 40 mm for unbonded screeds. BS 8204 is a bit more conservative, stipulating 25 mm for bonded screeds and 50 mm for unbonded screeds. 

However, these thicknesses should be used as a guide only. Many factors are required to be considered in order to determine the most appropriate screed/tile bed thickness. Each project is going to demand a unique screed thickness to ensure suitability, performance and durability based on the application, function and service conditions.

Read more: The Importance of Floor Screed Design