As strength was no concern once BS680 was written seventy years ago; a strength test was not integrated and our has been a downside in the face of weakened imported slates. Happily; the new standard deals with this issue. Firstly; that the stone strength; the ?modulus of rupture?; is actually calculated on a sample of 40 test pieces and from this specific strength values alongside plus over the slates are definitely determined. All values can then be used to a equation which gives a minimal basic slate thickness; EBI.