Because strength was no issue when BS680 was written 70 years ago; a strength test was not integrated and this particular has become a question in the face of weak imported slates. Gladly; the new standard deals with this issue. Firstly; that the rock strength; the ?modulus of rupture?; try assessed on the trial of 40 test pieces and from this specific strength values together and across the slates have always been determined. Such values can then be used to an equation which provides a lowest basic slate depth; ebi.